Electrical motor

Engi ngine nee erin ring g Ency ncyclo clope pedia dia Saudi Sa udi A ramco DeskTop Standards

Introdu ction To Motors

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 : El Electrical File Reference: EEX20301

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

C O NT E NT S

P AG E

Basic Electric Motor Operating Principles

1

DC Motors: Types, Major Components, and Operating Principles

6

Three-Phase AC Motors: Types, Major Components, Components, and Operating Principles

22

Single-Phase AC Motors: Types, Major Components, Components, and Operating Principles

39

GLOSSARY

49

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 contained in this document which 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.



C O NT E NT S

P AG E

Basic Electric Motor Operating Principles

1

DC Motors: Types, Major Components, and Operating Principles

6

Three-Phase AC Motors: Types, Major Components, Components, and Operating Principles

22

Single-Phase AC Motors: Types, Major Components, Components, and Operating Principles

39

GLOSSARY

49

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 contained in this document which 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.



Engineering Encyclopedia

Electrical Introduction Introduction of Motors

BASIC BASIC ELECTRIC MOTOR OPERATING OPERATING PRI NCIPLES All electrical motors act on the same basic operating principles. There are some differences in exactly how AC and DC motors operate, but the principle of motor action remains the same. This section will discuss the following topics pertinent to the principles of operation of electric motors: •

Basic Motor Action

•

Basic Principle for the Operation of DC Motors

•

Basic Principle for the Operation of AC Motors

Basic Motor Action Action The operation of an electric motor is based on the following three main principles: 1. An electric current passing through a conductor produces a magnetic field around the conductor. If the wire is wound in in a coil around an iron rod, rod, the magnetic field field around the wire becomes becomes strengthened strengthened and the the rod becomes becomes magnetized. magnetized. This arrangement of a rod and wire coil forms a simple electromagnet, with its two ends serving as north north and south poles. poles. The magnetic field field becomes stronger stronger as the number of conductor turns in a coil increases. 2. The direction direction of the the current current flowing through the coil determines determines the the location location of the magnetic poles (north (north or south) on the the electromagnetic. electromagnetic. If current passes passes through the coil in one direction, one end of the electromagnet is north and the other end is south. If the current is reversed, reversed, the poles will change change positions. 3. Magnetic poles either attract or repel each other. Like poles, such as two north poles, repel repel each each other. other. Unlike poles attract attract each other. other. For example, example, if a bar magnet is suspended between the ends of a horseshoe magnet, the bar magnet will rotate until its its north pole is opposite opposite the horseshoe horseshoe magnet's south south pole. The bar magnet's south pole then is opposite the horseshoe magnet's north pole. Figure 1 shows a simple electric motor diagram. This diagram illustrates the basic operating principles of a motor. motor. The north pole of the horseshoe horseshoe magnet attracts attracts the south pole pole of the bar magnet, and the south pole of the horseshoe magnet attracts the north pole of the bar magnet (Figure 1A). 1A). When the north and south south poles of the bar magnet are directly directly across across from the respective south and north poles of the horseshoe magnet, the bar magnet stops rotating (Figure 1B).

Saudi Aramco DeskTop Standards

1



If either of the bar magnet poles or horseshoe magnet poles were instantly reversed, like poles would be directly across across from each other and a repelling repelling action would occur. (In Figure 1C, the polarity of the bar magnet is reversed). reversed). This repelling action action would cause the bar magnet to turn again. If the timing is correct (continuously reversed) when the pole locations are switched, the bar magnet will keep turning. The repelling action that results from the interaction of the magnetic fields on the bar magnet and the horseshoe magnet produces a rotational rotational force on the bar magnet. This rotational force force is called torque. All electric motors are designed and are constructed so that two magnetic fields will be produced that that can interact with each other to produce produce torque. The torque that is produced is for use in the rotation of the shaft.

Simple Electric Motor Figure 1


2



Basic Basic Pr inciple inciple for for the Op erat ion ion of DC Motor s A permanent magnet generally is not not used in a motor. Electro-magnets Electro-magnets are created created through use of coils wrapped around an iron core for both the stationary and rotating magnetic fields. In a direct current (DC) motor, the current that flows through the rotating electromagnet is reversed at the appropriate time to obtain the desired attraction and repelling action. The current reversal is mechanically done. The field winding produces a stationary magnetic field, and the armature has many loops or coils that are insulated insulated from each each other. Each end of the armature armature coil is connected connected to the segmented commutator. Figure 2 shows the operation of a simple DC motor that has two field windings (north and south), one armature coil with two poles (north and south), and a commutator. The commutator is split in half (segmented). (segmented) . Each end of the armature coil connects to the commutator's commutator' s brushes. The current flowing in the armature coil produces its own magnetic field and creates both a north and a south pole. As Figure 2A shows, the north pole of the field winding will attract the south pole of the armature coil and cause cause the armature to turn. At the instant the north north and south poles are opposite of each other (Figure 2B), the brushes switch to the other segment of the commutator, commutator , and the current is reversed in the armature coil. This switching action causes the poles of the armature coil to instantly reverse. Now that the field winding and the armature coil poles are like like poles (Figure 2C), 2C), the poles repel each each other. This repulsion causes causes the armature coil to continue continue turning. The armature current is again again reversed by the commutator commutator when the unlike poles poles approach each other. other. This reversal of of armature current current will cause the armature coil to continue continue to turn. The motor action continues continues as long as power is supplied to both the field and armature circuit. The commutator acts as a mechanical switch to reverse the direction of the armature current at the appropriate appropriate time. time. Such action action is called commutation. commutation. In an actual actual motor, motor, there is more than one coil on the armature. Because each coil on the armature armature is connected to an adjacent commutator segment, the attracting and repelling actions are more uniform and more powerful than the simple one-loop motor that is shown in Figure 2.


