Aramco Engineering - Evaluating Motor Specifications

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Evaluating Motor Specifications

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Electrical File Reference: EEX20303

For additional information on this subject, contact W.A. Roussel on 874-1320

CONTENTS

PAGE

Specifying Motor Design Requirements

1

Specifying Motor Enclosure Requirements

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Specifying Motor Starting Methods

43

Specifying Motor Protection Requirements

61

WORK AID Work Aid 1: Motor Design Requirements for Saudi Aramco Installations Compiled from SADP-P-113, NEMA MG-1 and Established Engineering Practices

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Work Aid 2: Motor Enclosure Requirements for Saudi Aramco Installations Compiled from SADP-P-113, NEMA MG-1 and Established Engineering Practices

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Work Aid 3: Conditions Under Which the Various Types of Motor Starters Should be Specified for Use at Saudi Aramco Installations, Compiled from SADP-P-113, NEMA MG-1, and Established Engineering Practices Work Aid 4: Conditions Under Which the Various Types of Motor Protection Should be Specified for Use at Saudi Aramco Installations, Compiled from SADP-P-113, NEMA MG-1, and Established Engineering Practices

121

GLOSSARY

Saudi Aramco DeskTop Standards

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136

Engineering Encyclopedia

Electrical Evaluating Motor Specifications

SPECIFYING MOTOR DESIGN REQUIREMENTS This section will provide information on the following topics that are pertinent to specifying motor design requirements: _Stator _Rotor _Bearings _Vibration Monitoring _Mechanical Noise _Shaft Circulating Currents _Stator Windings and RTD's _Rotor Windings _Mounting Details _Cooling System _Control and Supply Leads _Nameplates _Space Heaters _Testing Requirements _Painting and Coating _Packing


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Stator A stator is defined as the stationary part of a machine that houses the windings. The stator is the integral unit that consists of the outerhousing and the baseplate. The only real design requirement for the stator frame and baseplate is that they be constructed of fabricated steel that is strong enough to withstand all of the stresses to which the stator will be exposed during shipping and operation. For motors that are above 150 kW (200 Hp), mechanical alignment devices must be installed in the baseplate to provide for accurate horizontal alignment. Examples of motor alignment devices are placement pins or foot pegs. The length of these devices can be varied to raise or lower one area of the motor. Stator mechanical alignment devices should not be the sole means of support for the stator. Shim material also should be provided with the motor to allow for accurate motor alignment and support prior to initial motor operation. Rotor A rotor is defined as the rotating component of a machine that has a shaft. The rotor of a motor must support the field winding. The following types of rotors are for use in Saudi Aramco motors: _Cylindrical Rotors _Salient Pole Rotors Cylindrical Rotors

Squirrel-cage induction cylindrical rotors are the only approved rotors that are used in induction motors for Saudi Aramco installations. The squirrel-cage induction rotor is a simple, sturdy design that allows the rotor to withstand arduous conditions. The construction of the squirrel-cage induction rotor begins with a simple shaft. Laminated supports are connected to the shaft, and, together with the shaft, they form the iron core of the rotor. The iron core increases the permeability of the rotor. The laminated supports are insulated from the rotor and from each other. Rotor bars, which are the material into which a voltage is induced, are attached to the outside of the laminated supports. An ending or shorting ring is attached at each end of the squirrel-cage induction rotor to electrically connect all of the rotor bars to complete the electrical circuit. Synchronous motors also can be designed with a cylindrical rotor, which is sometimes called a turbo rotor. The cylindrical rotor for the synchronous motor is constructed through the embedding of the windings in slots that are machined into the iron core. The embedding of the windings limits centrifugal force on the rotor and allows the cylindrical rotor to be operated at higher speeds without damage. The synchronous cylindrical rotor is used on


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motors that run at 3600 rpm or faster. Synchronous motors that have cylindrical rotors are very rarely applied in Saudi Aramco applications.


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Salient Pole Rotors

Salient pole rotors, which are used in synchronous motors, are available in the following types: _Laminated, salient pole rotor with a cage damper winding in each pole for starting. _Solid pole rotor with solid, bolted pole pieces. Either form of salient pole rotor is acceptable for use in Saudi Aramco applications, but most Saudi Aramco synchronous motor applications use the solid pole design. The solid pole design is preferred because of the very simple heavy duty construction and the rotor's high thermal capacity. The basic advantage that the solid pole rotor has over the laminated salient pole rotor is the absence of damper bars and end rings, and this absence ensures that there are fewer failure points on the solid pole rotor. Solid pole salient rotors for Saudi Aramco applications can be constructed through use of the following designs: _Solid, forged rotor shaft and pole body with solid pole shoes. _Cast steel body and hub with forged steel stub shafts and solid bolted shoes. When a rotor is specified for use in Saudi Aramco applications, the critical speed of the rotor must be examined. A rotor that operates at or near the critical speed of the motor will cause excessive vibration of the motor. The running speed of any motor must be different than the critical speed of the motor in order to prevent vibrational damage to the motor. The following two types of salient pole rotor shafts are acceptable in the analysis of critical speeds: _Rigid shaft rotors in which the first critical speed for vibration exceeds the running speed of the motor. _Flexible shaft rotors in which the first critical speed for vibration is less than the running speed of the motor. The first critical speed for ridged shaft rotors will be at least 115% of rated rotor speed. The first critical speed for flexible shaft rotors will be between 65% and 85% of rated motor speed. The second critical speed for both rigid and flexible shaft rotors must not be within plus/minus 10% of the second harmonic, which occurs at two times the rotor speed.


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Bearings Bearings are involved in a majority of motor failures. Because many motor failures are related to bearings, much attention should be paid to them as both the possible cause of a problem and a symptom of a problem. This discussion of bearings will include the following topics: _Bearing Types and Applications _Bearing Lubrication _Bearing Housing and Protection _Bearing Life Bearing Types and Applications

The following types of bearings are used in Saudi Aramco motors: _Antifriction _Sleeve Antifriction bearings are classified according to the type of rolling mechanism in the bearing. The rolling mechanism of an antifriction bearing can be a ball-type or a roller-type mechanism. The ball-type bearing that is shown in Figure 1A contains small balls, and the roller-type bearing that is shown in Figure 1B contains small rollers. The following is a comparison of the load capacity and the misalignment capabilities of balltype and roller-type antifriction bearings:


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Radial Bearing Type

Load Capacity

Ball Type Roller Type

Good Excellent

Thrust Load Capacity Fair Poor

Misalignment Capability Fair Fair

Each antifriction bearing application will have an equivalent ball-type and roller-type bearing that can be used. The type (ball or roller) of antifriction bearing that is selected should be based on the speed and load characteristics of the installation. Ball-type antifriction bearings have a small area of contact between the ball and the race. The small area of contact allows the ball-type bearing to operate at higher speeds, but the ability to carry load is reduced. Roller-type antifriction bearings have a much larger area of contact between the roller and the race. The larger area of contact allows the roller-type antifriction bearing to carry a higher load, but the speed capability of this bearing is reduced.

Antifriction Bearings Figure 1


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The sleeve bearing will be of the journal type. Sleeve bearings on horizontal machines are split to facilitate installation and maintenance. The two halves of horizontal motor sleeve bearings must be mechanically interchangeable. The applications for each type of bearing are based on the speed factor (Dn) of the bearing. The speed factor (Dn) is the product of the internal diameter of the bearing in millimeters (mm) and the motor speed. The application of bearing types for any given speed factor is based on Saudi Aramco experience, and it is presented in Saudi Aramco Design Practice SADP-P-113. The following list shows the type of bearing that should be applied for different speed factors: Speed Factor

Bearing Type

up to 250,000 up to 300,000 above 300,000

Antifriction, grease or oil lubricated Antifriction, oil lubricated Sleeve

For an example, the speed factor (Dn) for a motor that has a shaft diameter of 127mm and that operates at 3600 RPM can be calculated as follows: Dn = (Internal Bearing Diameter) (Motor Speed) Dn = 127mm x 3600 RPM Dn = 547,200 This calculation shows that the motor should have sleeve bearings because the speed factor exceeds 300,000. Bearing Lubrication

The lubrication of bearings can be accomplished with a variety of oils and greases that are applied through use of several methods. The type of lubricant and the method of lubricant application that best suits the installation will be determined by the lubricant's characteristics. This section will cover the following topics that are pertinent to bearing lubrication: _Bearing Lubricants: Types and Applications _Methods of Bearing Lubrication Bearing Lubricants: Types and Applications - Bearings can be lubricated through the use

of oil or grease. The selection of the correct type of lubricant, either oil or grease, depends on the properties of the lubricant and the specifications of the installation.


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The first type of bearing lubrication for use in Saudi Aramco applications is oil. Lubricating oils can be manufactured from mineral oil or from synthetic oil. Lubricating oil for use in Saudi Aramco installations must be manufactured from highly-refined turbine oil stocks, and it must be blended with additives to produce balanced oil stocks. Two of the main properties to consider in the selection of oil as a lubricant are the oil's viscosity and the oil's viscosity index. The viscosity of an oil is the oil's resistance to flow. An oil with a viscosity that is too high or too low can lead to the early failure of the motor. Saudi Aramco applications require that lubricating oils have a viscosity of 61.2 - 74.8 centistokes (cSt) at 40oC. Oils with a viscosity in this range are designated as ISO viscosity grade 68. The equivalent U.S. viscosity range is 317 - 389 saybolt universal seconds (SUS). The viscosity index is an empirical measurement of how the viscosity of an oil changes with temperature. An oil must be utilized that will meet the viscosity needs of the installation over the entire range of operating temperatures. The temperatures at which an oil can successfully perform its function will vary with the oil that is selected. Oils that have large viscosity indexes have the least change in viscosity for a given change in temperature. Greases, which are semisolid lubricants, are the other type of bearing lubrication that can be selected. Greases are used when the lubricant must stay in one place or must stick to a part. Most greases are made from mineral oil, but other materials such as waxes can be utilized. The lubricating properties of greases are determined by the following components from which greases are made: _Fluid base _Thickener _Additives _Fillers Mineral oil will be the fluid base for most greases. The fluid base will determine the viscosity of the grease. Greases that are designed for high temperature, low speed service are produced through the use of high viscosity oils, and greases that are designed for low temperature, high speed service are produced through the use of low viscosity oils. The minimum viscosity of the oil that is used as the fluid base for the grease must be 100 cSt at 40oC. The thickener is added to the fluid base to stiffen the grease. The most common type of thickener is soap. Soap is made from the combination of a fatty material and an alkali. Greases are generally named for the type of thickener that is used.


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Additives are the chemical compounds that are added to grease to change or add to the properties of grease. The additives in grease will increase the temperature range at which the grease can be utilized or will change the breakdown temperature of the grease. Additives can also increase the life span of the grease. Fillers are added to the grease to make the grease more solid and stable. Graphite is the most commonly used type of filler. Lubricating greases will have a variety of different properties that change dependent on the materials that are used during the production of the grease. To select the appropriate type of grease for an installation, the properties of the grease must be matched to the requirements of the installation. Lubricating greases for use in Saudi Aramco installations are required to perform under continuous temperatures of up to 120oC. The type of lubricant that should be specified for antifriction and sleeve bearings that are used in Saudi Aramco applications are as follows: Antifriction bearings Sleeve bearings

oil or grease oil

Methods of Bearing Lubrication - The method of bearing lubrication for use in a motor

must account for startup and rundown lubrication of the bearings. The method of bearing lubrication should be designed so that the bearing will be lubricated during startups that follow periods of extended shutdown, and it should permit the uncoupled motor to run down to standstill without damage to the bearings. The lubrication of antifriction bearings should be accomplished through the use of tapped holes in the bearing housing. Relief holes or drain plugs shall be located 180o from the grease point to provide for removal of old or excess lubrication. The lubrication of sleeve bearings can be accomplished in two ways. The method that is used depends on the velocity of the shaft journal as follows: Shaft Journal Velocity (Meters/Seconds) Below 11

Lubrication Method Uncooled

ring

or

disc

oil

lubrication Above 11

Circulated feed oil lubrication

The following formula is used to determine shaft journal velocity in meters per second from RPM:


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For example, the shaft journal velocity of a motor that has a shaft diameter of 100mm and that operates at 1800 RPM can be calculated as follows: The calculation shows that the bearings should be lubricated through use of the uncooled ring or disc method. The lubrication of sleeve bearings by an uncooled ring or disc involves the use of a loose ring that hangs on the motor shaft or a fixed disc that dips into an oil reservoir that is below the shaft. The ring or the disk also rotates as the shaft rotates, and this rotation transfers oil from the reservoir to the bearing surface. Heat is removed from the oil through use of natural heat transfer through the bearing housing to the ambient. The lubrication of sleeve bearings through the use of the circulation of feed oil requires an entire external system to support the bearing. When a circulating feed oil lubrication system is used, two separate pump units must be provided. Each of the two oil pumps must be able to supply 100% of the total operating oil requirements of the bearing. The circulated feed oil lubrication option for lubricating sleeve bearings is only chosen when it is required by the manufacturer. Bearing Housing and Protection

The bearing housings will contain the bearing and the lubrication that are necessary for the proper operation of the motor. The bearing housing should be designed to prevent physical damage to the bearing from external sources. All horizontal motors that are 3730 kW or above (5000 Hp and larger) must be equipped with pedestal bearings that are supported from the motor's baseplate. Bearing housings also must be designed to protect the bearing and the lubricant from contamination by external foreign matter. This contamination protection will also protect the bearing against the transfer of lubricant out of the bearing housing and into the surrounding atmosphere. Bearing Life

Because of the dispersion in life of identical bearings that are operated under identical conditions, a statistical result must be obtained for bearing life. Bearing life is expressed as the number of operational hours that 90% of a group of identical bearings will achieve or exceed under a given set of conditions, and it is referred to as the L10 life. There are multiple variables that are taken into account for a bearing life calculation. Because of the numerous variables, this section only discusses the basic bearing life equation.


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The following is the basic bearing life equation for an antifriction bearing: where: L10 N C P k

= Fatigue life for a 90% reliability = Operating speed = Dynamic load rating = Equivalent radial load in newtons or pounds = Constant that is equal to 3 for ball bearings and 10/3 for roller bearings

The dynamic load rating (C) is determined by the type of bearing that is used and by the number of active bearings that are mounted adjacent to one another. The equivalent radial load (P) is determined by the following factors: _Applied thrust load _Thrust load factors _Number of adjacent bearings _Basic static load There also are three life adjustment factors that could be placed into the basic bearing life equation. In most instances, the life adjustment factors can be assumed to be one, which will cancel out of the equation. The life adjustment factors that could be included are as follows: _Reliability _Bearing material _Application conditions The bearing life calculation will generally only be done by the manufacturer during the design of a new installation. The manufacturer should include the bearing life value with the bearing information. Bearing life calculations, although they are not routinely performed by the field Electrical Engineers, can be used for performance data. Maintenance of bearing life records can be used to evaluate the actual life of bearings against the calculated life expectancy, and they can be utilized to identify bearing application problems.


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Vibration Monitoring The vibration of motors to some degree is expected as a result of the motor's rotation. Vibration monitoring is employed to detect the occurrence of excessive vibration and to avoid damage to the motor or to adjacent equipment. There following types of vibration monitoring equipment are available for use in Saudi Aramco applications: _Seismic _Proximity Seismic-type vibration monitoring equipment is physically mounted so that the detector is connected to the bearing housing and moves with the motor. The movement of the motor causes a slug within the seismic detector to move back and forth, which changes the electrical coupling in the detector. The motor's vibration is proportional to the vibration in the electrical coupling of the detector. The advantages and disadvantages of the seismic probe result from the method of probe mounting. The advantages of the seismic probe are its rugged design and its ease of mounting. The seismic probe directly mounts to the bearing housing. The disadvantage of the seismic probe is that the failure rate of the seismic probes increases due to the extra moving parts that are used to physically mount seismic probes. Proximity probes are not connected to the bearing housing, and they will not move with the motor. The proximity probe measures the distance between the probe tip and the bearing casing. The proximity probe establishes a small magnetic field of the tip of the probe and, as the bearing casing vibrates in the magnetic field, the magnetic field will be distorted. The amount of distortion in the magnetic field is proportional to the amount of motor vibration. The advantages and disadvantages of the proximity probe also result from the method of mounting the probe. The advantages of the proximity probe are a much more accurate indication and a much lower failure rate. The accurate indication and lower failure rate are result from to the fact that the probe does not directly connect to the motor; therefore, the probe is not susceptible to damage and faults that result from the vibration of the motor. The disadvantage of the proximity probe is the elaborate mounting assembly that must be constructed. Because the proximity probe does not connect directly to the bearing housing, the extra mounting is necessary. Another disadvantage that results from the extra mounting assembly is the need to accurately align the probe with the motor bearing housing. Any misalignment between the probe and the motor bearing housing will result in an erroneous indication. Proximity probes can be used for frequency ranges of 1 to 1500 Hz, and seismic probes can be used for frequency ranges of 1 to 20,000 Hz. The actual requirements for determining when each type of probe should be used are contained in Work Aid 1.


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In general, any motor that operates at greater than 185 kW (250 Hp) will be supplied with vibration monitoring. The method and amount of vibration monitoring depends on the size of the motor and how the motor is mounted. Horizontal motors that operate from 750 kW (1000 Hp) to 3000 kW (4600 Hp) will have one seismic detector mounted on each bearing. Horizontal motors that operate above 3000 kW (4000 Hp) will have two proximity type detectors that are mounted 90o apart on each bearing. For vertical motors that operate at greater than 185 kW (250 Hp), two seismic detectors that are mounted 90o apart around the circumference of the top bearing housing are required. Proximity probes are never used with vertical motors. Mechanical Noise Mechanical noise will always be generated in a motor during operation. Different motor designs and motor mounting techniques will increase or decrease the mechanical noise that is produced by an operating motor. The following are the terms that are used to discuss mechanical noise: _Sound Power Level _Sound Intensity _Sound Pressure Level _Sound Level The Saudi Aramco noise limits are based on the sound level of the motor installation, and an understanding of the previously mentioned terms is necessary to facilitate this discussion. Sound Power Level

Sound power level (SWL) is a machine-related property that is independent of environmental conditions or distance from the machine. SWL is defined through use of the following equation: SWL = 10 log10 (P/Po) in decibels where:

P Po

= =

Measured sound Reference level of 10-12 watt (1 picowatt)

Because of environmental conditions, SWL cannot be directly measured. Surrounding equipment would add to any measured sound power level; therefore, another means of directly measuring SWL is necessary. Sound Intensity

Sound intensity is the density of sound power at a point away from the source, and it is expressed in watts per square meter. The sound power that is indicated by a source can be Saudi Aramco DeskTop Standards

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derived through integration of the sound intensity over an enclosed, hypothetical surface of measurement.


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Sound Pressure Level (SPL)

Sound Pressure Level (SPL) is the level of pressure in the sound conducting medium that results from the sound intensity at the concerned point. SPL can be expressed as follows: SPL = 20 log10 (P1/P2) dB where:

P2 = Reference pressure micropascals (2 x 10-5n/sqm) P1 = The sound pressure

that

is

equal

to

20

Sound Level

Sound level is a weighted measure of the amount of noise that is produced by a machine at a given point. Note that sound intensity and SPL at a point are a function of both the combined surroundings and the source of the noise. The following equation is for use in the direct calculation of SWL from measured free field sound: SWL = SPL + 20 log10r + 8 dB where:

SWL = Sound power level referred to 10-12 watts SPL = Average sound pressure level that is referenced to 20 micropascals r = Radius of hemisphere in meters

Saudi Aramco limits the sound level to a maximum of 90 dB when the sound level is referenced to a base of 20 micropascals for an eight-hour exposure period per day. Areas in which the SWL exceeds the 90 dB maximum must have the exposure time shortened to prevent injury to the personnel. Typical sound power levels from a motor will depend not only on the motor, but also on the type of enclosure of the motor. Certain types of enclosures such as dripoff, total-enclosed fan-cooled (TEFC), and weather protected type II (WPII) will tend to shield a portion of the sound. Figure 2 shows typical sound power levels for various motor horsepower and kilowatt ratings at various speeds and with different enclosure types. The sound levels are given in decibels.


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Typical Sound Power Levels Figure 2 Saudi Aramco DeskTop Standards

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Shaft Circulating Currents Shaft circulating currents are caused by stray voltages that are induced in the rotor during operation. The stray voltages that are induced in the rotor can form a closed loop for current flow. To complete the closed loop for current flow, circulating currents must bridge the oil film on the bearing surfaces. Because of the high resistance of the bearing supports to ground, the induced voltages cannot be shunted away. When the oil film on the bearing surfaces is bridged, a closed loop for current flow will exist from the rotor through the bearing housing, through the stator, through the other bearing housing, and back to the rotor. The circulating currents, if allowed to exist, will cause a problem in the form of damage to the bearing and shaft surfaces. The damage will occur in the form of pitting at the point where the current passes through the shaft/bearing connection. To prevent damage that results from shaft circulating current, a method of prevention must be obtained. On horizontal motors that are rated 375 kW (500 Hp) and above, both of the bearings must be electrically insulated from the motor frame. Vertical motors that are rated above 185 kW (250 Hp) only require insulation on the top bearing. The insulation resistance of the bearing must be greater than one megohm. Stator Windings and RTD's Stator windings are required to be designed to withstand environmental conditions that are common at Saudi Aramco installations. The stator windings must be treated to withstand the tropical conditions and the corrosive effects of industrial sulfurous atmospheres. The varnish impregnation should be a resin-rich type or a vacuum/pressure impregnation type process for form wound windings. The windings of weather-protected type motors should be provided with an additional protective coating to inhibit insulation abrasion by sand and salt that is entrained in the cooling air. Stator windings also need to be braced against excessive vibration to prevent damage to the stator insulation. Stator leads that require bracing within the motor enclosure should be provided with removable insulated supports to facilitate maintenance. Stator windings must be supplied with type F insulation systems that are designed so that the insulation will not exceed the class B temperature rise as measured by an RTD that is imbedded in the stator. The maximum temperature rise is based on a maximum ambient temperature of 50oC. To reduce the need for surge suppressors on all motors, the stator windings must be designed to withstand the surges that are caused by normal switching actions or lightning. In older motor designs, only coil to frame insulation was tested, and the minimum acceptable coil to frame insulation level was determined from the following equation:


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2V + 1 kV where:

V is the phase-to-phase rms voltage of the motor.

Recently, research has shown that in short time surges, most of the voltage will fall across the first turn of the stator winding. When existing motors are evaluated, an interturn basic insulation level (BIL) of only 25% of the coil to frame insulation level can be assumed; therefore, the allowable peak surge voltage can be determined by the following equation: New motors are specified with stator winding interturn insulation requirements that exceed 25% of the coil to frame insulation level to minimize the need for surge suppressors; however, Saudi Aramco still requires both high BIL level and surge suppression for 13.8 kV motors, regardless of the stator winding interturn insulation level. Rotor Windings The rotor windings of induction machines should be of the cage-type, they should be formed of copper, copper alloy, or aluminum bar, and they should be treated to withstand tropical conditions. End-ring connections on cage-type rotors should be of high mechanical strength. Filler metals that are part of the cage-type rotor should be resistant to attack by corrosive sulfurous gases. Copper alloy rotor construction should conform to American Welding Society (AWS) A5.8, and it should contain a minimum 40% silver. Copper-phosphorous, bronze-type fillers are unacceptable. The rotor body of synchronous machines should be of the salient pole type with windings of insulated copper wire or strip that also are treated to withstand tropical conditions. The insulation of rotor windings for both NEMA frame integral motors and form-wound motors will be class F. The temperature rise above 50oC must not exceed those values that are acceptable for class B insulation. Form-wound motor insulation systems should consist of low-hygroscopic materials. Mounting Details Motors that are manufactured to IEC and NEMA standards use "dimension letter" codes to define machine dimensions. To facilitate the replacement of IEC motors by NEMA motors, and vice versa, a comparison of dimensional code details is needed. Figures 3A and 3B show the dimensional measurements that are necessary for motor replacement and the IEC and NEMA dimension code letters that correspond to the measurements. The NEMA code letters are shown in parentheses.


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19



Motor Dimensions with NEMA and IEC Dimensions Figure 3A


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Motor Dimensions with NEMA and IEC Dimensions Figure 3B


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Vertical

Figure 4 is a list of the most common vertical motor measurements and their associated NEMA and IEC dimension code letters. This list can be used to determine the name of actual vertical motor measurements that were shown in Figures 3A and 3B.

NEMA and IEC Dimension Code Letters for Vertical Motors Saudi Aramco DeskTop Standards

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Figure 4


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Horizontal

Figure 5 is a list of the most common horizontal motor measurements and their associated NEMA and IEC dimension code letters. This list can be used to determine the name of the actual horizontal motor measurements that were shown in Figures 3A and 3B.

NEMA and IEC Dimension Code Letters for Horizontal Motors Saudi Aramco DeskTop Standards

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Figure 5


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Cooling System The IEC defines cooling as the means by which the heat that results from losses that occur in a machine is given up first to a primary coolant by means of an increase in coolant temperature. The heated primary coolant can be replaced by a new coolant at a lower temperature, or can be cooled by a secondary coolant in some form of heat exchanger. Because of the increased heat that is produced in larger motors, the importance of motor cooling increases as the size of the motor increases . The environmental conditions to which a motor is exposed will also dictate the amount of cooling that is required. The maximum temperature rise of Saudi Aramco motors cannot exceed the temperature rise that is approved for class B insulation. The maximum temperature rise that is allowed for Class B insulation is 80oC. Where totally-enclosed machines utilize heat exchangers, closed, air-circuit, air-cooled (CACA) heat exchangers should be mounted on the motor. Top-mounted heat exchanger assemblies should have flanges that extend downward to overlap the motor enclosure on all sides by a minimum of 10 mm (0.4 in). High-voltage motors with integral air-to-air heat exchangers should be provided with removable sections or doors to allow easy access to the motor and the cooling fan balance planes without dismantling the motor or rotor assembly. When air-to-air heat exchangers require auxiliary fan cooling, a shaft-mounted cooling fan or fans should be provided. Auxiliary motor-driven fans should not be specified. Internal and external cooling fans should be constructed of steel, bronze, or copper-free aluminum that is suitably treated to resist corrosion. Synthetic materials such as plastic are acceptable only for fractional kilowatt motors. Internal and external fans that are designed for dual rotation are preferred. When unidirectional fans are necessary to meet the motor performance specifications, preference will be given to fans of a reversible design that will facilitate future reversal of motor rotation.


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Control and Supply Leads The control and supply leads for Saudi Aramco motors must be designed to be moisture- and heat-resistant. The conductors of the control and supply leads should be made of copper, and they should be designed for operation at a maximum ambient temperature of 50oC. Control and supply leads must have a minimum conductor size of stranded 2.5 mm sq (14 AWG), and each lead should be clearly and permanently marked with a PVC sleeve wire marker. Resistance temperature detectors (RTD) are used for temperature monitoring. The RTDs should be of the platinum, three lead type, that are calibrated to a resistance of 100 ohms at 0oC (32oF). The RTDs should be located in the slot portion of stator winding coils as follows: _Motors that are rated above 150 kW (200 Hp) and below 1300 kW (1750 Hp) should have one RTD per phase. Motors that are rated 1300 kW (1750 Hp) through 7500 kW (10,000 Hp) should have two RTDs per phase. Motors that are rated above 7500 kW (10,000 Hp) should have three RTDs per phase. The hottest reading RTD should be identified by the vendor during factory testing. _Motors that are rated up to 1 kW (1.34 Hp) should be provided with a built-in thermal protective device that will open the motor supply circuit. RTDs should not be used for these motor ratings. Nameplates The nameplates of Saudi Aramco motors should include all the information that is required by NEMA MG1 and IEC 34-1 and the additional information that is required by SAES-P-113. The following is a list of the information that NEMA MG-1 requires on motor nameplates. _Manufacturer's name, serial number or date code, and suitable identification. _Horsepower output or kilowatt. _Time rating. _Temperature rise. _RPM at rated load. _Frequency. _Number of phases. _Voltage. _Rated-load amperes. _Code letter for locked rotor KVA.


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The following additional data is required by SAES-P-113 and can be supplied on a separate nameplate(s): _Buyer's Purchase Order number. _Year of manufacture. _Manufacturer's location. _Manufacturer's order reference number. _Anti-friction bearing number and manufacturer. _Class, Group, and Division (explosion-proof motors, only). _Maximum ambient temperature. _Insulation system designation. _Rotor weight. _Total weight of motor. Where two or more identical motors are supplied on one Purchase Order, the nameplates for all motors must show the temperature data and locked rotor current from the tested motor. Saudi Aramco also requires that a separate nameplate be supplied to show the direction of motor rotation. The direction of rotation should be indicated by an arrow and the nameplate should be located on the non-drive end of the motor. The nameplate(s) and rotation arrows must be made from 300 series stainless steel or monel, be securely fastened to the motor by pins of similar material, and be located for easy visibility. The entries on the nameplates must be marked by etching, engraving, or other permanent method of marking. Space Heaters The insulation of machines that are out of service for prolonged periods can absorb enough moisture to reduce the insulation resistance to a value that is below the allowable limit. Maintenance of the winding temperature 5oC above the surrounding ambient temperature will prevent moisture absorption of the insulation. Space heaters are used to maintain the winding temperature at 5oC above ambient. Electric strip heaters are the most common source of heat, and these heaters are convenient, easy to control, and inexpensive. The only inspection that is suggested for space heaters is an occasional measurement of heater circuit current to detect burned out units or loose connections. The space heaters should have no exposed elements.


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The amount of heat that is required to raise the winding temperature of a given enclosed horizontal motor approximately 5oC above ambient temperature, where the machine is closed except for a small vent at the top and bottom for circulation, is given by the following formula: H = 0.28 DL where:

H = D = L = meters

Heat in kilowatts Machine end-bell diameter in meters Machine stator length between end-bell centers in

For example, the heat (kW) that is required to raise the temperature of a horizontal motor with an end-bell diameter of one meter and a length of three meters to the specified 5oC above ambient can be calculated as follows: H H H

= = =

.28 DL (.28) (1M) (3M) .84 kW

The space heaters normally should be specified to operate on 120 Volt power supplies because such supplies are normally available in all locations. In some existing Saudi Aramco installations, it may be necessary to connect heaters in series to supply them from an existing 480V system. To ensure long operating life for a motor space heater, the heater nameplate voltage, as specified in 17-SAMSS-502, must be twice the supply voltage that is indicated in the data schedule. The following methods are available to control space heaters: _Manual Switching. _Thermostats that automatically energize and deenergize the heaters based on the temperature inside of the enclosure. _Auxiliary contacts that automatically energize the heaters when the motor is deenergized and that deenergize the heaters when the motor is energized. Saudi Aramco requires that auxiliary contacts in the switchgear be used for heater control whenever heaters are installed; therefore, manual switching or thermostats are unacceptable for use in Saudi Aramco installations.


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Heaters should be included with all motors that are supplied with motor operated valves and with all motors that are rated 2.3 kV or higher. Other motors that are installed outdoors and that are used only as standby equipment can also require heaters. The surface temperature of space heaters for motors that are installed in classified areas should not exceed the listed maximum allowable temperature for the area. The following are the maximum allowable surface temperatures for classified areas of Saudi Aramco installations: Area Classification

Maximum Surface Temperature

Class I, Group C Class I, Group D Class II, Group E Class II, Group G

Allowable

160oC (320oF) 215oC (419oF) 200oC (392oF) 120oC (248oF)

Testing Requirements The following tests should be made on machines that are completely assembled in the factory and that are furnished with a shaft and a complete set of bearings: _Measurement of winding resistance The motor winding resistance test is performed to ensure that the correct winding configuration and electrical connections have been made. _No-load measurements of current, power, and nominal speed at rated voltage and frequency. These measurements are performed to ensure that the motor operates within the no-load nameplate data. _High-potential test The high-potential test is performed to ensure that the motor's insulation system is adequate.


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_Vibration test The motor vibration test is performed during the no-load run test to ensure that the motor is balanced during operation. The vibration test also sets a baseline level of vibration for future comparison. _Measurement of bearing insulation resistance The bearing insulation resistance must be measured to ensure adequate insulation for the protection of personnel and equipment. Performance of the bearing insulation resistance test after all of the auxiliaries have been installed to the bearing housing will ensure that no breach in the bearing resistance system has been made. _Bearing/lube oil temperature measurement The bearing/lube oil temperature measurement is performed during the no-load run test to verify that the bearings operate within the established limits for the installation and for the lubricant. The following tests should be performed when specified in the motor installation description data sheet. _Performance Determination The performance determination test are performed to verify, after installation, that the motor is performing its design function within the limits that are established by the installation. _Temperature Tests The temperature test on the stator is performed to ensure that, under load, the insulation temperature does not exceed the maximum allowable temperatures. _Miscellaneous Tests Miscellaneous tests are any performance tests that the Engineer believes to be necessary for the installation. The exact tests that are performed and the acceptance criteria are established by the Cognizant Design Engineer.


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_Surge Tests The surge test is performed to ensure that the motor winding insulation is sufficient to protect the motor windings from harm during any expected surges. A detailed description of each of these test is included in Module EEX 203.04. Painting and Coating All steel surfaces must have the Vendor's standard finish with a minimum of 0.127 mm (5 mil) dry thickness. The purpose of painting and coating all steel surfaces is to protect the motor from the environment. Packing The packing of equipment should be suitable for shipment by sea and by vehicular transportation over unpaved, desert roads. Packing should be in accordance with Buyer's Packing Specification No. 1 and 1.1 of Vendor's standard export packing. Vendor's standard export packing should be subject to the approval of Saudi Aramco.


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SPECIFYING MOTOR ENCLOSURE REQUIREMENTS The motor enclosure requirements will vary with the type of motor that is installed and the area in which the motor is installed. There are many different degrees of protection that are afforded by the design of motor enclosures. The discussion here will concentrate only on those motor enclosures for use in Saudi Aramco applications, and it will cover the following topics: _Motor Enclosure Functions _General Motor Enclosure Requirements _Saudi Aramco Motor Enclosure Requirements _Motor Enclosures for Classified Areas _Enclosures for Motor Auxiliary Equipment _Connection Boxes _Conduit Boxes _Grounding Motor Enclosure Functions All motor enclosures must provide the following functions: _To protect personnel from the motor's energized and rotating parts. _To protect the motor from the injurious effects of the environment, such as sand, dust, rain, and water from cleaning operations (e.g., splashing). _To afford a reasonable degree of mechanical protection against external damage to the motor. _To protect a hazardous environment from a possible source of ignition. General Motor Enclosure Requirements All motor enclosures must protect against environmental and mechanical damage. Protection from the environment is afforded through use of an enclosed air circulation system to prevent fumes or gases from damaging the motor. Mechanical damage to the motor is prevented through use of screens and tight fittings that do not allow foreign material to enter the motor.


33



All enclosures for use in Saudi Aramco installations must meet a level of cooling and protection as defined in International Electrotechnical Commission 34-5 (IEC 34-5) and in International Electrotechnical Commission 34-6 (IEC 34-6). IEC 34-5 designates the degree of protection that the enclosure must provide for the motor. IEC 34-6 designates the degree and method of cooling a motor. IEC 34-5 has different protection codes that can be applied to motor enclosures. Figure 6 shows the degree of protection that is indicated by the first characteristic numeral of an IEC code. The first column of Figure 6 shows the possible first characteristic numerals (0-5). The second column of Figure 6 gives a brief description of the objects against which that particular enclosure will protect. The description of the object is based on the size of the object. The third column of Figure 6 gives a definition that further describes the degree of protection.


34



Degree of Protection Indicated by the First Characteristic Numeral Saudi Aramco DeskTop Standards

35



Figure 6


36



Figure 7 shows the degree of protection that is indicated by the second characteristic numeral of the IEC code. The second characteristic numeral indicates the degree of protection the enclosure provides from water ingress. The first column of Figure 7 shows the possible second characteristic numerals (0-8). The second column of Figure 7 gives a brief description of the type of water protection that is indicated through use of the second characteristic numeral. The third column of Figure 7 gives a definition of the type of protection that is indicated by each second characteristic numeral that further describes the degree of protection.

Degree of Protection Indicated by the Second Characteristic Numeral Saudi Aramco DeskTop Standards

37



Figure 7


38



All enclosures for Saudi Aramco installations must be designated as IP44. IEC 34-5 defines the protection code IP44 as follows: IP

=

Ingress protection.

4

=

First characteristic numeral indicates the degree of protection that is provided by the enclosure with respect to persons and to parts of the machine that are within the enclosure.

4

=

Second characteristic numeral indicates the degree of protection that is provided by the enclosure with respect to the harmful effects of the ingress of water.

Saudi Aramco enclosures also must be designed for cooling in accordance with IEC 34-6. The degree and method of cooling is also designated by an IEC code. The IEC designation code for the method of cooling of a machine consists of the following: _The letters IC that indicate an IEC designation. _A group of one capital letter and two characteristic numerals for each motor cooling circuit (e.g., A01). The capital letter designates the medium that is used as the coolant, the first characteristic numeral designates the circuit arrangement for circulating the coolant, and the second characteristic numeral designates the method that is used to supply power for circulating the coolant. The following list indicates both the possible mediums that can be used as coolants and the IEC code letters that are associated with those mediums. For other coolants that are not listed, the nature of the gas or liquid must be stated in full text. When the only coolant is air, the IEC code letter that designates the cooling medium can be omitted. Air is the only cooling medium that is approved for use in Saudi Aramco motors. Figure 8 is a complete list of the first characteristic numerals for IEC cooling method codes. The first characteristic numeral describes the physical arrangement of the coolant circulating system. The first column is a list of the possible first characteristic numerals (0-9). The second column is a short designation of the coolant system arrangement. The final column is a definition that further describes the short designation of the coolant circuit arrangement for each first characteristic numeral.


39



First Characteristic Numeral for Cooling Method Codes Saudi Aramco DeskTop Standards

40



Figure 8


41



Figure 9 is a complete list of the second characteristic numeral for IEC cooling method codes. The second characteristic numeral describes the method of supplying power for circulating the coolant. The first column is a list of the possible first characteristic numerals (0-9). The second column is a short designation of the method of supplying power for circulating the coolant. The final column is a definition of each second characteristic numeral that further describes the short designation of each code number.