3


Electrical Introduction of Motors

Operation of a Simple DC Motor Figure 2


4



Basic Pr inciple for the Op erat ion of AC Motor s In an alternating current (AC) motor, the current is reversed in the stationary electromagnets. The current reversal is done automatically because an AC waveform reverses direction every one-half cycle or 120 times per second on a 60 Hz power system. Figure 3 shows the process of reversing the magnetic field in an AC motor because of the AC power reversal. Most AC motors will have at least two coils, as shown in Figure 3A. Each coil produces its own magnetic field, and each magnetic field has a north pole and a south pole. The strength of the magnetic field varies with the magnitude of the AC waveform. As the current of the waveform increases, the magnetic field becomes stronger. As the current of the waveform approaches zero, the magnetic field becomes weaker. When the current of the waveform starts on the negative half cycle, the poles on the coils reverse because the direction of the current reverses (Figure 3B): the north poles become the south poles and the south poles become the north poles. The strength of this reversed magnetic field will increase as the current increases and will decreases as the current decreases, as occurred during the first half cycle.

Reversing of the Magnetic Field Figure 3


5



DC MOT ORS: TYPES, MAJ OR COMP ONENTS, AND OPER ATING PRINCIPL ES DC motors are used where high torque and controlled speed over a wide range are required. The DC motor is a complex machine that requires a high level of maintenance and thorough knowledge of its operation. The following topics are covered in this section: •

Types of DC Motors

•

Major Components of DC Motors

•

Operating Principles of DC Motors

Types of DC Motor s Direct current (DC) motors are classified as follows, according to the type of winding that is employed: •

Series-wound

•

Shunt-wound

•

Cumulative Compound Wound Motor

•

Differential Compound Wound Motor

In the series-wound DC motor, the field coils and armature are connected in series, and the entire DC current to the motor flows through the field coils. In the shunt-wound DC motor, the field coils and the armature are connected in shunt (parallel). Because of this parallel connection, the field current is only a small portion of the total or line current. The cumulative compound wound motor incorporates both the series-wound and the shuntwound windings. In other words, the cumulative-compound wound motor has both the series and the shunt windings. Both windings are placed in the motor so that their individual fluxes will be added. The differential compound wound motor is similar to the cumulative compound motor, except that the series-wound and shunt-wound windings produce fluxes that oppose each other.


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Maj or Components of DC Motors The major components of all DC motors are the same. The difference between DC motors occurs in the way the components are electrically connected. Figure 4 shows the components of a typical DC motor, the stator and the rotor. The stator is the stationary frame assembly of the DC motor. The stator assembly is made up of the frame, interpole windings, main field windings, brush-holder and brushes, and the end bells. The rotor is the rotating portion of a DC motor. The rotor assembly is made up of the armature, commutator, and the blower. The rotor and the stator are mechanically connected through use of the front and rear bearings that allow the rotor to rotate while providing mechanical support.

Components of a Typical DC Motor Figure 4


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Stator

As discussed, the stator is the stationary portion of a DC motor. The stator consists of the following: • • • •

Frame Field Winding Brush Rigging and Brushes End Bells

Frame - The frame of a DC motor provides the mechanical support for the stator

components. The frame also provides for a method of mounting and moving the DC motor. Field Winding - The field winding is wound around a field pole that usually is made

from laminated steel. The use of laminated steel reduces eddy current and hysteresis losses. Figure 5 shows a typical laminated field pole piece that would be bolted to the motor frame. The laminated field pole then is wrapped with coil wire to complete the main field winding assembly. When DC current is passed through these main field coils, a stationary magnetic field is produced. Several types of DC motors have an auxiliary winding that is mounted on the motor frame between the main field poles. This winding is called an interpole. The purpose of an interpole is to assist commutation and prevent sparking at the brushes.