Second Characteristic Numeral for Cooling Method Codes Saudi Aramco DeskTop Standards

42



Figure 9


43



The following is an example of an IEC code that designates the degree and the method of cooling a motor: ICA01 IC

-

Indicates that this designation is an IEC designation.

A

-

The coolant medium is air.

0

-

The circuit arrangement is free circulation.

I

-

The method of supplying power to circulate the coolant is self-circulation.

When more than one cooling circuit is needed to cool a machine, the IEC designation consists of the following: _The letters IC. _A group of one letter and two numerals for the circuit on the user's side that is at the lower temperature (secondary cooling circuit). _A group of one letter and two numerals for the circuit that is closer to the winding and that is at the higher temperature (primary cooling unit). The IEC cooling codes are the same as in the single system. The following is an example of an IEC code that designates the degree and the method of cooling a motor that requires two cooling circuits: ICA01A61 IC

-

Indicates that this designation is an IEC designation.

A01

-

Secondary cooling circuit (low temperature).

A

-


0

-

The circuit arrangement is free circulation.

-


-

Primary Cooling Circuit (high temperature)

A61 Saudi Aramco DeskTop Standards

44



A

-


6

-

The circuit arrangement is a machine-mounted heat exchanger that uses the surrounding medium.

1

-


Saudi Aramco Motor Enclosure Requirements NEMA MG-1 allows the use of numerous types of motor enclosures; however, only three types of enclosures are approved for use in Saudi Aramco applications. The following are motor enclosures that are allowed in Saudi Aramco applications: _Totally-enclosed fan-cooled (TEFC) _Environmental protection totally-enclosed air-to-air cooled (CACA) _Weather protect type II (WP-II).


45



TEFC

NEMA MG1 defines a TEFC enclosure as a totally-enclosed fan-cooled machine that is equipped for exterior cooling through use of a fan or fans that are integral with the machine, but that are external to the enclosing parts. The level of environmental protection that is provided by a TEFC enclosure will vary, and stringent environmental protection requirements cause this enclosure to be more complicated and more expensive. TEFC enclosures without heat exchangers are not permitted for motors that are rated above 11,000 kW (15,000 Hp). This requirement is due to the heat dissipation requirements of the motor. TEFC enclosures for use in Saudi Aramco applications must have the following IEC designation codes for environmental protection and cooling: _IP44 for protection _ICAO1A41 for cooling CACA

CACA is a variation of the simpler TEFC machine, but it includes an air-to-air heat exchanger to provide more effective cooling on larger machines. CACA is commonly known as a closed air-circuit, air-cooled type of enclosure. NEMA MG1 defines a CACA as a totally-enclosed air-to-air cooled machine that is cooled through circulation of the internal air through a heat exchanger that, in turn, is cooled through circulation of external air. The level of environmental protection that is provided by a CACA enclosure will vary, and stringent environmental protection requirements also cause this enclosure to be more complicated and expensive. A CACA enclosure is provided with an airto-air heat exchanger for cooling the internal air, a fan or fans that are integral with the rotor shaft or separate for circulating the internal air, and a separate fan for circulating the external air. CACA enclosures should be specified for induction motors and for salient pole synchronous motors that are rated up to 11,000 kW (15,000 Hp). CACA enclosures for use in Saudi Aramco applications must have the following IEC designation codes for environmental protection and cooling: _IP44 for protection _ICA01AG1 for cooling WP-II

NEMA MG1 defines a WP-II as an open machine with ventilating passages that are so constructed as to minimize the entrance of rain, snow and air-borne particles to the electric


46



parts, and with ventilated openings that are so constructed as to prevent the passage of a cylindrical rod that is 0.75 inch in diameter.


47



The WPII ventilating passages at the intake and the discharge are so arranged that highvelocity air and air-borne particles that are blown into the machine by storms or high winds can be discharged without entering the internal ventilating passages that lead directly to the electric parts of the machine itself. The normal path of the ventilating air that enters the electric parts of the machine should be arranged through use of baffles or separate housings to provide at least three abrupt changes in direction, none of which can be less than 90o. In addition, an area of low velocity that does not exceed 3 m/s (600 ft/min) should be provided in the intake air path to minimize the possibility of moisture or dirt being carried into the electric parts of the machine. The WP-II type of enclosure does not afford the same degree of protection as TEFC types, but it may be acceptable for synchronous motors with rated outputs that are above 11,000 kW (15,000 Hp) where the cost advantage over a TEFC type of enclosure is significant. WPII enclosures for use in Saudi Aramco applications must have the following IEC designation codes for environmental protection and cooling: _IP44 for protection _ICA01 for cooling Motor Enclosures for Classified Areas This section only covers the usual Class I, Division 1 and 2 locations with Group D hazards that are found in Saudi Aramco installations. For a Division 1 area, the motor enclosure must be explosion proof (Exd). The totallyenclosed flameproof motor is preferred for motor sizes up to about 500 kW (700 Hp) because of the TEFC ruggedness and simplicity. For larger motor sizes, the normal practice is to avoid Division 1 locations because of the cost of the enclosures.


48



For a Division 2 area, the motor enclosure must be non-sparking (Exn). The type of protection "n" is such that during normal operation, the motor is not capable of causing ignition, and a fault that is capable of causing ignition is not likely to occur; therefore, any type of enclosure that prevents sparks can be utilized. To verify that a motor that is installed in a hazardous area is permitted in that area, additional information must be included on the nameplate. All the information that is required by NEMA MG1 must be on the nameplate, plus the following additional information: _Class, division, and/or group of hazardous atmosphere type for which the machine is approved. _Type of protection that is provided. _Temperature class for which the motor is approved. _Maximum exposed temperature of the machine. Enclosures for Motor Auxiliary Equipment Motor heaters are mounted within the motor enclosure; therefore, no special enclosures are required for motor heaters. The surface temperature limitations of the area apply to all surfaces that are in contact with the air that is inside and outside the motor. The motor heaters must not exceed the maximum allowable exposed temperature in the area. Instruments that can be fitted into the motor enclosure also require no special enclosure. Where instruments are external to the motor enclosure, the instrument enclosure should be equal to, or better than, the motor enclosure. Instruments that are fitted to motors in Zone 1 and Zone 2 classified areas should be flameproof Exd, unless these instruments are certified or approved as part of the motor. Sparking devices must be housed in hermetically-sealed or Exd enclosures. Connection Boxes The terminal enclosure for termination of main windings, control/measurement circuitry, and auxiliary electrical supplies should meet or exceed the enclosure requirements for the main machine. At a minimum, the connection boxes must meet the requirements of NEMA 4. Terminal boxes and connectors must withstand the effect of faults within the enclosure as follows:


49



_To accommodate, without detriment, the maximum through fault current that is available at the terminals with a fault clearance time of 250 milliseconds. The prospective fault MVA's should be as follows: System Voltage 480 2,400 4,160 6,900 13,800

Fault MVA 40 250 350 500 750

_To contain, or relieve, the consequence of an internal terminal box fault without there being an external detrimental effect to personnel. All motors that are rated 1 kW (1.5 Hp) and above should be provided with a main connection box that is located on the right hand side of the motor frame as viewed from the non-drive end of the motor. When it enters the box, the conduit should be parallel to the shaft, and it should enter the box as follows: _The conduit should enter horizontal motors from the side that is opposite of the shaft extension. _The conduit should enter vertical motors from the side of the shaft extension. The connection box can be mounted on the end of the motor that is opposite the shaft extension on motors with rated outputs below 1 kW (1.5 Hp). Connections for auxiliaries are not permitted in the main connection box. Separate boxes, which should normally be mounted on the opposite side of the motor to the main terminal box, should be used for each type of instrument or auxiliary supply. A single box for all instruments or transducers that are of one type is preferred. No auxiliary wiring is to be taken through the main terminal box. Circuits that have different voltages are not permitted in the same box unless special precautions are taken and suitable warning labels are provided.


50



Connection Boxes (Cont'd) High voltage motors that are rated 2.3 kV and above must be supplied with separable connectors. The premolded slip-on type cable terminators are suitable for the cable size as specified in the purchase order. A vented terminal enclosure should be specified to provide mechanical protection and to terminate the conduit: it is not necessary to equal the protection that is provided to the motor itself. The terminal enclosure does not need to meet the enclosure requirements of the motor because there must be a seal between the two enclosures. The terminal enclosure must be metallic and meet or exceed the motor enclosure specification. The terminal enclosure must withstand fault conditions as noted above, and it may require anti-condensation heaters and a condensate drain. A rotatable, diagonally-split box or enclosure is preferred. Conduit Boxes All cabling must be enclosed in rigid or flexible steel conduit unless protection is provided through the use of armored cables. Flexible connections only can be used where movement is to be expected in service. The following conduit construction requirements must be met by all conduit installations: _All conduit connections to motor terminal boxes must be made through use of threaded conduit hubs that have tapered pipe threads of which at least five threads are fully engaged. _The conduit assembly must form a weatherproof and dust-tight system that is highly resistant to mechanical damage. Connection through the use of couplings can be installed, if necessary, when conduit boxes are installed in Class 1, Division 1, locations, and the conduit box must be sealed within 18 inches to complete the explosion-proof enclosure.


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Grounding Saudi Aramco motors must be grounded for the following reasons: _To safeguard a person from electric shock by ensuring that, under fault conditions, all surfaces with which the person is in contact (including the surfaces of metallic equipment and the ground) remain at safe relative potentials. _To reduce the possibility of static discharge and fire risk in hazardous areas. The main frames of motors that are rated up to 150 kW (200 Hp) should be provided with a corrosion resistant grounding stud for connection to the ground grid by means of 25 mm sq (No. 4 AWG) grounding cable. A grounding stud should be provided in the motor terminal box to ground the cable shield. Motors that are rated 150 kW (200 Hp) and above should be provided with flat corrosion resistant grounding pads that are drilled and tapped for NEMA two hole connectors and that are located on diagonally opposite corners of the non-removable portion of the motor main frame. The ground connection must accommodate the following minimum size of ground cable: Cable Size Motor Rating kW (Hp) mm sq (AWG/MCM) 185 < 370 (250 < 500) 70 (2/0) 370 < 3360 (400 < 4500) 120 (4/0) 3360 & above (4500 & above) 185 (350) The main connection box and all auxiliary connection boxes should be provided with an internal grounding clamp or bolt to provide cable grounding facilities. The dimensions of the grounding clamp or bolt should accommodate the grounding core of main or auxiliary cables, and they should provide a terminal point for the cable ground shield.


52



SPECIFYING MOTOR STARTING METHODS When specifying a method of motor starting, the Electrical Engineer must evaluate each possible method of starting the motor and the different limitations of each method of starting a motor. This section will cover the following topics that are pertinent to specifying motor starting methods: _Methods _Factor Considered in Starting Method Selection Methods There are many methods that can be used to start a motor. The designer must compare each method in order to specify the correct method of starting a motor. The following starting methods are available: _Full Voltage Starting _Autotransformer Starting _Primary Reactor Starting _Wye-Delta Starting _Primary Resistance Starting _Part Winding Starting _Variable Frequency Starting _Electronically-Controlled Reduced Voltage Starting


53



Methods (Cont'd) Full Voltage Starting

Full voltage starting is the preferred method of starting Saudi Aramco motors. Figure 10 is a simplified diagram of a full voltage motor starter. Contacts 1, 2, and 3 are shut through use of a circuit breaker or a contactor to start a motor that uses a full voltage motor starter. When contacts 1, 2, and 3 are shut, power from the line will be applied to the motor stator at full rated voltage.

Full Voltage Motor Starter Figure 10


54



Methods (Cont'd) Figure 11 shows the typical full voltage starting torque, speed, and kVA characteristics. Full voltage starting provides the most starting torque of the possible starting methods. A byproduct of the large starting torque of the full voltage starter is that it draws both the largest starting current of any of the methods of motor starting and a high initial kVA. The starting current of a motor that uses full voltage starting will remain relatively high until the motor's speed reaches about 90% of synchronous speed. In a motor with a long run up time, the large amount of current becomes a concern because of the extra heating effect of the large kVA and current values. The high torque that is created will reduce the time that the motor requires to reach rated motor speed. Full voltage starting is the least expensive and the simplest method of starting. Relative cost comparison tables for all types of motor starting are contained in Work Aid 3.

Full Voltage Starting Typical Torque, Speed, kVA Characteristics Figure 11


55



Autotransformer Starting

Figure 12 is a simplified diagram of an autotransformer motor starter. An autotransformer is placed in each phase (A, B, and C) of the supply voltage that will supply a percentage of full rated voltage to the motor stator during starting. Contacts 2, 3, 4, 6, and 7 must be shut to start a motor that uses the autotransformer starter. Shutting contacts 2, 3, 4, 6, 7 will apply a portion of the rated voltage to the motor stator to start the motor. The percent of full rated voltage that is applied to the motor is determined by the position of the autotransformer line taps. When the motor is started and running, an operator or an automatic control circuit will transfer the motor from start to operate. The transfer causes contacts 2 and 7 to open, contacts 1, 5, and 8 to shut, and contacts 3, 4, and 6 to open. This sequence of contact operation applies full voltage to the motor stator for running the motor.

Autotransformer Motor Starter Figure 12


56



The line current, motor starting torque, and motor maximum torque are proportional to the square of the motor's applied voltage. For this reason, the actual starting torque of a motor that uses an auto-transformer can be varied through a change in the position of the autotransformer line taps. Figure 13 shows the typical autotransformer starter torque, speed, and current/kVA characteristics. The motor torque for the autotransformer is compared to what the motor torque would be if full voltage starting was employed for comparison. The large spike in motor kVA, current, and torque at about 75% synchronous speed is the point at which the motor is switched from autotransformer starting to full line voltage. Autotransformer starting requires the least amount of starting kVA for an equal initial torque requirement as compared with other starting methods (part-winding excepted). Autotransformer starting results in a higher initial torque than resistor or reactor starting for an equal supply (line) current. The current that is drawn by the autotransformer starter is less than the current that is drawn in the full voltage starter method, but starting torque is proportionally lower. An autotransformer starter costs approximately two to five times the price of a full voltage starter. A comparison of cost of the autotransformer starting method with the other starting methods is included in Work Aid 3.

Autotransformer Starter Torque, Speed, and Current/kVA Curves Figure 13


57



Primary Reactor Starting

Figure 14 is a simplified drawing of a primary reactor starting circuit. The addition of the reactors to the motor circuit will lower the voltage that is applied to the motor. Such lowering of voltage will lower the starting current and torque. Contacts 1, 2, and 3 are closed to apply voltage from the line to the motor stator to start a motor with a primary reactor starter. The stator will be connected together through reactors that will limit the amount of current flow in the motor stator during starting. When the primary reactor starter is shifted from start to run, contacts 1, 2, and 3 will remain closed and contacts 4, 5, and 6 will close. Closing contacts 4, 5, and 6 will short out the reactors and lower the impedance of the stator. The lower impedance of the stator will allow full voltage to be applied to the motor for running operations.

Primary Reactor Starting Circuit Figure 14


58



Methods (Cont'd) Figure 15 shows typical motor torque, kVA, and speed curves for a motor with a primary reactor starter. The motor torque of a motor that is started with full-voltage is given for use as a comparison of different methods of starting. Notice the disturbance in KVA and torque during the transfer from start to run. A quick transfer from start to run will help provide a smooth run-up of the motor with primary reactor starting. Autotransformer motor starting will draw less line current for the same amount of initial torque as a primary reactor started motor; however, primary reactor or primary resistors give higher accelerating torque over the starting period for the same initial torque conditions because the voltage across the motor increases as the motor comes up to speed. With autotransformer starting, the motor's applied voltage is constant until the transition is made. Primary reactor starting provides a smooth run-up speed with only a slight disturbance at the transition from "start" to "run." The use of a variable reactor can further improve the run-up characteristics. The line current at starting is proportional to the motor's applied voltage, and the starting torque is proportional to the square of the motor's applied voltage. A primary reactor starter costs approximately 250% of the cost of a full voltage starter. A comparison of the primary reactor starting method with the other methods is contained in Work Aid 3.

Typical Motor Torque, kVA, Speed Curves of a Primary Reactor Starting Motor Saudi Aramco DeskTop Standards

59



Figure 15


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Wye-Delta Starting

Figure 16 is a simplified diagram of a wye-delta motor starting diagram. The motor is started in a wye configuration and is switched to a delta configuration at, or just below, full speed. Contacts 1, 2, 3, 4, 5, 6 are shut to start a wye-delta starting motor. These contacts connect the motor stator winding in a wye configuration. Connection of the stator windings in a wye configuration will reduce the starting current that is necessary to develop the required starting torque by the. When the motor is just below full speed, the motor is switched to a delta configuration. Contacts 4, 5, and 6 will open and contacts 7, 8, and 9 will shut to switch the motor from a wye to a delta configuration. When contacts 7, 8, and 9 are shut, the stator windings will be connected in a delta configuration, and this configuration applies full line voltage to the motor stator windings for normal running operation.

Wye-Delta Motor Starting Diagram Figure 16


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Figure 17 shows the torque, speed, and kVA curves for a wye delta starting motor. Motor torque at full voltage is shown for comparison. Through connection of the motor in a wye configuration during starting, the starting kVA and starting torque are reduced to approximately one-third of their full-voltage values. Wye-delta starting can be used where low motor torques are required. The motor's current will follow the kVA requirement when the wye-delta motor starting method is used. When the motor switches from a wye to a delta configuration, a large jump in motor kVA and torque will result. The jump in the motor kVA and torque can cause disturbances in the motor operation, and this jump in torque and kVA must be considered in the selection of this method because it can cause the motor speed run up to be rough. Some equipment cannot withstand the rough run-up when this starting method is used. A wye-delta starting motor will cost between three and six times the cost of a full voltage starting motor. A comparison of the relative cost of a wye-delta starting motor and the other methods of starting is given in Work Aid 3.

Torque, Speed, and kVA Curves for a Wye Delta Starting Motor Figure 17


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Primary Resistance Starting

The primary resistance starting method is essentially the same as a primary reactor starting method. The only difference between the primary reactors and primary resistance starters is that, in the primary resistance starting method, the reactors are replaced by resistors in the starting circuit. Figure 18 is a simplified diagram of a primary resistance motor starter. Notice that the diagram is the same as the diagram for a primary reactor starter, except that the reactors have been replaced by resistors. Contacts 1, 2, and 3 are closed to apply line voltage to the motor stator to start a motor with primary resistance starting. The resistors that are in series with the stator limit the current flow in the stator. When the motor accelerates to rated speed, contacts 4, 5, and 6 close to short out the resistors and to allow full current to flow.

Primary Resistance Motor Starter Figure 18


63



Figure 19 shows the typical motor torque, speed, and kVA characteristics for a primary resistance motor starter. Motor torque at full voltage is shown for comparison. Notice that the primary resistance motor starter curves are identical to the starter curves of a primary reactor motor starter. The line current at starting is proportional to the motor's applied voltage; starting torque is proportional to the square of the motor's applied voltage. Motor starting kVA is high for the amount of starting torque that is developed by the motor. The motor speed run-up will be smooth until the motor is switched from start to run. The rapid jump in motor torque and kVA can cause a disturbance in the motor's speed that is not acceptable for certain loads. A primary resistance motor starter costs approximately 520% of the cost of a full voltage starter. For economic reasons, reactors rather than resistors are used with all but the smaller sizes of motors. A comparison of the relative cost of a primary resistance motor starter and other motor starting methods is given in Work Aid 3.

Torque, Speed, kVA Curves for Primary Resistance Motor Starter Figure 19


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Part Winding Starting

Figure 20 is a simplified diagram of a part winding motor starter. The part winding motor starter has two sets of contacts to supply the motor. Part-winding motors are similar in construction to standard cage motors except that two parallel windings are provided in the stator and six leads must be included. Contacts 1, 2, 3 are closed to start a part winding motor. This closing will energize one set of windings in the motor. After the motor is almost at full speed, contacts 4, 5, 6 are closed. This closing will energize the other set of windings to supply full line voltage to the motor stator.

Part Winding Motor Starter Figure 20


65



Figure 21 shows the typical torque, speed, and kVA curves for a part winding starting motor. Motor torque at full voltage starting is shown for comparison. Starting torque on the first starting point varies from 48% to 72% of full load torque. The actual percentage depends on motor design, size, and speed. Notice the spike in motor torque and kVA when the second set of windings is energized. The sudden change in motor torque and KVA can cause a large disturbance in motor speed and run-up. Starting current varies from 50% to 80% of the locked-rotor current with both windings. The actual percentage depends on motor design, size, and speed. Part-winding starting can only be used with limited types of loads because of the small amount of starting torque that is generated on the first step of acceleration. Part-winding starters are used with motors that drive low inertia, low-torque starting loads such as airconditioning compressors, refrigeration compressors, centrifugal pumps, fans and blowers. This method also is used where reduced starting torque is necessary. A part-winding motor starter will cost approximately two to four times a full voltage starter. A cost comparison of part-winding motor starting with other motor starting methods is given in Work Aid 3.

Typical Torque, Speed, and kVA Curves of a Part Winding Starting Motor Figure 21


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Variable Frequency Starting

Figure 22 is a simplified diagram of a variable frequency motor starter. Variable frequency starting is essentially the same as full voltage starting except that the full voltage and frequency input to the motor stator can be converted to different values. The frequency and voltage converter may be a short-time rated motor-generator set or a solid-state unit. Both the output voltage and frequency of a variable frequency motor starter must be adjusted proportionally to each other. The output voltage and frequency must vary proportionally to ensure that the motor does not draw excessive current as a result of the lower impedance that is present at low frequencies. Contacts 1, 2, and 3 are shut to start a motor that uses a variable frequency starter. When these contacts shut, the voltage and frequency at the output of the voltage and frequency converter will be applied to the motor stator.

Variable Frequency Motor Starter Figure 22


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One frequency and voltage converter may be used for several motors on one bus if only one motor is to be started at any one time. A smooth motor speed run-up is possible through the steady increase of the supply frequency to the motor from the starting value to 60 Hz. Because of the rise in the frequency that is applied to the motor, no current surge is imposed on the supply system. The torque, speed, kVA, and current curves of a variable frequency motor will follow the same shape as for full voltage starting. The values of torque, speed, kVA, and current will change proportionally with the applied value of voltage and frequency. Maximum motor torque up to the full voltage, full frequency, breakdown torque is possible throughout the runup period. Variable frequency drives are mainly used for speed control. The use of a variable frequency driver only as a starting device is generally cost prohibitive. Electronically-Controlled Reduced Voltage Starting

Electronically-controlled reduced voltage starters employ back-to-back, phase-controlled thyristors in two or three of the lines to the motor as shown in Figure 23. The thyristors (G1 G6) are controlled during the starting period to maintain the starting current at about 300% of the full load current through the gradual increase of the motor voltage from the initial value up to full line voltage.

Electronically Controlled Reduced Voltage Starter Figure 23


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The torque, speed, kVA, and current can be easily adjusted through change in the amount of time that the thyristor will conduct, and this change will change the applied voltage. The motor run-up will be smooth because the applied voltage will gradually increase as the motor speeds up and because there is no mechanical switching. The electronically-controlled reduced voltage starter is applied where the line current is critical and where repetitive motor starting would limit the life of electromagnetic contactors. The cost of an electronically-controlled reduced voltage starter is prohibitively high in most instances. Factor Considered in Starting Method Selection Full voltage starting is used for the majority of Saudi Aramco induction and synchronous motor applications. Reduced voltage starting only should be considered when one of the following conditions exist: _The calculation of the voltage drop that results from motor starting indicates that the applied voltage at the motor terminals will be less than 80% of the motor nameplate voltage. _The load and/or the connection between the load and the motor may be damaged by the sudden application of full voltage starting torque. _The motor will be started several times an hour or the motor draws excessive starting current. In certain applications, the motor must be capable of starting under the worst case conditions. The characteristics of a motor that one chooses for a particular application must match an entire range of load torque characteristics; however, one must avoid the unnecessary expenditures that can result from over-specification. The worst case conditions are assumed because, if a motor will start under these conditions, the motor will start under any conditions. Worst case conditions are hypothetical conditions that assume that all possible detriments simultaneously occur. The worst case condition is when there is maximum load on the motor, while there is the lowest possible bus voltage, all other loads are running, and the largest motor starts. The resulting voltage drop can cause longer acceleration time for the largest motor and a heating that can result in deterioration and/or failure of the insulation. Also, the voltage drop can trip breakers and deenergize other loads. Many more possible conditions that depend on the conditions that are present at a given installation could exist; therefore, each installation must be individually evaluated.


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Factor Considered in Starting Method Selection (Cont'd) The following factors are considered in the selection of a starting method: _Permissible system voltage drop _Required starting torque of the load/load connection _Load current (heating) limitations _Comparative cost Terminal voltage that drops below 80% of full voltage value can still result in successful starting of the motor; however, the drop in the terminal voltage will cause the necessary current draw to increase in proportion to the drop in terminal voltage. The problem of excessive voltage drops when motors are started is that other loads on the system can be effected. Checks should be made to ensure that the motor controllers for any running motors that are on the same bus or on any other bus that is affected by the voltage depression remain held-in, and that the running motors do not stall. Figure 37 in Work Aid 3 shows the approximate voltage drop that results from full voltage starting of a motor. Motors and their respective loads must be connected by some means. The method of connection can be a hard permanent connection, a spider contact, or a simple belt. The method of motor/load connection must be considered in the selection of a motor starting method. Application of full voltage at starting will cause a large amount of torque on the motor/load connection. Starting torque that exceeds the rating of the motor/load connection will cause the motor/load connection to fail. Motors that are started with full voltage will develop the largest amount of starting current possible. The large starting current will add to the heating of the motor. A motor that is started several times an hour or that has an excessively high starting current could heat up and fail. The use of a reduced voltage starting method will reduce the starting current and thereby reduce the heating of the motors. Starting currents will vary with the method of starting the motor. Starting current will be proportional to the applied voltage and the developed starting torque of the motor starting method. Starting current will cause the motor to heat, and consideration must be given to how much starting current will be drawn and how often the motor will be started. A comparison of torque and of starting values for various starting methods is given in Figure 38 of Work Aid 3.


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Factor Considered in Starting Method Selection (Cont'd) For normal applications, full voltage direct-on-line starting is preferred for economic reasons. When it is necessary to use reduced voltage starting, the selection of appropriate starting methods will generally be made on economic grounds. A cost comparison for the different methods of motor starting for various size motors is given in Work Aid 3. Because other methods of starting are unusual in Saudi Aramco installations, approval must be sought from the Technical Services Department before implementation of a reduced voltage starter.


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SPECIFYING MOTOR PROTECTION REQUIREMENTS Motor protection requirements will vary with the type, size, duty, and application of motors. Protective devices should be specified with care to allow correct coordination with other protective devices and to avoid the possibility of misapplication in the field. Particular care is required with the specification of non-adjustable devices such as fuses. The sections that follow provide information on the following topics that are pertinent to specifying motor protection requirements: _Overload Protection _Short-Circuit Protection _Ground-Fault Protection _Current Unbalance _Vibration Protection _Bearing Failure Protection _Stator Winding High Temperature Protection _Undervoltage Protection _Overvoltage Protection and Surge Protection _Motor Stalling Protection _Differential Protection _Additional Protection for Synchronous Motors _Saudi Aramco Motor Protection Schemes Overload Protection Overload protection requirements and methods will vary with the size of the motor that is to be protected. The reasons for overload protection and range settings of overload protection will remain relatively constant, but the type of overload protection that is provided will change as motor size changes. The following aspects of motor overload protection will be discussed in this section: _Reasons for Overload Protection _Settings for Overload Protection _Types of Overload Protection Reasons for Overload Protection

The effect of an overload is a rise in temperature in the motor windings. Large overloads cause the temperature of the motor to quickly increase to a point where damage to the insulation and the lubrication of the motor occur; therefore, an inverse relationship exists between current and time (e.g., for higher currents, motor damage or "burnout" can occur in a shorter period of time than for lower current).


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Overload Protection (Cont'd) The increase in motor winding temperatures that is caused by overloads shortens motor life by the deterioration of the insulation. Relatively small overloads of short duration cause little damage but, if sustained, these small overloads could be just as harmful as overloads of greater magnitude. The relationship between the magnitude (percent of full load) and the duration (time in minutes) of an overload is illustrated by the Motor Heating Curve that is shown in Figure 24. At a 300% overload, the particular motor for which this curve is characteristic would reach its permissible temperature limit in three minutes. Overheating or motor damage would occur if the overload persisted beyond this time limit.


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Motor Heating Curve Data Figure 24 Settings For Overload Protection

Overload devices should be sized and specified at the design stage with trip points that can be set in the field. Overload devices should be the adjustable type with a minimum range of adjustment of 90% to 110% of the nominal trip point. The sizing of the overload device should be based on the motor nameplate full load current.

Overload Protection (Cont'd) A knowledge of motor starting current, total run-up time, and permissible stall time is required to correctly select motor overload protective devices. For most of the run-up period, the motor draws a current that is approximately the same as locked rotor current. Motor current sensing protection can only discriminate between stalling and normal run-up where the permissible stall time exceeds the run-up time. Where the run-up time exceeds the permissible stall time, extra protection for stalling is required. Extra protection for stalling is discussed later in this section. The NEC requirement for running overload protection is that tripping should ultimately occur at motor currents of not more than 115% of nameplate full load current. Excessive damage to the motor will be prevented if the motor is tripped at less than 115% of full load. Alternatively, where, because of the high starting current or the long run time, this 115% of full load setting does not allow correct motor starting. A higher trip setting of up to 130% of nameplate full load current is usually permissible (NEC trip point should be finally set by Field Engineers). The field setting should allow limited duration overloads that are essential to process continuity, and it should be a setting that prevents nuisance tripping during starting or as a result of nominal supply voltage fluctuations.


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Overload Protection (Cont'd) The key to setting an overload relay is shown in Figure 25. The figure shows a motor heating curve and a curve of the time that an overload requires to trip. Setting the overload relays to trip at the value on the time required to trip curve will ensure that the overload device will always trip at a safe value before the motor overheats.

Motor Heating Curve vs. Time Required to Trip Figure 25 Types of Overload Protection

The type of overload protection that is provided for a motor depends on the size of the motor. The two types of overload protection that are available are fuses and relays. Fuses are not Saudi Aramco DeskTop Standards

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acceptable in Saudi Aramco applications because a fuse cannot be adjusted. The following are the types of overload relays that are available: _Magnetic _Thermal


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Overload Protection (Cont'd) Figure 26 shows a magnetic overload relay. The magnetic overload relay consists of a movable iron core plunger that is inside of a solenoid coil. Current is supplied to the solenoid coil from the circuit's sensing network. As current passes through the solenoid coil, a flux is established in the solenoid coil. The flux that is established in the solenoid coil will pull the iron core plunger upward. The moving contact that is attached to the end of the iron core plunger also will move upward. Movement of the moving contact will bridge the gap between the two fixed contacts that are in line with the circuit trip coil. The circuit to the breaker trip coil will be completed when the gap is bridged between the fixed contacts, and the breaker will operate.

Magnetic Overload Relay Figure 26


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Overload Protection (Cont'd) The movement of the core can be slowed by an oil-filled piston (called a dashpot) that is attached to the core. The rate that the oil passes the piston is adjusted through opening or closing oil ports on the piston to adjust the tripping point of the overload relay. Closing the oil ports on the piston causes more resistance to the movement of oil past the piston, and the relay will take longer to trip. Conversely, opening the oil ports on the piston will reduce the resistance to movement of the piston, and the relay will trip sooner. A thermal overload works on the principle of heat buildup, which is caused by current flow. When an overload is placed on a motor, the motor will draw more current. The increase in current flow will cause an increase in the heat that is produced by the current flow. This increase in heat is the basis for thermal overload protection. All thermal overload protection relays operate on the principle of tripping the motor at a preset heating value of current flow. The thermal overload relay will sense the amount of current that flows to the motor, and this current flow will cause the thermal overload to heat up and trip the motor. There are two types of thermal overloads: _Melting alloy _Bimetallic Figure 27 shows a melting alloy type thermal overload relay. In the melting alloy thermal overload relay, the motor current passes from the equipment connection terminals through a heater winding. The heat that is developed by the heater winding causes a special solder in the solder pot assembly to melt. The melting of the solder allows a ratchet wheel that was being held by the solder to spin free and open a set of contacts. The motor will trip when the contacts open.


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Melting Alloy Type Thermal Overload Relay Figure 27 Overload Protection (Cont'd) Figure 28 shows a bimetallic thermal overload relay. The bimetallic thermal overload relay senses current through the use of a u-shaped bimetallic strip that is associated with a current carrying heater coil. An overload condition will cause an increase in the current that passes through the heater coil. The increase in current will cause more heat to be produced in the heater coil. The increased heating of the heater coil will cause the u-shaped bimetal strip to heat up. The u-shaped bimetal strip is constructed of two different metals; as the temperature of the metals increase, each metal will expand at a different rate. The difference in the rate of expansion of the two metals will cause the u-shaped bimetal strip to deflect and close a set of contacts on the contact assembly. Opening the contacts will cause the motor to trip.


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Bimetallic Thermal Overload Relay Figure 28 Overload Protection (Cont'd) Motors that are rated less than 1 kW (1.5 Hp) should be provided with thermal protective devices that are built into the starter. Built-in thermal protective devices should be of the selfresetting type except for open motors. The manual reset types are acceptable in open motors. Note that open motors (e.g., motors with drip-proof enclosures) are no longer acceptable in Saudi Aramco applications, but existing units need not be replaced. Automatic-reset protective devices should not be applied where the sudden restarting of a motor could be hazardous to personnel or equipment. Low voltage integral kW motors up to 600V and above 1.5 Hp require three thermal overload devices, of which one device should be in each phase. A trip action from any one device will cause all three phases of the starter to open. Manual reset is required, and the trip setting should be capable of field adjustment. Thermal overload devices must be of the ambient-compensated type because, in most Saudi Aramco installations, the motor starter/breaker is in an air conditioned environment and the motor is in an outdoor environment. The trip setting of ambient-compensated thermal Saudi Aramco DeskTop Standards

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overload devices will not vary significantly for ambient temperatures up to 60oC. Ambient temperatures of up to 60o will cover the majority of Saudi Aramco installations; for higher ambient temperatures, the manufacturer's advice on heater selection should be sought. Where power circuit breakers or molded case circuit breakers are used as motor starters, three current transformer-fed, ambient-compensated, thermal overload relays should be used, of which one relay should be in each phase. The relays should be reset by hand, and they should be arranged to trip the circuit breaker through use of a shunt trip device. Direct-acting trip devices should not be used to provide motor overload protection. Motors that operate on 2300 and 4000 V are split into the following two size categories: _1100 kW (1500 Hp) and below _Above 1100 kW (1500 Hp) For motors that are 1100 kW (1500 Hp) and below, three current transformer-fed, ambientcompensated, thermal overload relays should be used, of which one relay should be in each phase. The relays should be reset by hand and arranged to open the supply line to the motor. For motors that are above 1100 kW (1500 Hp), motor overload protection should be provided by RTDs. The overload protection should be designed to activate a visible alarm in the control room and to trip the motor supply when the stator RTDs reach a preset value.

Overload Protection (Cont'd) Overcurrent relays should also be included as a backup for the RTD overload protection. The temperature value of the RTDs that will trip the motor should be set so that the motor does not overheat. For 6.6 and 13.2 kV motors, only circuit breaker starters should be used. Motor overload protection should be by RTD-based protection schemes and induction-disc, overcurrent relays as backup for the RTD overload protection. Short-Circuit Protection The reason for short-circuit protection is to protect the motor branch circuit conductors, control apparatus, and motor windings from damage that is caused by excessive current. Short circuit protection can be provided through use of overcurrent relays or fuses. The limitation of fuses is that the trip setpoint cannot be adjusted.


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Protection against short circuits in motors or motor circuits will, in general, be through use of instantaneous trip devices. When used in combination with overload protective devices, the instantaneous elements should not be set to trip above 13 times the motor full load current, per NEC requirement. The most suitable setting for instantaneous trip devices is just above the maximum motor starting current. Maximum motor starting current should be determined as 1.8 times the motor locked rotor current to allow for DC offset and relay operating tolerances. When the devices are installed, the setting should be adjustable from 90 to 110% of the nominal trip setting. The type of motor short circuit protection that is employed will vary with the size of the motor and the type of starter that is installed. Motors 600V and below that use combination controllers with molded case circuit breakers as the motors fault protecting device should be equipped with adjustable pick-up, instantaneous, magnetic trip units, of which there should be one of these units in each phase. For motors 600V and below that use power circuit breakers or molded case circuit breaker starters, short circuit protection will be provided through use of three adjustable pick-up, instantaneous, overcurrent relays (device 50), of which there should be one of these relays in each phase. The relays should be arranged to trip the circuit breaker and should, in general, be integral with the overload protection employed by the motor.