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Laminated Field Pole Piece Figure 5


9



Brush Rigging and Brushes - Current flows into the armature winding through contacts

called brushes. These brushes ride on the commutator bars. The brushes are made from a carbon compound and are mounted in a brush holder, as shown in Figure 6. The entire assembly is referred to as a brush rigging. The brush holder keeps the brush properly aligned with the commutator and maintains a constant pressure on the brush through use of a spring. The armature circuit consists of a current path from the power supply through the brushes, through a commutator bar, through a set of coils, through another commutator bar, through a second set of brushes, and back out to the power supply. End Bells - The end bells are on either end of the motor. The end bells complete the

frame of the motor and also house the bearing support for the motor.

Brush and Holder Figure 6


10



Rotor

The rotor is the rotating portion of a DC motor. components: •

Armature

•

Commutator

•

Blower

The rotor consists of the following

Armature - The armature consists of a group of coils that are imbedded in a laminated

iron core. When a DC current is applied to the armature, a magnetic field will be produced. Commutator - The commutator, shown in Figure 7, is the mechanical means by which

the direction of current is switched to the armature coils. The armature coils are connected to a commutator by copper bars that are called risers. A commutator is a copper cylinder that is divided into many sections or segments that are called bars. The segments are insulated from each other with mica. Blower - The blower is a fan that is mounted on the rotor shaft. The blower rotates with

the rotor and forces air to pass through the DC motor. The blower is used to cool a DC motor.

Cutaway View of a Commutator Figure 7


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Oper ating Pr inciples of DC M otor s The major components of all DC motors are the same, as discussed in the previous section. Particular types of DC motors differ in the way in which the fields are connected and in operating principles. The following types of DC motors will be discussed: •

Series Motors

•

Shunt Motors

•

Cumulative Compound Motor

•

Differential Compound Motor

Series Motor s

The series motor is the simplest of the DC motors. The series motor is internally connected so that the series field and the armature are in series. When these components are in series, all of the armature current flows through the series field windings, and that all of the flux in the motor is proportional to the armature current. Figure 8 shows a series DC motor and the connections that are between the armature, the field, and the power supply. Note that the armature is identified as A1 and A2, that the field is identified as S1 and S2, and that the connections to the line (power supply) terminals are identified as L1 and L2.

Series DC Motor Figure 8


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When voltage is applied to the motor, current begins to flow from the negative power supply terminal, through the series winding and armature winding, and back to the positive power supply terminal. Because the armature is not rotating when voltage is first applied, the only resistance in this circuit is provided by the large conductors that are used in the armature and field windings. The conductors have a very small amount of resistance that causes the motor to draw a very large amount of current for the power supply. When the very large amount of current begins to flow through the field and armature windings, a very strong magnetic field starts to develop. The current is so large that the armature and series coils will reach saturation, which will produce the strongest magnetic field possible. The strength of the series magnetic field and the armature magnetic field provides the armature shaft with a great amount of torque. The large amount of torque causes the armature to begin to spin, which causes the armature to begin to produce a voltage. In accord with the basic theories of magnetism, any time a coil of wire passes through a magnetic field, a current will be produced. More current will be generated when the magnetic field increases or when the speed with which the coil passes through the flux increases. When the armature begins to rotate, it will produce a voltage that is of opposite polarity to that of the power supply. The voltage that is produced is called counter voltage or counterelectromotive force (CEMF). The overall effect of the CEMF is that it will be subtracted from the supply voltage so that the motor windings will see a smaller voltage potential. When Ohm's Law is applied to the motor circuit, it is easy to see that when the voltage is reduced, the current also will be reduced. Such a reduction in voltage means that the series motor will see less current as the motor's speed increased. The reduced current will cause the motor to lose torque as the motor's speed increases. Once the load is moving, less torque is required to keep the load moving. The lower torque works to the motor's advantage by automatically reducing the motor current as soon as the load begins to move. The lower circuit also allows the motor to operate with less heat buildup. The direction of rotation of a series motor can be changed by change to the polarity of either the armature winding or series field winding. Change to polarity of the applied voltage will change the polarity of both the series field winding and the armature winding, and the motor's rotation will remain the same. Because only one set of windings needs to be reversed to change direction, the armature winding typically is changed because the armature winding terminals are readily accessible at the brush rigging. Because the armature winding receives its current through the brushes, change to the polarity at the brushes will cause the armature winding's polarity to also change.