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Short-Circuit Protection (Cont'd) Motors of 2300V and 4000V are divided as either NEMA Class E2 or circuit breaker starters. In NEMA Class E2 starters, short circuit protection is provided through use of fuses. Manufacturer's application guides allow correct selection of fuse ratings to be made when the motor starting characteristics are known. In combination with the thermal overload relays or induction disc overcurrent relays that are used for overload protection in NEMA Class E2 starters, the fuse operation forms a composite trip-current/time characteristic. The specified fuse should not melt or be damaged by motor starting, but it should not be too large to prevent satisfactory coordination with other protection schemes. In Class E2 starters, the fuse must interrupt all fault currents that exceed the limited fault interrupting capacity of the contactor to prevent damage to the motor. For 2300V and 4000V motors that use circuit breaker starters, short circuit detection is provided through use of three adjustable pick-up, instantaneous, overcurrent relays, of which there should be one of these relays in each phase. The relays shall be integral with the thermal overload relays that are specified for overload protection. For 6.6 kV and 13.2 kV motors, short circuit protection should be provided through use of three adjustable pick-up instantaneous, overcurrent relays, one in each phase. The relays should be integral with the induction disc over-current relays that are specified for overload protection. Ground-Fault Protection Ground-fault protection is necessary to prevent excessive overcurrent and burning damage to the motor that results from a ground-fault. All ground-fault protection is of the instantaneous type to accomplish the rapid removal of power in a fault condition. Modern industrial practice favors solid grounding of 480V systems and low resistance grounding at 4.16 kV, 6.6 kV, and 13.8 kV motor-bus voltages. Saudi Aramco also has several ungrounded installations at 480V and 2400V. This discussion only applies to grounded systems. For motors that are larger than 30 kW (40 hp), ground fault detection should be provided through use of a ground sensor device (device 50GS) that consists of a window-type current transformer (C.T.) and an adjustable pick-up, instantaneous current relay. The relay should be arranged to trip the circuit power line portion of the combination controller through use of a shunt trip device. In a 50/5 window type current transformer, a current relay pick-up setting of 0.5 A will be satisfactory. (After allowance for C.T. inaccuracies, this setting will give a primary current pick-up of approximately 13 A). The window-type current transformer should be carefully installed in accordance with manufacturer's instructions. During installation, particular attention must be paid to the termination of cable sheaths and armoring Saudi Aramco DeskTop Standards

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where applicable. Motors that are less than 30 kW do not require any ground fault protection in Saudi Aramco installations Short-Circuit Protection (Cont'd) Current Unbalance A current unbalance is caused by a voltage unbalance if the assumption is made that all three phases of the motor will have the same impedance. The current will become unbalanced if the voltage in any phase varies from the voltage in another phase. The most common cause of supply current unbalance is the voltage unbalance that follows a blown fuse in one phase of the primary supply to a step down transformer. Unequal supply phase impedance from untransponded transmission lines can also produce a voltage unbalance and thereby cause a supply current unbalance. The overall effects of a current unbalance that consisted solely of positive sequence currents would be minimal unless the value of unbalance became to large. The presence of negative sequence currents will increase the effects of the current unbalance on the motor. There are two reasons why the presence of negative sequence currents will adversely effect the motor because of the current unbalance: (1) The rapid rise in negative sequence current flow for a low change in voltage unbalance and (2) the increased heating that is caused by the negative sequence current. The frequency of the negative sequence current is higher than the motor's frequency. At the higher frequency, the motor resistance is high because of the skin effect, and the motor heating is correspondingly increased. The overall effect of a current unbalance in a motor is that the higher current in one phase will cause the heating effect on the motor to increase. A current unbalance can cause one phase current to be larger than the current for which the motor is designed. The phase with the higher current will cause an increase in the heating of the motor. The heat that is produced will be localized, and it may not be indicated by other means. If the higher temperature persists, it can damage the motor. Operation of a motor in excess of five percent current unbalance is not allowed. Above five percent current unbalance, local heating can occur that may not be detected by temperature sensing methods. This local heating can cause damage to the motor that is not detected until a catastrophic failure occurs.


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Current Unbalance (Cont'd) The following protective devices can be employed to protect against current unbalance: _Thermal Overload Relays _Current Balance Relays _Negative Sequence Voltage Relays _Thermal Synthesis Relays Thermal overload relays will tend to either overprotect or underprotect motors that only use thermal overload relays for current unbalance protection. For a motor that is running at full load, an unbalance current will cause at least one phase to exceed normal full load current. The thermal overload device in that phase will operate. To use thermal overload devices for current unbalance protection, one device must be installed in each phase of the supply. For motors that are running at near full load, thermal overload relay protection will generally overprotect because the magnitude of the largest current gives a pessimistic indication of the actual motor heating that is averaged over the three phases. The degree of protection is reduced for motors that run well below full load and that use thermal overload relays for current unbalance protection because a large proportion of negative sequence current is required before the thermal overload trip current is reached. Current balance relays operate when a preset (typically 25 percent) difference in any two supply currents is exceeded. Current balance relays are not entirely satisfactory protection against negative sequence currents because the magnitude in difference of the supply currents is not a direct indication of actual motor heating. To effectively apply negative sequence voltage relays, a sensitive relay with time delayed operation is required to prevent operation on momentary supply unbalance. The measurement of negative sequence voltage is not a direct indication of actual motor heating, but one relay can provide effective whole-bus protection against single phasing of the bus. Thermal synthesis relays provide the only satisfactory protection against the effects of unbalanced supply because the thermal synthesis relay correctly simulates the actual motor heating. The thermal synthesis relay is now only available as a static unit, and it generally incorporates short circuit and running overload protection in one unit. Thermal synthesis relays sense both positive and negative sequence current components in the motor supply


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lines. The actual motor heating effect is simulated through the derivation of a quantity of heating that is proportional to the following:

Current Unbalance (Cont'd) where:

I1 = positive sequence current I2 = negative sequence current K = increased heating effect of negative sequence current, typically 3-6.

Saudi Aramco policy in the application of unbalanced supply current protection is dictated by the size of installation, the likelihood of single phasing, and the present limited applicability of static protective schemes at Saudi Aramco. For motors 600 V and below, no additional protection is applied for current unbalance. The thermal overload devices in all three phases are expected to provide adequate protection for current unbalance. For 2.3 kV motors, no additional protection against current unbalance is necessary in each individual motor. The thermal overload devices in all three phases are expected to provide adequate protection for current unbalance. On 2300 volt installations where single phasing may commonly occur, one negative sequence voltage relay and timer should be used for whole-bus protection. 4 kV, 6.6 kV, and 13.2 kV installations require unbalanced supply voltage protection if single phasing can commonly occur. Whole-bus protection should be applied through use of a negative phase sequence voltage relay and auxiliary timer. This arrangement is expected to be eventually superseded by the application of thermal synthesis relays when such devices are fully approved for Saudi Aramco applications. Vibration Protection There are two types of vibration probes available: proximity and seismic. The proximity probe works on the principle of moving a permeable material through a magnetic field. The distortion in the field is representative of the amount of vibration in the motor. The tip of the proximity probe will be placed close to, but not touching, the motor bearing casing. Power that is supplied to the proximity probe will establish a small magnetic field at the end of the probe. The magnetic field will interact with the motor's bearing housing The vibration of the Saudi Aramco DeskTop Standards

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motor will cause the bearing casing to move in the magnetic field. The movement of the bearing casing in the magnetic field will change the coupling of the magnetic field and produce an output that is proportional to the amount of motor vibration.

Vibration Protection (Cont'd) The seismic probe works on a similar principal. In the seismic probe, a core is attached to the bearing housing and is mounted in a coil. The vibration of the motor will cause the core to move back and forth in the coil. The movement of the core in the coil will change the magnetic coupling in the coil and produce an output that is proportional to the motor's vibration. The following are the preferred types of vibration detectors for use in Saudi Aramco applications: Detector Equipment Seismic Type Bently Nevada 16699-10-05-02 or equal Proximity Type

Bently Nevada series 21000 or equal

The recommended limits of vibration for all Saudi Aramco motors are those limits that are stated in NEMA MG 1, except when proximity type vibration sensors are fitted. For all motors, the maximum rotor shaft peak-to-peak amplitude of vibration, as measured in tests on completely assembled motors at the vendor's factory at rated no-load conditions, should be as follows: _Horizontal Motors with Proximity Probes Maximum Shaft Maximum Combined Motor Speed Vibration Level Runout Allowance rpm Micrometers Mils Micrometers Mils 3600 50 2 8.86 0.35 1800 63 2.5 12.66 0.5 1200 or less 76 3 12.66 0.5 When vendors can demonstrate that run-outs due to shaft material anomalies are present, the value of these run-outs may be added to the above acceptable vibration levels up to a maximum of 6.3 micrometers (0.25 mil.). _Vertical and Horizontal Motors with Seismic Velocity Transducers.


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Maximum allowable vibration level should be 4.6 mm/s (0.18 in/s) zero-topeak, and it should be measured in three planes that are as close as possible to the bearing housing.


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Vibration Protection (Cont'd) Alarm and shutdown levels for horizontal motors with proximity type shaft probes or casing mounted seismic probes are derived from the maximum allowable vibration for acceptance of new machinery. Alarm levels for all horizontal motors are determined through the addition of 25 micrometers (1 mil) peak-to-peak to new machinery acceptance vibration levels. In the case of proximity probes, allowable "runout" is not included in the determination of alarm levels. The shutdown levels for motors are determined by doubling new machinery acceptance vibration levels. Vertical motor alarm and shutdown levels are based on acceptance criteria for vertical pumps. Because most, if not all, vertical pumps are actually an integral part of a vertical pump/motor system, they must have alarm and shutdown levels set accordingly. Vertical motor/pump alarm and shutdown levels are determined from the following: where:

n

=

motor speed in RPM

Tables that show the actual Saudi Aramco alarm and shutdown levels for different speed horizontal and vertical motors are given in Work Aid 4. Bearing Failure Protection Resistance temperature detectors (RTDs) are for use in the provision of bearing temperature monitoring. RTD's for bearing temperature monitoring should be supplied and fitted by the motor vendor. The RTD should be the platinum, three-wire type, and it should be calibrated to 100 ohms at 0oC (32oF). When embedded elements are used for bearing temperature measurement, extra elements should be installed in the bearing oil throw-off lines. These extra elements should be wired as unconnected spares to the monitor through use of the common connection box. Bearing failure protection monitors should consist of a separate alarm unit for each temperature set point and a single, time-shared temperature indicator. The alarm units should have dual set points and outputs, and they should accept the signal directly from the RTD element. The alarms are displayed on a separate annunciator.


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Bearing Failure Protection (Cont'd) The indicator should have an RTD location selector switch and a nameplate that shows the corresponding switch positions and RTD locations. The monitor will provide a fault alarm for open or short circuits in the control wiring between the RTD and the monitor. Temperature switches (thermostats) should not be utilized for Saudi Aramco applications. Maximum bearing temperatures must not exceed 40oC rise above ambient. The oil temperature rise for pressure lubricated bearings must not exceed 30oC when the inlet temperature is 60oC. The actual trip settings must be in accordance with the recommendations of the rotating machinery manufacturer, and they should be confirmed on the Buyer's Specification Sheet. One RTD should be provided in each bearing of horizontal motors that are rated 370 kW (500 HP) and larger. Horizontal motors that are rated 185 kW (250 HP) up to but not including 370 kW (500 HP) should only include a mounting provision in each bearing for resistance temperature detection. Vertical motors are not to be provided with RTD's. Stator Winding High Temperature Protection The monitoring of motor supply currents does not provide complete protection against stator winding overheating. In particular, monitoring of the supply current does not guard against overheating that is caused by inadequate ventilation. To protect against stator winding overheating, RTD's are used to detect stator temperature. The RTD's should be of the platinum, three-wire type, and they should be calibrated to 100 ohm at 0oC (32oF). Stator mounted RTDs are satisfactory indicators of motor winding temperature under steady state or slow load change conditions, but they cannot accurately indicate motor temperatures under rapid heating conditions such as stalling. RTD-based protection schemes should always by supplemented through use of overload relays that are operated from the motor supply current. The hottest reading RTD must be continuously monitored by the temperature relay (device 49T). The remaining RTDs should be terminated in the motor auxiliary connection box as future spares. This equipment should be used in place of the temperature relay to monitor the remaining stator-mounted RTDs. The actual requirements for the use of stator winding RTD's are given in Work Aid 4. The stator winding RTD trip settings for electrical motors must be in accordance with the recommendations of the motor manufacturer. Recommended alarm and trip setpoint for various motor insulation classes are given in Work Aid 4. Saudi Aramco DeskTop Standards

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Undervoltage Protection Undervoltage protection is necessary to protect the motor from damage that would be caused by higher current levels than the motor would experience if the voltage was below the nameplate design voltage level. A motor will draw enough current to provide the necessary power to operate the load because power is proportional to voltage and current as shown below: Power _ Voltage X Current If voltage drops below the designed voltage level, current will rise accordingly to maintain the proper level of power. The undervoltage relay is a simple magnetic relay that will deenergize when the supply voltage drops below a preset value. The deenergized undervoltage relay will open a set of contacts that will prevent the motor from restarting. The time delay dropout of the relay is a function of the relay's design. Certain relay designs will maintain the relay coil field for a given amount of time after power has been removed from the relay. Saudi Aramco uses instantaneous or time-delay undervoltage relays to provide motors with undervoltage protection. Motors of 2300 V and above should be provided with short time-delay under-voltage relay protection. The time delay is a function of the drop in voltage, and it typically ranges from two seconds for complete loss of supply to 15 to 20 seconds on voltage drops to 67%. The circuit breaker is tripped or the magnetic contactor is prevented from reclosing after the appropriate time delay. Except for motors that experience excessive starting torques because of out of phase residual rotor fields, the undervoltage time settings for complete loss of voltage should be four seconds. Thus, for a voltage depression with subsequent restoration within four seconds, the motor would be allowed to be immediately restarted. For voltage restoration after four seconds, the motor would be permanently locked out until it was manually reset. In some cases, especially with large motors, reconnection of the supply before the rotor magnetic field has decayed significantly will result in excessive restarting torque and currents. Typically, the field takes a few tenths of a second to decay, and restoration of voltage within this time will cause high torques if the residual rotor field has become out-of-phase with the (reapplied) stator field. Saudi Aramco DeskTop Standards

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These factors must be considered in schemes where the contactor or circuit-breaker is not tripped immediately on undervoltage. Magnetically-held contactors will open on undervoltage. A timer may be needed to prevent reclosure within the critical time. Latched breakers or contactors will need to be tripped instantaneously and reclosed after a suitable time delay. For such applications, an instantaneous undervoltage relay should be used for this application. Overvoltage Protection and Surge Protection The operation of an electric motor above the rated voltage will cause a deterioration of the motor insulation. Continuous operation of an electric motor above the motor's rated voltage will shorten the life the motor because of the slow deterioration of the motor's insulation. Motors that comply with NEMA MG 1 are capable of satisfactory operation at sustained supply voltages up to ten percent above nameplate rated voltage; however, overvoltage protection should be applied to each motor bus for all voltage and power ratings. The overvoltage protection should be of the time-delay type and capable of providing a tripping action at voltages that exceed approximately 110% of motor nameplate rated voltage. Motors are also susceptible to surge overvoltages. High voltage motors may require surge overvoltage protection to prevent damage to the motor. Significant voltage surges may occur on the motor terminals because of the following conditions: _Contactor/circuit breaker switching surges _Lightning induced surges on incoming overhead lines _High frequency voltage spikes from fast switching, solid-state type devices These surges can cause a failure of the motor's major or ground-wall insulation, and/or a failure of the motor's turn-to-turn insulation. The type of insulation failure that occurs is dependent upon the crest value of the overvoltage surge and the rate-of-rise of the overvoltage surge. A failure of a motor's major insulation can occur when the crest value of an overvoltage surge that gradually rises (e.g., rise time of 5 _s) exceeds the impulse strength of the motor's major insulation. The generally accepted impulse strength of a motor's major insulation can be calculated through use of the following equation: where V is the one minute rms high potential acceptance test voltage for the motor. For example, the impulse strength of 4000V, 3000 hp induction motor is calculated as follows: Saudi Aramco DeskTop Standards

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If the example motor is likely to be subjected to a voltage surge that rises to a crest value that is _ 15.91 kV in 5 _s, a surge arrestor should be installed to limit the magnitude of the overvoltage surge. Installation of a surge arrestor will protect the motor's major insulation from damage.

Overvoltage Protection and Surge Protection (Cont'd) A failure of a motor's turn-to-turn insulation can occur for lower crest value overvoltage surges when the rate-of-rise of the overvoltage surge is less than 5 _s. Such failures occur because of the large capacitive coupling that exists between the turns of the winding and the grounded core. When the motor is subjected to an overvoltage surge that has a high rate-ofrise (<5_s), 70% to 90% of the voltage will appear across the first two adjacent turns. Because the insulation that is between the adjacent turns has a much lower impulse strength than does the major insulation, the turn-to-turn insulation will fail. Such failures can be prevented through installation of a surge capacitor to reduce the rate-of-rise of the overvoltage surge. Saudi Aramco requires that surge protection be provided for motors that are rated 3730 kW (5000 hp) and above, and for motors that operate at 13,200 volts. The surge protection is to consist both of surge capacitors and of surge arrestors. The surge capacitors must be the last devices that are connected to the motor leads before the leads enter the stator. The connection leads to the surge capacitors must be 120 sq. mm (4/0 AWG) conductors. The following are the required surge capacitor ratings: Motor Rated Voltage (kV) Capacitance (micro-farads) Maximum Operating Voltage (kV) Continuous Temperature Rating (oC)

4.0 0.5 4.57 60

6.6 0.5 7.9 60

13.2 0.25 15.2 60

The only type of surge arrestor that is acceptable for use in the protection of Saudi Aramco motors is the gapless zinc oxide type. The surge arrestors must be mounted in the terminal box, and all of the connection leads must be less than 60 cm (24 in) from the surge arrestor to the motor terminal. The following are the required surge arrestor ratings: Saudi Aramco DeskTop Standards

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Motor Rated Voltage (kV) Arrestor Voltage Rating (kV rms) Maximum Discharge Voltage at 1500A (Crest kV) Continuous Temperature Rating (oC)

4.0 4.5 7.4 60

6.6 7.6 12.2 60

13.2 15 24.2 60

Motor Stalling Protection Should a motor stall during operation or be unable to start because of excessive load, the motor will draw from the supply a current that is equivalent to the locked rotor current. The excessive current that is drawn during stalling can rapidly damage the motor windings; therefore, quick disconnection of the machine is desirable to avoid damage.

Motor Stalling Protection (Cont'd) Relays cannot distinguish a stalling condition from a healthy starting condition on the sole basis of current magnitude. The only way to discriminate between a healthy starting condition and a stalled condition is to arrange the protective device that disconnects the motor to a time delay to sense when the current continues for longer than the normal starting time. Where the possibility of motor stalling exists, the stalling protection should be applied in view of the following criteria: _Where the normal start time of motor, including the effect of reduced terminal voltage, exceeds the safe stalled (locked rotor) time of the motor or _Where the normal start time is less than the safe stall (locked rotor) time. With regard to the second of these two criteria, three methods of protection are acceptable: _Set the thermal overload to trip after the normal start time has elapsed, but set it so that it trips before the safe stall time of the motor elapses. _Add a separate relay that will activate upon the starting of a motor but that will be inoperative for normal or overload current levels. This relay will prevent tripping of the motor during starting, but it will allow tripping if the motor stalls. _Use a start sequence timer that is designed to operate if normal full load current or speed is not achieved in the normal run-up time.


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For motors that are rated above 4000 kW (5360 Hp) or where the start time exceeds the safe locked rotor time, the use of one or more of the previously started protective elements should be supplemented by the following stalled rotor protection: _A start timer with a timer setting that is less than the safe stalled rotor time of the motor and that is designed to operate if rotation has not occurred within the time that is set for the timing relay. Speed measurement of the rotor should be by direct monitoring of shaft speed through use of proximity type monitoring equipment and a digital tachometer. The digital tachometer also should provide an alarm (voltage free contact) at approximately ten percent of motor full load speed to be used in conjunction with the above mentioned start timer. The tachometer alarm device should employ a time range of typically 0.3 to 3 seconds. If the motor does not reach ten percent speed within the timer setting, the motor circuit breaker or contactor would be tripped.


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Differential Protection Differential protection provides sensitive detection and rapid removal of in-zone ground faults and phase-to-phase faults while being insensitive to motor starting inrush. The differential protection will protect a motor from damage that is caused by internal faults. The differential relay works on the principle of comparison of the input current of one phase with the output current of the same phase. The differential relay will receive a signal from a current transformer on the input to one phase of the motor and from a current transformer on the output of the same phase of the motor. The current transformers are wound so that the currents that are induced in the transformer secondaries will oppose each other and no current will flow through the differential relay. The in-zone protection is between the two current transformers. Any fault that occurs to the motor outside of the in-zone protection will not be sensed by the differential relay. A fault that occurs in-zone will cause the current that flows out of the protected zone to be different than the current that flows into the protected zone. The difference in the two currents results because some of the current that flows into the protected zone flows out through the fault. The difference in the current flows causes a differential current to flow through the operating winding of the differential relay. The differential relay will operate and send a signal to the trip circuit of a circuit breaker when the differential current exceeds the relay setpoint. Motors that are 3700 kW (5000 Hp) and above should be equipped with motor differential protection. The self-balance type of differential protection should be used in preference to other types. Motors that are less than 3700 kW (5000 hp) do not require differential protection. Additional Protection for Synchronous Motors Synchronous motors require more protection than induction motors because synchronous motors are more complex. The additional protection that is required by synchronous motors relates specifically to the unique nature of how a synchronous motor operates. The following is a list of the additional protection that is required for synchronous motors: _Out of step _Sudden restoration of supply power _Overvoltage _Reverse power _Loss of excitation _Over-excitation _Under-excitation _Diode failure


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Additional Protection for Synchronous Motors (Cont'd) Synchronous motors can pull out-of-step because of sudden loading of the shaft or because of temporary depressions of the supply voltage. The slip cycle time will depend on the initial cause and also the inertia of the drive. The effect of loss of synchronism is induced currents in rotor circuits and subsequent overheating of the motor. Out-of-step protection should be applied to detect pull-out or loss of synchronism. A relay will be provided that detects pull-out conditions through measurement of stator current and voltage, either in the form of power reversals or by the power factor of stator current. The protective relay must allow some time for the motor to attempt to pull into step before initiating a trip. The time delay that is permissible will vary, but it will allow either three half slip cycles or a total pull-out time of 10 to 15 seconds, as determined by the motor's manufacturer. Upon a loss of the supply to a synchronous motor, the motor breaker must be tripped as quickly as possible if there is any possibility of the supply being either automatically restored or restored without the knowledge of the machine operator. Tripping is necessary in order to prevent the supply from being restored out-of-phase with the motor-generated voltage. Restoration of the supply voltage out of phase with the motor-generated voltage will generate an excessive torque that could damage the motor or the attached equipment. To trip the breaker, an undervoltage relay is placed on the power supply line. The undervoltage relay will trip the circuit breaker if the power supply voltage drops below a preset value. The voltage of the synchronous motor can rise and exceed design limits when no load or minimal load is connected to the busbars that supply the motor and when there is no load on the motor itself. Under these conditions, the voltage could rise instantaneously from 120% to 130% of the nominal values. Busbar overvoltage protection will provide overvoltage protection, and instantaneously trip the motor. The overvoltage protection will be provided through use of a voltage sensing relay on the bus work. Reverse power protection is only applicable where power reversals do not occur under normal operating conditions, which is the case for all Saudi Aramco installations. A reverse power relay will be provided to prevent the motor from delivering power to other loads on the same busbar. A slight time delay should be incorporated into the reverse power relay to provide for the momentary reversals of power that result from system faults. Failure to trip the motor on a power reversal can damage the motor bearing and possibly cause an excessive current to flow in the motor.


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Additional Protection for Synchronous Motors (Cont'd) Under-excitation or complete loss of excitation in a synchronous motor will cause the motor to run as an induction motor at a slip frequency. The machine's reactive power is then provided by means of the system, and it results in a larger stator current that operates at a lagging power factor. The larger stator current conditions may not be satisfactory for either the supply system or the motor. Loss of excitation will not necessarily be detected through use of any of the previously described protective devices. A separate protective device should be provided that can be arranged either to initiate an alarm or shut down the motor. For large machines that are rated at above 7500 kW (10000 hp), an additional relay (device 40) should be provided as part of the motor switchgear. This relay utilizes an offset mho-type of impedance measuring relay that continuously monitors motor impedance as seen from the motor terminals. If the motor excitation fails or falls to a very low value, the machine impedance enters the relay operating characteristic circle, and the relay operates to start a time delay relay. Operation of the time delay relay should cause tripping of the main circuit breaker and initiation of field suppression. The time delay is provided to prevent false-operation of the protection, which can result from transient surges. Over-excitation of a synchronous motor can occur in the event of failure of the field excitation control circuit. Over-excitation causes the continuous field current to exceed its rating. Field current protection (over-excitation) can be applied through use of a normal thermal overload relay. To prevent operation at low field levels and to minimize the problems that are associated with instability and stator overcurrent, under-excitation protection should be provided. Underexcitation protection will be accomplished through use of an undervoltage relay that is set up to monitor the motor excitation. Diode failure is detected by monitoring the waveform of the exciter field current and operating a time delay relay from the harmonic current that is produced by open circuits or short circuits in the rotating diodes. Operation of the device should trip the main circuit breaker.


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Additional Protection for Synchronous Motors (Cont'd) The following list shows the protective equipment that should be provided at the machine circuit breaker and the excitation control cubicle of a synchronous motor: Machine Circuit Breaker Instantaneous Overcurrent Time Delayed Overcurrent Instantaneous Ground Fault Differential Current Overvoltage Repeat Start Blocking Incomplete Sequence (including running stall) Stalled Rotor Out of Step Reverse Power Loss of Excitation

Excitation Cubicle Field Overcurrent Diode Failure Field Failure Under-excitation

The following list shows the metering equipment that should be provided at the machine circuit breaker and the excitation control cubicle of a synchronous motor: Machine Circuit Breaker

Excitation Cubicle

Line Voltage

Excitation Current

Line Current Power Factor kW var

Power Factor

The machine vendor should supply, as a minimum, the equipment that is associated with the excitation control cubicle. The supply of remaining equipment that is located at the machine circuit breaker should be subject to Project requirements and the capabilities of the prospective vendor. Saudi Aramco Motor Protection Schemes Protective schemes should, in general, employ electromechanical or thermal-type relay elements in preference to static type protection. The use of static type protection requires the approval of the Technical Services Department, except when no alternatives to the static type are available. Saudi Aramco DeskTop Standards

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Saudi Aramco Motor Protection Schemes (Cont'd) Protection against overvoltage, vibration, and bearing failure is not shown and should be added to the basic protection as appropriate for the given size of the motor. Protection schemes for the following motor circuits will be discussed in this section: _Motors 600V or less with combination magnetic starters 1 to 100 hp. _Motors 600V or less with circuit breaker starters >100 hp. _Motors 4000V or greater with NEMA E2 starters _1500 hp. _Motors 4000V or greater with circuit breaker starters <10,000 hp. _Motors 4000V or greater with circuit breaker starters _10,000 hp. _Synchronous motors <10,000 hp. Figure 29 shows the protection scheme for motors 600V or less with combination magnetic starters that are 1 to 100 hp. The protective devices are integral units to the combination magnetic starter. The combination magnetic starter houses both the circuit breaker and the motor's control contacts. The protective relays that are shown in Figure 29 are identified through the use of the following standard device function numbers: _49 is the thermal overload. _50 is the instantaneous overcurrent relay. _50GS is a ground fault overcurrent with an instantaneous element. Device 49 (thermal overload) provides overload protection for the motor as follows: _An increase in load on the motor above the full load rated value will result in an increase in current flow to the motor stator. _The increase in current flow will cause the heating element in the thermal overload to produce more heat. _If the increase in current flow persists for a sufficient length of time, relay 49 will operate and cause the motor's control contacts to open. _When the control contacts open, power to the motor stator will be interrupted and the motor will stop.


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Saudi Aramco Motor Protection Schemes (Cont'd) Device 50 (instantaneous overcurrent relay) provides overcurrent protection against faults that occur downstream of the circuit breaker as follows: _A fault downstream of the circuit breaker will cause an increase in current flow through device 50. _A large increase in current flow will cause the instantaneous element of device 50 to operate. _When the instantaneous element of device 50 operates, the circuit breaker will trip (open). _When the circuit breaker opens, the power supply to the motor will be disconnected and the motor will stop. Device 50GS (instantaneous ground fault overcurrent relay) provides overcurrent protection against ground faults that occur downstream of the circuit breaker as follows: _A ground fault downstream of the circuit breaker will cause an increase in current flow through the circuit breaker and CT-1. _The increase in current flow is transmitted to the 50GS device from CT-1. _A large increase in current will cause the instantaneous element of the 50GS device to operate. _When the instantaneous element of the 50GS device operates, the device will send a signal to the circuit breaker that causes the circuit breaker to trip open. _When the circuit breaker opens, the power to the motor stator will be interrupted and the motor will stop.


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Saudi Aramco Motor Protection Schemes (Cont'd)

Motors 600 V or Less with Combination Magnetic Starters 1 to 100 hp Figure 29


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Saudi Aramco Motor Protection Schemes (Cont'd) Figure 30 shows the protection scheme for motors 600V or less with circuit breaker starters that are >100 hp. The protective devices in this protection scheme are identified through the use of the following standard device function numbers: _27 is the undervoltage relay. _42 is the starting circuit breaker. _49 is the thermal overload. _50/51 SST is the phase and ground solid state trip device _50GS is the instantaneous ground fault overcurrent relay. Device 27 (undervoltage relay) provides undervoltage protection for the motor as follows: _The voltage transformer continuously monitors the voltage that is supplied to the motor from the power source, and it transmits this voltage signal to the undervoltage relay (device 27). _If the voltage drops to a level that would cause the motor to draw excessive currents (normally about 67% of rated voltage), the undervoltage relay operates and sends a trip signal to the starting circuit breaker (device 42). _The starting circuit breaker (device 42) trips and secures power to the motor to prevent motor damage that can result from excessive current flows. Device 49 (thermal overload) provides overload protection for the motor as follows: _An increase in load on the motor above the full load rated value will result in an increase in current flow through CT-2. _The increase in current flow is transmitted to the 49 device from CT-2. _If the increase in current flow persists for a sufficient length of time, relay 49 will operate and send a signal to the starting circuit breaker (42). _The starting circuit breaker (device 42) will trip and deenergize the motor.


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Saudi Aramco Motor Protection Schemes (Cont'd) The 50/51 SST device is a solid state trip unit that provides the following types of protection to the motor: _Long time adjustable phase overcurrent protection. _Short time adjustable phase overcurrent protection. _Instantaneous phase overcurrent protection. _Instantaneous ground fault overcurrent protection. _The starting circuit breaker (device 42) will trip (open), which secures power to the motor and removes the overcurrent. Device 50GS (instantaneous ground fault overcurrent relay) provides protection against ground faults that occur downstream of the circuit breaker as follows: _A ground fault that is downstream of the breaker will cause an increase in current flow through the circuit breaker and CT-1. _The increase in current flow is transmitted to device 50GS from CT-1. _A large increase in current flow will cause the instantaneous element of the 50GS device to operate. _When the instantaneous element of the 50GS device operates, the device will send a signal to the starting circuit breaker (42). _The starting circuit breaker (device 42) will trip (open), which secures power to the motor and removes the ground fault.


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Motors 600 V or Less with Circuit Breaker Starters that are >100 hp. Figure 30


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Saudi Aramco Motor Protection Schemes (Cont'd) Figure 31 shows the protection scheme for motors 4000 volts or greater with NEMA E-2 starters that are 1500 hp. Overcurrent protection in this scheme is provided through use of current limiting fuses that are integral to the NEMA E-2 starter. The protective relays that are shown in Figure 31 are identified through the use of the following standard device function numbers: _27 is the undervoltage relay. _42 is the motor contactor. _47 is the negative sequence voltage relay. _49 is the thermal overload. _49T is the RTD actuated thermal/overload relay. _50G is the instantaneous ground fault overcurrent relay. _51 LR is the locked rotor relay. _86 MI is the auxiliary lockout relay. Device 27 (undervoltage relay) provides undervoltage protection for the motor as follows: _The voltage transformer continuously monitors the voltage that is supplied to the motor from the power source, and it transmits this voltage signal to the undervoltage relay (device 27). _If the voltage drops to a level that would cause the motor to draw excessive currents (normally about 67% of rated voltage), the undervoltage relay operates and sends a trip signal to the motor contactor (device 42). _The motor contactor (device 42) trips and secures power to the motor to prevent motor damage that can result from excessive current flows. Device 47 (negative sequence voltage relay) provides bus protection against negative sequence voltages as follows: _When a fault occurs that results in a negative sequence voltage, the negative sequence voltages will be sensed through the use of the voltage transformer. _The voltage transformers will transmit the negative sequence voltage to the negative sequence relay (device 47).


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Saudi Aramco Motor Protection Schemes (Cont'd) _When the negative sequence voltage value exceeds the setpoint of the negative sequence relay (device 47), the relay operates and sends a signal to the motor contactor (device 42). _The motor contactor (device 42) will operate and secure power to the motor. Device 49 (thermal overload) provides overload protection for the motor as follows: _An increase in load on the motor above the full load rated value will result in an increase in current flow through CT-2. _The increase in current flow is transmitted to the 49 device from CT-2. _If the increase in current flow persists for a sufficient length of time, relay 49 will operate and send a signal to motor contactor (device 42). _The motor contactor (device 42) will operate and deenergize the motor, which removes the overload. Device 49T (RTD actuated thermal/overload relay) provides additional overload protection to the motor as follows: _An increase in load on the motor above the full load rated value will result in an increase in current flow to the motor. _The increase in current flow will cause the temperature in the motor stator to increase. _When the temperature of the motor stator, as read by RTDs that are embedded in the stator reaches a preset value, device 49T will operate. _When device 49T operates, a signal is sent to the motor contactor (device 42). Device 42 will secure power to the motor, and the motor will stop.


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Saudi Aramco Motor Protection Schemes (Cont'd) Device 50G (ground overcurrent relay) provides protection against ground faults that occur downstream of the circuit breaker as follows: _A ground fault downstream of the breaker will cause an increase in current flow through the circuit breaker and CT-1. _The increase in current flow is transmitted to device 50G from CT-1. _A large increase in current flow will cause the instantaneous element of the 50G device to operate. _When the instantaneous element of the 50G device operates, the device will send a signal to the auxiliary lockout relay (device 86 MI). _When the auxiliary lockout relay (device 86 MI) operates, the device will send a signal to the motor contactor (device 42). _The motor contactor (device 42) will operate to secure power to the motor and to remove the ground fault. Device 51 LR (locked rotor relay) provides protection against locked rotor conditions as follows: _A locked rotor condition will cause an increase in current flow through CT-3. _The increase in current flow is transmitted to device 51 LR from CT-3. _If the increased current flow persists for a sufficient length of time, device 51 LR will operate and send a signal to the auxiliary lockout relay (device 86 MI). _The auxiliary lockout relay (device 86 MI) will operate and send a trip signal to the motor contactor (device 42). _The motor contactor (device 42) trips and secures power to the motor.


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Motors 4000 Volts or Greater with NEMA E2 Starters _ 1500 hp Figure 31


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Saudi Aramco Motor Protection Schemes (Cont'd) While not shown in the protection scheme of Figure 31, a lockout relay (86) and a stalling relay can be added as the application dictates. Figure 32 shows the protection scheme for motors 4000 volts or greater with circuit breaker starters that are < 10,000 hp. The protective relays that are shown in Figure 32 are identified through use of the following standard device function numbers: _6T is the motor starting circuit breaker. _27 is the undervoltage relay. _47 is the negative sequence voltage relay. _49 is the thermal overload. _49T is the RTD-actuated thermal overload relay. _50 are the phase A and phase C instantaneous overcurrent relays. _50G is the ground overcurrent relay. _50/51 LR is the phase B instantaneous overcurrent/locked rotor relay. _86 M1 is the auxiliary lockout relay #1 _86 M2 is the auxiliary lockout relay #2. _87 are the differential relays. Device 27 (undervoltage relay) provides undervoltage protection for the motor as follows: _The voltage transformer continuously monitors the voltage that is supplied to the motor from the power source, and it transmits this voltage signal to the undervoltage relay (device 27). _If the voltage drops to a level that would cause the motor to draw excessive currents (normally about 67% of rated voltage), the undervoltage relay operates and sends a trip signal to the motor starting circuit breaker (device 6T). _The motor circuit breaker (device 6T) trips and secures power to the motor to prevent motor damage that can result from excessive current flows. Device 47 (negative sequence voltage relay) provides bus protection against negative sequence voltages as follows: _When a fault occurs that results in a negative sequence voltage, the negative sequence voltages will be sensed through use of the voltage transformer.


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Saudi Aramco Motor Protection Schemes (Cont'd) _The voltage transformer will transmit the negative sequence voltage to the negative sequence relay (device 47). _When the negative sequence voltage value exceeds the setpoint of the negative sequence relay (device 47), the relay operates and sends a trip signal to the motor starting circuit breaker (device 6T). _The motor starting circuit breaker (device 6T) trips (opens) and secures power to the motor. Device 49 (thermal overload) provides overload protection for the motor as follows: _An increase in load on the motor above the full load rated value will result in an increase in current flow through CT-2. _The increase in current flow is transmitted to the 49 device from CT-2. _If the increase in current flow persists for a sufficient length of time, relay 49 will operate and send a trip signal to motor starting circuit breaker (device 6T). _The motor starting circuit breaker (device 6T) trips (opens) and secures power to the motor. Device 49T (RTD-actuated thermal overload relay) provides additional overload protection to the motor as follows: _An increase in load on the motor above the full load rated value will result in an increase in current flow to the motor. _The increase in current flow will cause the temperature in the motor stator to increase. _When the temperature of the motor stator, as read by the RTDs that are embedded in the stator, reaches a preset value, device 49T will operate. _The operation of Device 49T will send a signal to the motor auxiliary lockout relay #1 (device 86M1). _Device 86M1 will operate, which causes the motor starting circuit breaker (device 6T) to trip (open) and secure power to the motor. Saudi Aramco DeskTop Standards

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Saudi Aramco Motor Protection Schemes (Cont'd) Device 50 (instantaneous overcurrent relay) provides overcurrent protection against faults that occur downstream of the circuit breaker as follows: _A fault downstream of the circuit breaker will cause an increase in current flow through CT-4. _The increase in current flow is transmitted to the 50 device from CT-4. _A large increase in current flow will cause the instantaneous element of device 50 to operate. _The operation of device 50 will send a signal to the motor auxiliary lockout relay #1 (device 86M1). _The motor auxiliary lockout relay #1 (device 86M1 will operate and send a signal to the motor starting circuit breaker (device 6T). The motor starting circuit breaker (device 6T) will trip (open) and secure power to the motor. Device 50G (ground overcurrent relay) provides protection against ground faults that occur downstream of the circuit breaker as follows: _A ground fault that occurs downstream of the breaker will cause an increase in current flow through the circuit breaker and CT-1. _The increase in current flow is transmitted to device 50G from CT-1. _A large increase in current flow will cause the instantaneous element of the 50G device to operate. _When the instantaneous element of the 50G device operates, the device will send a signal to the motor auxiliary lockout relay #1 (86M1). _The motor auxiliary lockout relay #1 (device 86M1) will operate and send a trip signal to the motor starting circuit breaker (device 6T). The motor starting circuit breaker (device 6T) will trip (open) and secure power to the motor.