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Shunt Motor

The shunt wound motor is the most common type of DC motor. The shunt motor is internally connected so that the shunt field and the armature are in parallel (shunt), as shown in Figure 9. The applied voltage forces two parallel currents to flow in the motor circuit: an armature current (I a) and a field current (I f ). Because the armature circuit has a very low impedance and the field circuit has a high impedance, armature current is high and the field current is low. The magnetic field that is produced by the shunt field is proportional to the field current. Because the shunt field is directly connected across the power supply, the field current has a constant value regardless of the armature current; therefore, the magnetic field that is produced by the shunt field also is constant. The magnetic field that is produced by the shunt motor armature is proportional to the armature current. The amount of armature current that flows through a shunt motor armature varies with the amount of CEMF that is produced and with the size of the load; therefore, the magnetic field that is produced by the shunt motor armature also varies. The interaction between the magnetic fields that are produced by the shunt field and by the armature produce torque that causes the shaft of the shunt motor to rotate. The amount of torque that is produced by the shunt motor and therefore the speed of rotation of the shaft depends on the strength of the produced magnetic fields. As an example, an increase in the strength of one or of both magnetic fields causes more torque to be produced and causes the speed of rotation of the shaft to increase for a constant load. In Figure 9, the armature circuits (A1 and A2) and the shunt field (F1 and F2) circuits are directly connected across the power supply bus (L1 and L2). When voltage is applied to the motor, the high resistance of the shunt field keeps the overall current flow low. The armature and the shunt field will draw enough current to produce magnetic fields. The strengths of these magnetic fields will be sufficient to interact and produce torque. The amount of torque that is produced will cause rotation of the shaft and the connected load. The shunt motor is similar to the series motor in that CEMF is produced as the armature begins to turn. The CEMF causes the current in the armature to drop as the motor accelerates to full speed. This drop in current causes a resultant drop in torque. The drop in torque as the motor accelerates does not cause a drop in speed because less torque is required to maintain a load in motion than is required to start a load in motion. When the shunt motor reaches full speed, the amount of torque that is produced by the interaction of the two magnetic fields will remain fairly constant and therefore the motor's speed will remain fairly constant.


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Shunt DC Motor Figure 9


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The shunt motor's speed can be controlled. The ability of the motor to maintain a constant speed when the load changes is due to the characteristic of the shunt field and armature. Because the shunt field is connected in parallel to the armature, the shunt field's strength is constant and does not change with load. The only things that change with load are CEMF and armature current. If the load slightly increases and causes the armature shaft to slow down, less CEMF will be produced. Slowing the shaft will allow the difference between the CEMF and the applied voltage to become larger, which will cause more current to flow. The extra current provides the motor with the extra torque that is required to regain its speed when this load is slightly increased. The direction of rotation of a DC shunt motor can be reversed by changing the polarity of either the armature field or the shunt field. In this application, the armature usually is changed, as was the case with the series motor. Figure 10 shows an electrical ladder diagram for a shunt motor that depicts how the reversing of the rotation if a shunt motor is accomplished. Notice that the F1 and F2 terminals of the shunt field are directly connected to the power supply, and the A1 and A2 terminals of the armature are connected to the reversing starter. When the forward pushbutton is pushed while the stop pushbutton is closed and the reversing starter contact is closed, the forward starter will energize. When the forward starter is energized, the F contacts in the armature circuit will connect the A1 lead to the negative power supply terminal and the A2 lead to the positive power supply terminal. The F1 motor lead for the shunt field is directly connected to the positive terminal of the power supply and the F2 lead of the shunt field is connected to the negative terminal. Since connections will cause the motor to run in the forward direction. Notice that the F contact in the circuit to the reverse pushbutton opens to lock out the reverse until the forward starter is turned off. Also, the F contact that is under the forward pushbutton closes to lock in the forward starter after the forward pushbutton is released. When the reversing starter is energized, the contacts reverse the armature so that the A1 lead is connected to the positive power supply terminal and the A2 lead is connected to the negative power supply terminal which reverses the polarity of the armature. The shunt field remains connected directly to the power supply so the polarity of the shunt field is not changed. Because the armature polarity has reversed, the motor will begin to rotate in the reverse direction.


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Reversing of a Shunt Motor Figure 10


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Cumulative Compound Motor

The cumulative compound DC motor is a combination of the series motor and the shunt motor. The cumulative compound DC motor has a series field winding that is connected in series with the armature and a shunt field that is in parallel with the armature. The combination of series and shunt windings allows the motor to have the high torque characteristics of the series motor and the regulated speed characteristics of the shunt motor. The cumulative compound motor is one of the most common DC motors because it provides high starting torque and good speed regulation at high speeds. Since the series field is wired with the same polarity as the shunt field, the motor is called cumulative. Figure 11 shows a cumulative compound motor and the connections between the armature, shunt field, and series field. When the motor is connected in this way, the motor can start, even with a large load, and then operate smoothly when the load slightly varies at full rpm. Cumulative compound motors use the interaction of the following magnetic fields to produce the necessary torque to operate the motor: •

Series field

•

Shunt field

•

Armature field

The strength of the magnetic fields that are produced by the series field, the shunt field, and the armature are proportional to the current that flows through the circuit. Figure 11 shows that the series field (S1 and S2) is connected in series with the armature (A1 and A2); therefore, all of the current that flows through the armature also flows through the series field. This current flow causes a strong magnetic field to be developed by the series field. Figure 11 also shows that the shunt field (F1 and F2) is connected in parallel with the armature; therefore, a smaller portion of the total current flow through the circuit will flow through the shunt field. This smaller current flow causes a proportionally weaker shunt field strength. The interaction of the strong series field, the weak shunt field, and the armature field produces a large torque to cause rotation of the shaft and the connected load. The interaction of the magnetic fields in a cumulative compound motor produces more torque than either the shunt motor or the series motor. The increase in the amount of torque that is produced is a result of connection of the series winding so that its magnetic field aids the strength of the magnetic field that is produced by the shunt winding.