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Saudi Aramco Motor Protection Schemes (Cont'd) Device 51LR (locked color relay) provides protection against locked rotor conditions as follows: _A locked rotor condition will cause an increase in current flow through CT-4. _The increase in current flow is transmitted to device 51LR from CT-4. _If the increased current flow persists for a sufficient length of time, device 51LR will operate and send a signal to the auxiliary lockout relay (device 86M1). _The auxiliary lockout relay (device 86M1) will operate and send a trip signal to the motor starting circuit breaker (device 6T). _The motor starting circuit breaker (device 6T) trips and secures power to the motor. Device 87 (differential relays) provides internal motor fault protection as follows: _CT-3 will establish the protected zone on the motor. _A fault in the protected zone will cause a difference in the magnitude of the current that flows into and out of the protected zone. _The difference in current flows will cause a current to flow through the operating coil of the differential relay (device 87). _Device 87 will operate and send a trip signal to the motor auxiliary lockout relay #2 (device 86M2). _Device 86M2 will operate and cause the motor starting circuit breaker (device 6T) to trip (open) and secure power to the motor.


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Motors 4000 Volts or Greater with Circuit Breaker Starters < 10,000 hp Figure 32


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Saudi Aramco Motor Protection Schemes (Cont'd) While not shown for the protection scheme in Figure 32, a current balance and reverse phase relay, a stalling relay, and a repeat start block relay can be added as the application dictates. Figure 33 shows the protection scheme for motors 4000 volts or greater with circuit breaker starters that are _ 10,000 hp. The protective relays that are shown in Figure 33 are identified through use of the following standard device function numbers: _6T is the motor starting circuit breaker. _27 is the undervoltage relay. _46 is the current balance and reverse phase relay. _47 is the negative sequence voltage relay. _50 are the instantaneous phase overcurrent relays. _50G is the ground-fault overcurrent relay. _50/51G is the residual ground overcurrent relay. _51OL are the phase A and phase C overload. _51LR is the locked rotor relay. _86M1 is the auxiliary lockout relay #1. _86M2 is the auxiliary lockout relay #2. _87 are the differential relays. Device 27 (undervoltage relay) provides undervoltage protection for the motor as follows: _The voltage transformer continuously monitors the voltage that is supplied to the motor from the power source, and it transmits this voltage signal to the undervoltage relay (device 27). _If the voltage drops to a level that would cause the motor to draw excessive currents (normally about 67% of rated voltage), the undervoltage relay operates and sends a trip signal to the motor starting circuit breaker (device 6T). _The motor starting circuit breaker (device 6T) trips and secures power to the motor to prevent motor damage that can result from excessive current flows. Device 46 (current balance and reverse phase relay) provides protection against unbalanced phase currents and reversed phase currents as follows: _When a fault such as single-phasing occurs, unbalanced phase currents will flow through CT-3.


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Saudi Aramco Motor Protection Schemes (Cont'd) _The unbalanced phase currents are transmitted to the 46 device from CT-3. _If a sufficient percentage of unbalance exists between any two phases, the 46 device operates. _The operation of the 46 device will send a signal to the auxiliary lockout relay #2. _The auxiliary lockout relay #2 will operate and send a trip signal to the motor starting circuit breaker (device 6T). _The motor starting circuit breaker (device 6T) trips and secures power to the motor. Device 47 (negative sequence voltage relay) provides protection against negative sequence voltages as follows: _When a fault occurs that results in a negative sequence voltage, the negative sequence voltages will be sensed through use of the voltage transformer. _The voltage transformer will transmit the negative sequence voltage to the negative sequence relay (device 47). _When the negative sequence voltage value exceeds the setpoint of the negative sequence relay (device 47), the relay operates and sends a signal to the motor starting circuit breaker (device 6T). _The motor starting circuit breaker (device 6T) will trip (open) and secure power to the motor. Device 50 (instantaneous phase overcurrent relay) provides overcurrent protection against faults that occur downstream of the circuit breaker as follows: _A fault that occurs downstream of the circuit breaker will cause an increase in current flow through CT-1. _The increase in current flow is transmitted to the 50 device from CT-1. _A large increase in current flow will cause the instantaneous element of device 50 to operate. Saudi Aramco DeskTop Standards

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Saudi Aramco Motor Protection Schemes (Cont'd) _The operation of device 50 will send a signal to the auxiliary lockout relay #1 (device 86M1). _The auxiliary lockout relay #1 (device 86M1) will operate and send a signal to the motor starting circuit breaker (device 6T). The motor starting circuit breaker (device 6T) will trip (open) and secure power to the motor. Device 50G (ground fault overcurrent relay) provides protection against ground faults that occur downstream of the circuit breaker as follows: _A ground fault that occurs downstream of the breaker will cause an increase in current flow through the circuit breaker and CT-2. _The increase in current flow is transmitted to device 50G from CT-2. _A large increase in current flow will cause the instantaneous element of the 50G device to operate. _When the instantaneous element of the 50G device operates, the device will send a signal to the auxiliary lockout relay (86M1). _The auxiliary lockout relay (device 86M1) will operate and send a trip signal to the motor starting circuit breaker (device 6T). The motor starting circuit breaker (device 6T) will trip (open) and secure power to the motor. Device 50/51G (residual ground overcurrent relay) provides additional protection against ground-faults as follows: _When a ground-fault occurs, ground-fault current will flow through CT-5. _The ground-fault current is transmitted to device 50/51G from CT-5. _If a sufficient magnitude of ground-fault current flows, the 50/51G device operates and sends a signal to the auxiliary lockout relay #2 (device 86M2). _The auxiliary lockout relay #2 will operate and send a trip signal to the motor starting circuit breaker (device 6T).


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_The motor starting circuit breaker (device 6T) trips and secures power to the motor. Saudi Aramco Motor Protection Schemes (Cont'd) Device 51OL (overload relay) provides overload protection for the motor as follows: _An overload on the motor will result in an increased current flow through CT1. _The increase in current flow is transmitted to device 51OL from CT-1. _If the increase in current flow persists for a sufficient length of time, device 51OL will operate and send a signal to the auxiliary lockout relay #1 (device 86M1). _Device 86M1 will operate and cause the motor starting circuit breaker (device 6T) to trip (open) and secure power to the motor. Device 51LR (locked color relay) provides protection against locked rotor conditions as follows: _A locked rotor condition will cause an increase in current flow through CT-1. _The increase in current flow is transmitted to device 51LR from CT-1. _If the increased current flow persists for a sufficient length of time, device 51LR will operate and send a signal to the auxiliary lockout relay (device 86M1). _The auxiliary lockout relay (device 86M1) will operate and send a trip signal to the motor starting circuit breaker (device 6T). _The motor starting circuit breaker (device 6T) trips and secures power to the motor. Device 87 (differential relays) provides internal motor fault protection as follows: _CT-4 will establish the protected zone on the motor. _A fault in the protected zone will cause a difference in the magnitude of the current that flows into and out of the protected zone. Saudi Aramco DeskTop Standards

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_The difference in current flows will cause a current to flow through the operating coil of the differential relay (device 87). _Device 87 will operate and send a signal to the auxiliary lockout relay #2 (device 86M2). _Device 86M2 will operate and cause the motor starting circuit breaker (device 6T) to trip (open) and secure power to the motor.


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Motors 4000 Volts or Greater with Circuit Breaker Starters _ 10,000 hp Figure 33


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Saudi Aramco Motor Protection Schemes (Cont'd) While not shown for the protection scheme in Figure 33, a stalling relay and a repeat start block relay can be added as the application dictates. Figure 34 shows the protection scheme for synchronous motors that are < 10,000 hp. The protective relays that are shown in Figure 34 are identified through use of the following standard device function numbers: _6T is the motor starting circuit breaker. _27 is the undervoltage relay. _46 is the current balance and release phase relay. _47 is the negative sequence voltage relay. _48 is the incomplete sequence relay. _49 are the thermal overload relays. _50 are the instantaneous phase A and phase B overcurrent relays. _50G is the ground-fault overcurrent relay. _50/51LR is the instantaneous phase B overcurrent relay and the locked rotor relay. _55 is the power factor (loss of field) relay. _86M1 is the auxiliary lockout relay #1. _86M2 is the auxiliary lockout relay #2. _87 are the differential relays. Device 27 (undervoltage relay) provides undervoltage protection for the motor as follows: _The voltage transformer continuously monitors the voltage that is supplied to the motor from the power source and transmits this voltage signal to the undervoltage relay (device 27). _If the voltage drops to a level that would cause the motor to draw excessive currents (normally about 67% of rated voltage), the undervoltage relay operates and sends a signal to the motor starting circuit breaker (device 6T). _The motor starting circuit breaker (device 6T) trips and secures power to the motor to prevent motor damage that can result from excessive current flows. Device 46 (current balance and reverse phase relay) provides protection against unbalanced phase currents and reversed phase currents as follows: _When a fault such as single-phasing occurs, unbalanced phase currents will flow through CT-3. _The unbalanced phase currents are transmitted to the 46 device from CT-3. Saudi Aramco DeskTop Standards

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Saudi Aramco Motor Protection Schemes (Cont'd) _If a sufficient percentage of unbalance exists between any two phases, the 46 device operates. _The operation of the 46 device will send a signal to the auxiliary lockout relay #2. _The auxiliary lockout relay #2 will operate and send a trip signal to the motor starting circuit breaker (device 6T). _The motor starting circuit breaker (device 6T) trips and secures power to the motor. Device 47 (negative sequence voltage relay) provides protection against negative sequence voltages as follows: _When a fault occurs that results in a negative sequence voltage, the negative sequence voltages will be sensed through use of the voltage transformer. _The voltage transformer will transmit the negative sequence voltage to the negative sequence relay (device 47). _When the negative sequence voltage value exceeds the setpoint of the negative sequence relay (device 47), the relay operates and sends a signal to the motor starting circuit breaker (device 6T). _The motor starting circuit breaker (device 6T) trips (opens) and secures power to the motor. Device 48 (incomplete sequence relay) secures power to the motor in the event that the motor fails to start within its allotted time as follows: _When the motor starting circuit breaker (device 6T) shifts to apply power to the motor, a timer that is internal to device 48 starts to time out. _If the motor fails to reach its normal speed of operation before device 48 times out, device 48 operates and sends a trip signal to the motor starting circuit breaker (device 6T). _The motor starting circuit breaker (device 6T) trips (opens) and secures power to the motor. Saudi Aramco DeskTop Standards

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Saudi Aramco Motor Protection Schemes (Cont'd) Device 49 (thermal overload relay) provides overload protection for the motor as follows: _An increase in load on the motor that is above the full load rated value will result in an increase in current flow through CT1. _The increase in current flow is transmitted to the 49 device from CT1. _If the increase in current flow persists for a sufficient length of time, relay 49 will operate and send a signal to the auxiliary lockout relay #1 (device 86M1). _The auxiliary lockout relay #1 will operate and send a trip signal to the motor starting circuit breaker (device 6T). _The motor starting circuit breaker (device 6T) trips (opens) and secures power to the motor. Device 50 (instantaneous phase overcurrent relay) provides overcurrent protection against faults that occur downstream of the circuit breaker as follows: _A fault that occurs downstream of the circuit breaker will cause an increase in current flow through CT-1. _The increase in current flow is transmitted to the 50 device from CT-1. _A large increase in current flow will cause the instantaneous element of device 50 to operate. _The operation of device 50 will send a signal to the auxiliary lockout relay #1 (device 86M1). _The motor auxiliary lockout relay #1 (device 86M1) will operate and send a signal to the motor starting circuit breaker (device 6T). The motor starting circuit breaker (device 6T) will trip (open) and secure power to the motor. Device 50G (ground-fault overcurrent relay) provides protection against ground faults that occur downstream of the circuit breaker as follow:


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_A ground fault that occurs downstream of the breaker will cause an increase in current flow through the circuit breaker and CT-2.

Saudi Aramco Motor Protection Schemes (Cont'd) _The increase in current flow is transmitted to device 50G from CT-2. _A large increase in current flow will cause the instantaneous element of the 50G device to operate. _When the instantaneous element of the 50G device operates, the device will send a signal to the auxiliary lockout relay (86M1). _The auxiliary lockout relay (device 86M1) will operate and send a trip signal to the motor starting circuit breaker (device 6T). The motor starting circuit breaker (device 6T) will trip (open) and secure power to the motor. Device 51LR (locked color relay) provides protection against locked rotor conditions as follows: _A locked rotor condition will cause an increase in current flow through CT-1. _The increase in current flow is transmitted to device 51LR from CT-1. _If the increased current flow persists for a sufficient length of time, device 51LR will operate and send a signal to the auxiliary lockout relay (device 86M1). _The auxiliary lockout relay (device 86M1) will operate and send a trip signal to motor starting circuit breaker (device 6T). _The motor starting circuit breaker (device 6T) trips and secures power to the motor. Device 55 (power factor relay) provides loss of field protection for the motor as follows: _Synchronous motors normally are designed to operate at power factors that range form 0 to 0.8 leading.


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_The 55 device continuously monitors the power factor of the motor through use of a voltage input from the voltage transformer and a current input from CT-1.

Saudi Aramco Motor Protection Schemes (Cont'd) _Upon a loss of a significant decrease in the motor's field, the motor's power factor will become lagging. _The lagging power factor will cause the 55 device to operate and send a signal to the auxiliary lockout relay #1. _The auxiliary lockout relay #1 will operate and send a trip signal to the motor starting circuit breaker (device 6T). _The motor starting circuit breaker (device 6T) trips (opens) and secures power to the motor. Device 87 (differential relays) provides internal motor fault protection as follows: _CT-4 will establish the protected zone on the motor. _A fault in the protected zone will cause a difference in the magnitude of the current that flows into and out of the protected zone. _The difference in current flows will cause a current to flow through the operating coil of the differential relay (device 87). _Device 87 will operate and send a trip signal to the motor starting circuit breaker (device 6T). _The motor starting circuit breaker (device 6T) trips (opens) and secures power to the motor.


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Synchronous Motors < 10,000 hp Figure 34 Saudi Aramco DeskTop Standards

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WORK AID 1: MOTOR DESIGN REQUIREMENTS FOR SAUDI ARAMCO INSTALLATIONS COMPILED FROM SADP-P-113, NEMA MG-1 AND ESTABLISHED ENGINEERING PRACTICES Use Work Aid 1 to complete Exercise 1. Stator The stator frame should be of fabricated steel construction, with sufficient strength and rigidity to withstand the stresses to which the stator may be subjected in handling, transport, or due to short-circuit or other forces when in service. Rotor Induction motors should have a cylindrical rotor of the squirrel-cage type. Synchronous motors are available in cylindrical rotors or salient pole rotors. Cylindrical synchronous motors are used in speeds in excess of 1800 rpm. Salient pole rotors come in two types: _The laminated salient pole rotor with a cage damper winding in each pole face for starting. _The solid pole rotor with solid bolted pole shoes. The solid pole rotor is the preferred type of synchronous salient pole rotor. Critical Speed The critical speeds of motors shall be established by the following chart.

Rigid Shaft

First Lateral Critical Speed 115% rated motor rpm

Flexible Shaft

65 to 85% rated motor speed

Second Lateral Critical Speed Not within _10% of 2 times rated motor rpm Not within _10% of 2 times rated motor rpm

WORK AID 1 (Cont'd) Saudi Aramco Evaluating Motor Specifications

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Bearings Select bearing type by the chart below. Motor Size < 15 hp (11 kW) _ 15 hp (11 kW) > 200 hp (150 kW) * * Horizontal Motors

Bearing Type 200 or 300 series ball bearings 300 series ball bearings Sleeve bearings

Type of bearing lubricant to use. Bearing Type Anti-friction Sleeve

Lubrication Oil or Grease Oil

When grease or oil can be used, use what is most economical. Grease will usually be utilized because it is usually more economical to use. Method of Bearing Lubrication Sleeve bearing lubrication methods should be as follows: Shaft Journal Velocity m/S Below 11

Type of Lubricant Uncooled ring or disc oil lubrication

11 and above

Circulated feed oil lubrication

Ball bearings should be of the regreasable shielded type, furnished without grease fittings, but equipped with plugs in the tapped holes that are normally provided for such fittings. Relief holes or drain plugs that are located 180o from the grease point should be provided. Pre-lubricated sealed anti-friction bearings are not acceptable.


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WORK AID 1 (Cont'd) Bearing Housing Protection Horizontal motors that are rated <3730 kW (5000 hp) should have the bearings housed in the endbells of the motor. Horizontal motors that are rated 3730 kW (5000 hp) and larger should be provided with pedestal bearings that are supported by the motor baseplate. The bearing housing for horizontal motors with oil-lubricated bearings should be equipped with labyrinth type end seals and deflectors where the shaft passes through the housing. Synthetic gaiter (lip) type seals should not be used. Vertical motors with oil or grease-lubricated bearings should be provided with a positive shaft seal on top and bottom bearings. Vibration Monitoring Vibration monitoring should be used as required below: _Vibration monitoring equipment should not be used for motors below 185 kW (250 hp). Horizontal Motors

_750 kW (10000 hp) - 3000 kW (4000 hp): one seismic detector should be mounted horizontally at each bearing. _3001 kW (4001 hp) and above: two proximity detectors should be mounted 90o apart at each bearing. Vertical Motors

_Seismic type vibration detectors should be mounted 90o apart around the circumference of the top bearing housing. Mechanical Noise The sound pressure level at one meter should not exceed 90 dB.


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WORK AID 1 (Cont'd) Shaft Circulating Current Horizontal motors that are rated 375 kW (500 hp) and larger should have bearings that are electrically insulated from the motor frame or baseplate. A removable test link should be provided for the drive end bearing. Vertical motors that are rated 375 kW (500 hp) and larger should have an electrically insulated top bearing. Stator Winding The stator insulation system, including the leads, should consist of low-hygroscopic materials. The system should have Class F insulation. The stator windings should be treated to withstand tropical conditions and the corrosive effects of industrial sulfurous atmospheres. Stator temperature should be monitored as follows: _Motors that are rated 150 kW (200 hp) up to but not including 1300 kW (1750 hp) should have one RTD per phase. _Motors that are rated 1300 kW (1750 hp) through 7500 kW (10,000 hp) should have two RTD's per phase. _Motors that are rated above 7500 kW (10,000 hp) should have three RTD's per phase. _The hottest reading RTD should be identified by the vendor during factory testing. Rotor Windings The rotor windings of induction machines should be of the cage type and formed from copper, copper alloy, or aluminum bar. The rotor body of synchronous machines should be of the salient-pole type with the windings of insulated copper wire or strip that are treated to withstand tropical conditions.


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WORK AID 1 (Cont'd) In addition to induction motor rotors, salient-pole rotors of synchronous motors may incorporate a copper starting cage to meet a desired torque characteristic. On squirrel-cage rotors, end-ring connections should be of high mechanical strength. Filler metals that are used should be resistant to attack by corrosive sulfurous gases. Copper alloy rotor construction should conform to American Welding Society AWS A5.8 and contain a minimum 40% silver. Copper-phosphorous bronze type fillers are technically unacceptable. The motor insulation system, including the leads, should consist of low-hygroscopic materials. The system should have Class F insulation. Mounting Details Enough measurements must be taken to ensure proper mechanical fit of the new motor. The number of measurements that are taken are determined by the designer according to his judgement. Cooling If the motor insulation exceeds the limits of class B insulation, a heat exchanger must be installed. Motor Insulation Class B F H

Maximum Allowable Insulation Temperature in oC 130 155 180

The cooling of all motor components and lubricating oil (if required) should be by air. Liquid cooling is not acceptable. Where totally-enclosed machines utilize heat exchangers, closed-air circuit-air cooled (CACA) exchangers should be provided and mounted on the motor. Top-mounted heat exchanger assemblies should have flanges that extend downward to overlap the motor enclosure on all sides by a minimum of 10 mm (0.4 in). When air/air heat exchangers require auxiliary fan cooling, shaft-mounted cooling fan or fans should be provided. Auxiliary motor-driven fans should not be specified.