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Cumulative Compound Figure 11


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Differential Compound Motor

Differential compound motors use the same motor and windings as cumulative compound motors, but differential compound motors are connected to provide slightly different operating speed and torque characteristics. Figure 12 shows the diagram for a differential compound motor with the series field that is connected in reverse polarity to the armature and shunt field. Notice that S1 and S2 leads are reversed from those of the cumulative compound motor. This reversal indicates that the direction of the current flow through the series field is reversed. In this diagram, the series field is connected so that its magnetic field opposes the magnetic fields in the armature and the shunt field. When the series field is connected as shown in the diagram, less torque is produced by this motor than is produced by the cumulative compound motor. Differential compound motors produce torque through use of the same means as the cumulative compound motor. The only difference is in the relative strength of the combined series and shunt magnetic fields. In the differential compound motors, the series field opposes the shunt field; therefore, the strength of the resultant combined field is lower. This weaker combined field cannot produce the same amount of torque through interaction with the armature field as is produced in the cumulative compound motor. This drop in produced torque makes the differential compound motor less desirable than the cumulative compound motor for most applications.


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Fans Stator End Bracket

Rotor

Armature Windings

Differential Compound Figure 12


21



All compound motors can be reversed simply by a change in the polarity of the armature winding. The speed of a compound motor can be changed very easily through adjustment to the amount of voltage applied to the compound motor. When the voltage to the compound motor is decreased, the decreased voltage will cause the armature current to decrease, which causes the armature rotation to slow down. Because the motor has a series winding, the motor will be able to carry the load on the armature shaft even though the speed has been reduced. When the voltage is increased again, the current in the armature's coil will cause the armature's rotation to increase.


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THRE E-PHASE AC MOT ORS: OPERATING PRINCIPLES

TYPES,

MAJO R

COM PONENTS,

AND

Three-phase AC motors are frequently used in industrial applications. The following three types of three-phase AC motors will be covered in this section: •

Three-Phase Squirrel-Cage Induction Motors

•

Three-Phase Wound Rotor Inductor Motor

•

Three-Phase Synchronous Motors

The three-phase squirrel-cage induction motor is the most common and simplest construction. There are no electrical connections to the rotor in the three-phase squirrel-cage induction motor. The three-phase wound rotor induction motor is very similar to the three-phase squirrel-cage induction motor. The outstanding difference between these two motors is in the rotor winding. The three-phase synchronous motor stator is very similar to the other three-phase motors. The major difference between the three-phase synchronous motor and the other two three-phase motor types is that an external power source is applied to the rotor to create the rotor field. Thr ee-Phase Squir r el-Cage Indu ction Motors The three-phase squirrel-cage induction motor is the most common of all three-phase motors. The simple, yet rugged construction of the three-phase squirrel-cage induction motor makes this motor so popular. The following topics pertinent to the three-phase squirrel-cage induction motor will be discussed in this section: • •

Major Components Operating Principles

Major Components

Figure 13 shows the major components of a three-phase squirrel-cage induction motor: the stator, field coils, rotor, and fan. Each of the major components will be discussed in further detail.


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Major Components of an AC Squirrel-Cage Motor Figure 13


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In an actual motor, the stator core is made of stacked iron laminations held together by rivets. Each lamination is punched from silicon alloy steel sheets that are protected with an insulation coating. This insulation coating lowers the eddy current and hysteresis losses of the motor. Figure 14 shows the coil polar region for an AC motor. The stacked lamination iron core has slots cut into the iron core. The field coils lay in these slots and are uniformly distributed around the periphery of the core for each phase of the three-phase winding. These field coils are connected and grouped to form polar areas. Because the span of each coil is smaller than the full span of the magnetic pole, a group of several coils in adjacent slots partially overlap and produce one pole.

Coil Polar Region for an AC Motor Figure 14


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Figure 15 shows a squirrel-cage rotor. A squirrel-cage rotor is constructed of rotor bars that are made of copper, aluminum, or a suitable alloy and that are placed in slots on the rotor iron core. The rotor bars are connected together at the ends by end rings that are made of similar material. The conductor bars carry large currents at low voltages. It is not necessary to insulate the bars from the core because the current will follow the path of least resistance and is confined to the cage windings. Most rotors also will contain a fan to circulate air around the motor for cooling purposes.

Squirrel-Cage Rotor Figure 15


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Operating Principles

The basic operation of a three-phase, AC, squirrel-cage induction motor is similar to the simple motor that was described earlier. The supply of three-phase power to the stator windings (field coils) produce the rotating magnetic field. Figure 16 shows the three-phase motor stator windings. The individual windings for each phase (phase 1, phase 2, and phase 3) are shown alone, and all three phases are shown tied together in a Y-connected stator. The dot in each diagram indicates the common point of the Y-connection. As shown in Figure 16, the individual phase windings are spaced 120 o apart around the stator.


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Three-Phase Motor Windings Figure 16


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The three-phase rotating field that is produced by the three-phase stator that was shown in Figure 16 is shown in Figure 17. Phase 1 is connected to field coils 1 and 1A; phase 2 is connected to field coils 2 and 2A; and phase 3 is connected to field coils 3 and 3A. At point 1, the magnetic field in coils 1 and 1A is maximum, with polarities as shown. Note that on the pictorial representation of the three-phase power, phase 1 is at its maximum positive value at point 1. At the same time, negative voltages are being felt in the 2 and 2A and 3 and 3A windings, but these voltages are not yet at their maximum value. These negative values create weaker magnetic fields, which tend to aid the 1 and 1A field. At point 2, the maximum negative voltage is being felt in the coils 3 and 3A. This maximum negative voltage creates a strong magnetic field that is aided by the weaker fields in 1 and 1A and 2 and 2A. As each point on the voltage graph is analyzed, the resultant magnetic field is rotating in a clockwise direction. When the three-phase voltage completes one full cycle (point 7), the magnetic field has rotated through 360 o. This pattern will continue as long as power is supplied to the stator. Both the sine wave and the field have rotated 360 degrees in a locked manner. This manner is called the synchronous speed, because the sine wave and the field are synchronized to the power supply frequency.

Three-Phase Rotating Field Figure 17


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The formula for determining the synchronous speed of a motor is: where:

N f P

= = =

Speed of the field Frequency of the applied voltage Number of poles in a motor

If the power supply frequency is constant, the speed of the rotating field always will be independent of load changes on the motor. As an example, calculate the field speed of a two-pole motor that is connected to a 60 Hz power supply. Use the following formula: The rotor is not electrically connected to the power supply. The induction motor derives its name from the mutual inductance that takes place between the stator and the rotor during operation. As this stator field involves, it cuts the squirrel-cage conductors, and voltages are set up in the squirrel-cage conductors. These voltages in the squirrel-cage conductors cause current to flow in the squirrel-cage circuit through the bars that are under the north poles, into the ring, back to the bars that are under the adjacent south poles, into the other ring, and back to the original bars under the north pole. The current that is flowing in the squirrel-cage establishes magnetic fields with north and south poles in the rotor core. There are several conductors in parallel, and the currents may be large. These poles in the rotor are attracted by the poles of the revolving field and follow the revolving field around. The interaction of the rotor field and the stator field produces torque on the rotor and causes the rotor to rotate in the direction of the stator field rotation. Figure 18 shows the rotor field and stator field interactions. The torque on the rotor of a squirrel-cage induction motor tends to turn the rotor in the same direction as the rotating field. If the motor is not connected to a load, the motor speed will accelerate to nearly the same speed as the rotating field. As the rotor accelerates, the magnitude of the induced voltage in the rotor decreases because the relative motion between the rotating field and the rotor conductors is reduced. An induction motor cannot operate at synchronous speed (exactly equal to the power supply frequency) because there would be no relative motion between the rotating field and the rotor. There would be no induced voltage, no rotor current, no rotor magnetic field, and no torque without relative motion.


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Rotor Field and Stator Field Interaction Figure 18


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The rotor always rotates at a speed less than synchronous speed in a squirrel-cage induction motor. The rotor speed is such that sufficient torque is produced to balance the restraining torque that is caused by motor friction and mechanical load. The difference between synchronous speed and rotor speed is called slip. Slip is mathematically expressed as the following formula: where:

S N NR

= = =

Slip Synchronous speed Rotor speed

To express slip as the quantity of a percent, multiply the resultant slip by 100. As an example, calculate the slip of a six-pole, 60 hertz motor with a synchronous speed of 1200 rpm. The motor's rotor speed is 1164 rpm. The slip is calculated as the following formula: Three-Phase Wound Rotor Induction Motors The three-phase wound rotor induction motor is not as popular as the three-phase squirrelcage induction motor because the more complex design of the three-phase wound rotor induction motor causes the motor to become undesirable in many applications. This section will cover the following topics pertinent to three-phase wound rotor induction motors: •

Major Components

•


Major Components

The stator of a three-phase wound rotor induction motor is the same as the stator of a threephase squirrel-cage induction motor, but the rotor is different. Figure 19 shows a three-phase wound rotor induction motor. Figure 19(A) shows the physical construction of the wound rotor, and Figure 19(B) shows the electrical circuit of the motor. Instead of rotor bars the wound rotor has a three-phase winding that is embedded in a laminated iron core. The windings on the rotor are brought out to a set of three slip rings (one per phase) so the windings are connected to an external variable resistor. Brushes ride on the slip rings, similar to the arrangement of the brushes that ride on the commutator bars of the DC motor. Operating Principles

The stator rotating field of a three-phase wound rotor induction motor is developed and applied in the same way that the stator rotating field is developed in a three-phase squirrelcage induction motor. The rotating fields of the three-phase wound rotor and the squirrelcage motors are identical.