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WORK AID 1 (Cont'd) Control and Supply Leads Wiring for auxiliary equipment should be moisture and heat resistant and should conform to the following: _Conductors should be made of copper and rated in accordance with the National Electrical Code (NEC) for operation at 50oC ambient temperature. _Minimum conductor size should be stranded 2.5 mm sq. (No. 14 AWG). _Conductors should be clearly and permanently identified with sleeve type PVC wire markers. _All conductors that are passing through an internal baffle or partition that is within the motor enclosure should be protected by a bushing. Nameplates The following is the minimum information that is required to be on motor nameplates: _Manufacturer's name, serial number or date code, and suitable identification _Horsepower output or kilowatt rating _Time rating _Temperature rise _RPM at rated load _Frequency _Number of phases _Voltage _Rated-load amperes _Code letter for kVA _Buyer's purchase order number _Year of manufacture _Manufacturer's location _Manufacturer's order reference number _Anti-friction bearing number and manufacturer class, group, and division (explosion-proof motors, only) _Maximum ambient temperature Saudi Aramco DeskTop Standards

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_Insulation system designation _Rotor weight _Total weight of motor _Direction of rotation arrow WORK AID 1 (Cont'd) The nameplate(s) and rotation arrows should be 300 series stainless steel or monel, they should be securely fastened by pins of similar material, and they should be located for easy visibility. Entries should be marked by etching, engraving, or by another permanent method of marking. Space Heaters Heaters should be installed in all motors that are rated 2.3 kV or higher. Other motors that are installed outdoors and used only as standby motors may also require heaters. In such cases, the designer should check with the proponent. Motors should be equipped with space heaters that are completely wired, with leads that are brought out to a separate terminal box. Heaters with exposed elements are prohibited. Heater nameplate voltage should be twice the supply voltage that is indicated on the Data Schedule. The heaters will maintain the temperature of the motor windings at approximately 5oC (9oF) above ambient. Surface temperature of the heater elements should not be greater than shown below: Area Classification Class I, Group C

Surface Temperature 160oC (320oF)

Class I, Group D

215oC (419oF)

Class II, Group E

200oC (392oF)

Class II, Group G

120oC (248oF)

The amount of heat that is required to raise the winding temperature of a given enclosed horizontal motor approximately five degrees C above ambient temperature, where the machine is closed except for a small vent at the top and bottom for circulation, is given by the formula: H


=

0.28 DL

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where: D L

H = =

= Heat in kilowatts Machine end bell diameter in meters Machine stator length between end bell centers in meters

WORK AID 1 (Cont'd) Heaters should be suitable for supply voltages of 120 VAC, and they should be controlled by auxiliary contacts in the breaker. Testing Requirement The following are the minimum tests that are required: _Measurement of winding resistance. _No-load motoring readings of current, power, and nominal speed at rated voltage and frequency. _Measurement of open-circuit voltage ratio on wound-rotor machines. _High-potential test. _Bearings/lube oil temperature measurement. When determined by the designer, the following test should be performed: _Performance determination _Temperature tests _Miscellaneous tests _Surge tests Painting and Coating All steel surfaces should have the vendor's standard finish with a minimum of 0.127 mm (5 mil) dry thickness. Packing


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The packing of equipment should be suitable for shipment by sea and also for vehicular transportation over unpaved desert roads. Packing should be in accordance with Buyer's Packing Specifications No. 1 and No. 1.1 of Vendor's standard export packing. Vendor's standard export packing should be subject to the buyer's approval.


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WORK AID 2: MOTOR ENCLOSURE REQUIREMENTS FOR SAUDI ARAMCO INSTALLATIONS COMPILED FROM SADP-P-113, NEMA MG-1 AND ESTABLISHED ENGINEERING PRACTICES Use Work Aid 2 to complete Exercise 2. Technical requirements for specification of a motor enclosure type. _Totally Enclosed Fan-Cooled (TEFC) -

Allowed on motors that are rated less than 11,000 kW (15,000 hp) Permitted in zone 1, zone 2, and unclassified locations

_Totally Enclosed Air to Air-Cooled (CACA) -

For use on all induction motors and for salient pole synchronous motors that are rated up to 11,000 kW (15,000 hp) when the motor's temperature rise exceeds the allowable temperature rise for class B insulation.

-

For use on induction motors that are rated greater than 11,000 kW (15,000 hp)

-

For use on synchronous motors where the cleaning cost over the life of the machine plus the capital cost of an open motor would be more than the cost of the CACA enclosure.

-

Permitted in zone 1, zone 2, and unclassified areas.

_Weather Protected (WPII) -

Specified for synchronous motors that are rated above 11,000 kW (15,000 hp) where the lifetime motor cleaning costs plus motor capital costs are less than the capital cost of a CACA motor.

-

Permitted in zone 2 and unclassified locations.

Technical requirements for specification of a motor-enclosed level of protection. _All Saudi Aramco motor enclosures will be protected to a level that is equal to the IEC level designation IP44.


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WORK AID 2 (Cont'd) Technical requirements for the specification of a motor enclosure level of cooling. _TEFC motors will be cooled to an IEC level ICA01A41. _CACA motors will be cooled to an IEC level ICA01A61. _WPII motors will be cooled to an IEC level ICA01. Technical requirements for specification of a motor enclosure level of hazard protection. _Zone 1 area requires a flameproof EXd motor. _Zone 2 area requires a non-sparking EXn motor. _Unclassified area does not require a level of hazard protection for a motor. Main and auxiliary equipment boxes should meet or exceed the main motor enclosure requirements. _Grounding Requirements -

Motors up to 150 kW (200 hp) _ grounding stud Motors at or above 150 kW (200 hp) _ grounding pad

Ground connections should accommodate the following minimum size of grounding cable: Motor Rating kW (HP) 185 < 370 (250 < 500) 370 < 3360 (500 < 4500)


Cable Size mm2 (AWG/MCM) 70 (2/0) 120 (4/0) 3360 & above (4500 & above) 185 (350)

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WORK AID 3: CONDITIONS UNDER WHICH THE VARIOUS TYPES OF MOTOR STARTERS SHOULD BE SPECIFIED FOR USE AT SAUDI ARAMCO INSTALLATIONS, COMPILED FROM SADP-P-113, NEMA MG-1, AND ESTABLISHED ENGINEERING PRACTICES Use Work Aid 3 to complete Exercise 3. 1.

Determine if the voltage drop on the system upon motor start at full voltage is greater than the voltage drop that is permissible for the system. Use the graph that is shown in Figure 35 to approximate the voltage drop on a system when starting a motor. a.

Find on the bottom axis the motor kVA or the motor hp.

b.

Find the power supply's rated kVA curve for the power supply that will supply the motor.

c.

Trace the motor rating line up from the bottom and the power supply's rated kVA curve. Find the point of intersection of these two lines.

d.

Trace a line to the left axis (power supply's voltage) and read the percentage value of initial voltage.

e.

Subtract from 100 the value that is read on the left axis to arrive at the system voltage drop upon motor starting.

If the voltage drop is less than 15%, full voltage starting should be used. If the voltage drop is greater than 15%, the Consulting Services Department should be consulted to perform a more accurate calculation on a computer. On voltage drops greater than 15%, reduced voltage starting should be considered. Note: For the purpose of performing Exercise 3, the approximate voltage drop should be assumed to be the actual voltage drop. For this Exercise, there is no reason to contact the Consulting Services Department. WORK AID 3 (Cont'd)


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Approximate Voltage Drop Due to Full Voltage of the Motor Figure 35 WORK AID 3 (Cont'd) 2.

Choose the reduced voltage starting to be performed when voltage drop exceeds the allowable value. A table of comparable torque and current values for various starting methods is shown in Figure 36. Figure 36 shows the method of starting, the motor voltage as percent of full voltage, the starting torque as percent of full voltage starting torque, and the line starting current as percent of full voltage starting current. a.

b.

Determine the starting torque that is required by the load and what types of starting methods will provide the necessary starting torque. -

If only one method of reduced voltage starting will provide enough starting torque, select that starting method.

-

If more than one method of reduced voltage starting will provide the necessary starting torque, proceed to the next selection factor.

Determine if motor heating is a concern because of excessive motor starts. -

If heating is a concern, select the reduced voltage starting method with the lower starting current that meets the motor starting torque requirements.

-

If heating is not a concern, proceed to the next selection factor.


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c.

If more than one method of reduced voltage starting can meet the starting torque requirement of the load and the motor heating limitations, choose the lower cost method of starting.


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WORK AID 3 (Cont'd)

Comparable Torque and Current Values for Various Starting Motors Figure 36


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2400/4160V NEMA E2 Air Break Cost comparisons are for floor-mounted starters that are complete with overload and short circuit protection and all necessary starting contents. Percent Full Voltage Primary Reactor Auto-transformer

100 250 285

Typical Sizes (2400/4160 V) up to 1100/3000 kW up to 1100/3000 kW up to 1100/3000 kW

WORK AID 3 (Cont'd) Average Relative Cost of Starting Methods 480V Magnetic Starters (Individual Enclosures) The starters are costed complete with built-in starting components, overload protection, and housed in NEMA general purpose enclosures. Short circuit protection is not included.

Full Voltage Part Winding Primary Resistor Auto-transformer Wye-Delta

Percent 100 360 520 560 630

Typical Sizes 18.5 kW to 75 kW 18.5 kW to 75 kW 18.5 kW to 75 kW 18.5 kW to 75 kW 18.5 kW to 75 kW

480V Magnetic Starters (in Motor Control Centers)


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Cost comparisons are for starter units that are suitable for mounting in NEMA I motor control centers. The starters are complete with overload and short circuit protection (to 22,000 A) and include all necessary starting components.

Full Voltage Part Winding Auto-transformer Wye-Delta


Percent 100 180 285 360

Typical Sizes 37 kW to 185 kW 37 kW to 140 kW 37 kW to 185 kW 37 kW to 185 kW

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WORK AID 4: CONDITIONS UNDER WHICH THE VARIOUS TYPES OF MOTOR PROTECTION SHOULD BE SPECIFIED FOR USE AT SAUDI ARAMCO INSTALLATIONS, COMPILED FORM SADP-P-113, NEMA MG-1, AND ESTABLISHED ENGINEERING PRACTICES Use Work Aid 4 to complete Exercise 4. Overload Protection Requirements Motors less than 1 kW (1.5 HP) Built-in thermal protection devices. Low voltage (<600 volts) motors above 1 kW (1.5 HP) Combination starters Three thermal overload devices that are ambient-compensated. One device should be located in each phase. Power circuit breakers Three transformer-fed, ambient-compensated, thermal overload relays. One device should be located in each phase. 2.3 and 4 kV motors NEMA Class E starters Three current transformer-fed, ambient-compensated thermal overload relays. Power circuit breakers - <1,000 kW (1500 HP) Three current transformer-fed, ambient-compensated thermal overload relays. Power circuit breakers >1,000 kV (1500 HP) RTD-based protection - stator RTDs. 6.6 and 13.2 kV RTD-based backed-up by overcurrent relays.


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WORK AID 4 (Cont'd) Short Circuit Motors 600V and below Adjustable pickup, instantaneous, and overcurrent relays. One device should be located in each phase. Motors 2.3 kV and 4 kV Nema Class E2 starters Fuses in combination with thermal overload relays. Circuit breaker starters Three adjustable pickup, instantaneous, overcurrent relays. One device should be located in each phase. Motors 6.6 kV and 13.2 kV Three adjustable pickup, instantaneous, and overcurrent relays. One device should be located in each phase. Ground Fault One instantaneous overcurrent ground sensing relay for all motors with a 50/5 window type current transformer. Current Unbalance 2.3 kV motors and below No additional protection is required. 4 kV, 6.6 kV, and 13.2 kV Negative phase sequence voltage relay with timer. Vibration Horizontal motors 750 kW (1000 HP) - 3000 kW (4000 HP) - 1 seismic detector on each bearing. Vertical motors Two seismic detectors on the top bearing.


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WORK AID 4 (Cont'd) Alarm levels and shutdown levels are listed below: Horizontal Motors with Proximity Probes Motor Synchronous Speed RPM


Alarm Level Micrometers

Shutdown Level Micrometers

147



3600 1800 1200

75

100

90 125

100


150

148



Horizontal Motors with Seismic-Type, Casing Mounted Probes 3600 1800 1200 900 and below

50

75

100 75

125 90

150 100


149




150



Vertical Motors with Seismic-Type, Casing Mounted Probes Motor Synchronous Speed RPM

Alarm Level Micrometers

Shutdown Level Micrometers

3600 1800 1200 900 600 and below

100

150

275 125

190 140

200


151



150

225

175


152



WORK AID 4 (Cont'd) Bearing Failure RTD's will be used for detection and protection purposes. RTD's will be platinum, 3-wire type, calibrated to 100 ohms at 0oC. Horizontal Motors 185 kW (250 HP) to 370 kW (500 HP) - mounting provision must be included, but no RTD's. -

>370 kW (500 HP) - one RTD for each bearing.

Bearing alarm and motor shutdown for all motors that are larger than 370 kW (500 HP). Stator Winding Protection Temperature detection is accomplished by RTD. Motors 375 kW (500 HP) to 7500 kW (1000 HP) - two RTD's per phase. Motors 7500 kW (10,000 HP) and above - three RTD's per phase. RTDs will be the following type. RTDs will be platinum, 3-wire type, calibrated to 100 ohms at 0oC. Shutdown and alarms Plant-attended - alarm only Plant-unattended - alarm and shutdown Shutdown and alarm levels Motor Insulation Class

Alarm or Trip Setting Degrees C

B F H

120 145 170


Maximum Allowable Insulation Temperature Degrees C 130 155 180

153



WORK AID 4 (Cont'd) Undervoltage Motors less than 2.3 kV require no special undervoltage protection. Motors 2.3 kW and higher require a short time delay undervoltage relay. Overvoltage and Surge Protection The following surge protection requirements are found in 17-SAMSS-502: Motors that are rated 3730 kw (5000 hp) and larger and motors that are rated 13.2 Kv will be furnished with surge protection. The surge capacitors will be the last devices that are connected to the motor leads before the leads enter the stator. The connection leads to the capacitors will be 120 SQ MM (4/0 AWG). The surge capacitors will be rated as follows: Motors Rated Voltage (KV) 4.0 6.6 13.2 Capacitance (Micro-Farads) 0.5 0.5 0.25 Maximum Operating Voltage (KV)

Continuous Temperature Rating (Deg C)


4.57 7.9 15.2 60 60 60

154



Surge arresters will be of the gapless zinc-oxide type, and they will be rated as follows: Motor's Rated Voltage (KV) 4.0 6.6 13.2 Arrester Voltage Rating (KV RMS) 4.5 7.5 15 Maximum Discharge Voltage at 7.4 12.2 24.2 1500 (CREST KV) Continuous Temperature Rating (O C) 60 60 60 Surge arrestors will be mounted in the terminal box and all connection leads will be less than 60 cm (24 in) from arrestor to motor terminal. All motors should have a relay to trip the motor at 110% of the motor's nameplate rated voltage as indicated on the nameplate.


155



WORK AID 4 (Cont'd) Motor Stalling Where the possibility of motor stalling exists and start time is less than still time, use one of the following: -

Thermal overload set to trip after starting current delay time.

-

Start sequence timer to prevent tripping overload upon start if motor comes up to speed.

-

Start timer that prevents tripping during starting.

For motors that are rated above 4000 kW (5360 HP) or that have a start time that is longer than the allowable locked rotor time, apply one of the above methods and use a timer that is set so that, if rotation does not begin within a set time, the motor will trip. Differential Protection Motors >3700 kW (5000 HP) will require differential protection. One relay will be applied for each phase. Synchronous Motors Extra Protection If necessary, provide the following protection. -

Out of step Sudden restoration power Over voltage Reverse power Loss of excitation Over-excitation Under-excitation Diode failure


156



GLOSSARY ANSI

American National Standards Institute

AWG

American Wire Gauge

CACA

Closed Air Circuit Air-Cooled

centistokes (cSt)

The International Standards Organization's metric measure of a lubricant's viscosity.

centrifugal force

The force that tends to impel a thing or parts of a thing outward from the center of rotation.

critical speed

A speed at which the amplitude of the vibration of a rotor that results from shaft transverse vibration reaches a maximum value.

diode

A semiconductor device with two terminals that exhibits a nonlinear voltage - current characteristic.

dynamic load rating

The load to which the bearings can be subjected while achieving a rated life of one million revolutions.

EXD

Explosion Proof Motor. This type of motor is enclosed in a case that is capable of withstanding an explosion of a specified gas or vapor that may occur within it. It prevents the ignition of a specified gas or vapor surrounding the enclosure by sparks, flashes, or explosion of the gas or vapor within the enclosure. This type of motor operates at such an external temperature that a surrounding flammable atmosphere will not be ignited.


157



EXN

Non-sparking motor.

heat exchanger

A device that is used to transfer heat between two fluids without direct contact between them.

hygroscopic

Readily taking up and retaining moisture.

IEC

International Electrotechnical Commission.

ingress

The act of entering.

IP

Ingress Protection.

journal bearing kVA

A bearing that supports the cylindrical journal of a shaft. Kilovolt Ampere.

locked rotor current

The steady-state current that is taken from the line while the rotor is locked and the rated voltage is being applied to the motor.

natural frequency

The frequency at which an object vibrates because of its own physical characteristics when it is distorted and then released while it is restrained or supported at specified points.

NEMA

National Electrical Manufacturers Association.

overload

Operation of equipment in excess of normal, full load rating.

saybolt universal seconds (SUS)

A standard measure of a lubricant's viscosity.

second harmonic

The second component frequency of a harmonic motion that is an integral multiple or the fundamental frequency.

short circuit

An abnormal connection of relatively low impedance, that is made either accidentally or intentionally between two points of different potential that results in a high current flow.


158



sleeve bearing

A bearing with a cylindrical inner surface in which the journal of a rotor shaft rotates.

sound intensity

The density of sound power at a point away from the source. This density of sound power is expressed in watts per square meter. Sound power that is radiated by a source may be derived by integration of the intensity over an enclosed hypothetical surface of measurement.

sound level

A weighted measure of the amount of noise that is produced by a machine at a given point.

sound pressure level (SPL)

The level of pressure in the sound-conducting medium that results from the sound intensity at the concerned point.

sound power (SWL)

A property of the noise that is produced by a machine that is independent of the environmental conditions or the distance from the machine.

TEFC

Totally Enclosed Fan-Cooled.

thyristor

A bistable semiconductor device that comprises three or more junctions that can be switched from the off-state to the on-state or vice versa. Such switching occurs within at least one quadrant of the principal voltage-current characteristic.


159

Aramco Engineering - Evaluating Motor Specifications

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