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In the three-phase wound rotor induction motor the rotor field is produced in the same way that this field is produced in the squirrel-cage induction motor. However, in the wound rotor motor, the current that is produced in the rotor flows through the slip rings and brushes to the external resistors, as shown in Figure 19(B). These external resistors are used for motor starting and for speed control. When a three-phase wound rotor induction motor is started, insertion of external resistance in the rotor circuit results in the development of a high torque with a comparatively low starting current. As the motor accelerates up to speed, the resistance is gradually reduced until, at full speed, the external resistance is reduced to 0_ and the rotor is short-circuited. Through variance of the resistance in the rotor circuit, the motor speed can be regulated within practical limits.

Three-Phase Wound Rotor Induction Motor Figure 19


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Thr ee-Phase Synchronous Motors A three-phase synchronous motor, as the name implies, operates at synchronous speed. The rotor is constructed so that definite north and south poles are developed. These magnetic poles "lock-in" with the opposite poles that are rotating around the stator field. The rotor thus rotates at synchronous speed. The three-phase synchronous motor differs from the two types of induction motors in two main ways. The synchronous motor requires a separate source of DC for the rotor field and special starting methods or components. This section will cover the following topics pertinent to three-phase synchronous motor's: •

Major Components

•

Operation Principle

Major Components

In a synchronous motor, the stator winding is similar to the stator winding of a squirrel-cage induction motor. The difference between a squirrel-cage and synchronous motor is in the construction and operation of the rotor. The rotor of a three-phase synchronous motor has a winding that is made from copper wire that is constructed so that alternate north and south poles exist when DC power is applied to the winding. The windings on the rotor of a threephase synchronous motor are called field windings. The following basic types of rotor construction will be discussed: •

Laminated Salient Pole

•

Solid Salient Pole

•

Cylindrical Rotor - Turbo Type

Laminated Salient Pole - The most common type of rotor design that is used on a

synchronous motor is the salient pole type, as shown in Figure 20. All salient pole rotors have three main components: the shaft, the slip rings, and the windings. The salient pole type of rotor is constructed in two forms. The laminated salient pole rotor has a cage damper winding in each pole face for starting. This starting winding is known as an Amortisseur or "damper" winding. The starting winding behaves exactly like a squirrel-cage winding when the motor is started. However, the damper winding will not support the machine load at synchronous speed and will overheat if the DC field is lost or removed. Also, because the stator winding gets very hot on normal startup, the number of starts per unit of time are limited to the manufacture's specifications. The thermal rise in the winding depends on the "run-up" time or acceleration period of the motor. Saudi Aramco DeskTop Standards

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Typical Laminated Salient Pole Rotor Figure 20


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Figure 21 shows the actual pole assembly of the laminated salient pole type rotor. The poles consist of iron core pieces that are separated by laminations, as seen in Figure 21A. As shown in Figure 21B, the poles then are wrapped with the coil winding to create a pole assembly. Then, each of the motor's pole assemblies are placed on the squirrel-cage bar assembly.

Laminated Salient Pole Assembly Figure 21


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Solid Salient Pole Type - Most Saudi Aramco synchronous motor applications for pumps

and turbo compressors can be accomplished by the solid salient pole design of the rotor. In this design the pole pieces are a solid piece of iron and not laminated, which results in a very simple, heavy-duty construction. The solid pole design depends on eddy currents setup in the solid pole faces by the rotating stator AC field for starting torque. Cylindrical Rotor - Turbo Type - Figure 22 shows a typical cylindrical rotor-turbo rotor.

This type of rotor should be used on motors that are running at 3600 rpm. The cylindrical rotor-turbo rotor windings are placed in slots that are machined into a cylindrical iron core. Because the windings are embedded into the core, centrifugal force and other stresses are minimized at high speed.

Typical Cylindrical Rotor - Turbo Rotor Figure 22


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A three-phase current is supplied to the stator winding of a synchronous motor to produce a rotating magnetic field, as previously discussed. A direct current is supplied to the rotor winding to produce a magnetic field with fixed polarities at each end. If the rotor of a synchronous motor has no inertia and no load, the rotor would rotate in step with the rotating field, as soon as power was applied. However, in actual applications, the rotor does have inertia, and a load is connected to the motor. As explained previously that one of the differences between an induction motor and a synchronous motor is that the synchronous motor requires a special starting method. The reason for the special starting method is illustrated in Figure 23. When the stator and rotor fields in a synchronous motor are first energized,north and south poles are established in the stator and rotor fields. The poles of the rotating field approach the rotor poles of opposite polarity as shown in Figure 23(A). The attracting force of the stator and rotor fields tends to turn the rotor in a direction that is opposite to the direction of the rotating field (counterclockwise). As the rotor starts to move counterclockwise, the rotating field poles move past the rotor poles. Such movement will pull the rotor in the same direction as the direction of the rotating field (clockwise), as shown in Figure 23(B). The result of this movement is that a synchronous motor does not develop starting torque. In order to start a synchronous motor, an auxiliary method must be used. Usually, a synchronous motor is started as a squirrel-cage motor. Once the rotor is brought up to a high speed (close to synchronous speed) by the auxiliary starting method, the rotor will "lock" with the rotating stator field. A running torque is developed under these conditions. The rotor will rotate at synchronous speed in a direction and at a speed that is determined by the stator field.


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Synchronous Motor Operation at Start Figure 23


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The two rotating fields will not line up perfectly when the motor is running. The rotor pole always will lag behind the stator pole, which is referred to as an angle. This angle shown in Figure 24 is called the synchronous motor's torque angle. The synchronous motor's torque angle between the rotating stator field in the stator windings and the rotor depends on the motor load. As the load on the motor's shaft increases, the torque angle increases, even though the rotor continues to turn at synchronous speed. This increasing torque angle continues until the torque angle is approximately 90 o. At 90o, the motor is developing a maximum torque. Any further increase in load will cause one of the following to occur: •

If the increase in load is momentary or very small, the rotor will slip a pole. The rotor will electrically "stand-still" until the rotation field of the stator can regain control. A noticeable straining sound can be heard.

•

If the increase in load is large enough and not momentary, the motor will lose synchronism and stall. A noticeable straining sound can be heard.

Synchronous Motor Torque Angle Figure 24


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SINGLE-PHASE AC MOT ORS: OPERATING PRINCIPLES

TYPES,

MAJO R

COM PONENTS,

AND

This section will discuss the following four types of single phase AC motors: •

Split-Phase Induction Motors

•

Repulsion Induction Motors

•

Capacitor Start Motors

•

Universal Motors

Although Single-phase AC motors are used in a variety of applications, these motors are nearly always used in low voltage and low horsepower capacities. Each type of single-phase AC motor has its unique way of starting. The single-phase AC motors require a special means to create a rotating magnetic field to produce a rotational force on their rotors. Each type of single phase AC motor uses a different method of starting. The following section will present the major components and operating principles of each type of single-phase AC motor. Split-Phase Indu ction Motors Because of its simple construction and wide variety of sizes, the split-phase induction motor is the most popular of all single-phase AC motors. The following topics will be discussed pertinent to the split-phase induction motor: •

Major Components

•


Major Components

Figure 25 shows a split-phase induction motor with the rotor, starting windings, run windings, and centrifugal switch that is connected across a single-phase power source. The rotor (a squirrel-cage rotor) is a cast cylinder on a machined shaft. The cylinder has cast-in bars that are welded to a peripheral ring on each end so that the rotor resembles a cage. The cast-in bar material determines the rotor resistance and the speed and torque of the rotor.


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Split-Phase Motor Figure 25


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When the split-phase induction motor is energized with single-phase AC, the two windings (run and start) are physically different enough in position and construction to produce respective magnetic fields that are of different strengths and that are not in phase. The variations in the magnetic fields give the illusion of a revolving field that rotates around the stator at a synchronous speed. As the rotating field rotates around the stator, the field cuts across the rotor conductors and induces a voltage in them. The interaction of the rotor currents and the stator field causes the rotor to accelerate in the direction of the rotating field. When the rotor has come up to about 75% of synchronous speed, the centrifugal switch disconnects the starting winding from the single-phase power supply, and the motor continues to run on the run winding. Repulsion Ind uction M otors The repulsion induction motor is a combination of an induction motor and a repulsion motor. The combination of two types of motors allows for better operating conditions. The following topics will be covered pertinent to repulsion induction motors: •

Major Components

•


Major Components

Figure 26 shows a diagram of a repulsion induction motor. The field windings make up the stator, which is very similar to the stator of other induction motors. The rotor has the following two parts combined on the same shaft: •

Wound rotor with brushes

•

Squirrel-cage

The brushes are shorted together so that the wound rotor becomes a physically and electrically shorted circuit on the rotor.


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Diagram of a Repulsion-Induction Motor Figure 26


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Figure 27 shows a repulsion induction motor without rotation. When the brush axis of the repulsion induction motor is aligned with the poles, the stator induces equal and opposite currents in the two halves of the rotor windings. Because the induced current establishes a north pole on the rotor directly under the stator north pole and a rotor south pole directly under the stator south pole, no torque is produced and no rotation results.

Repulsion Motor Without Rotation Figure 27


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If the brush axis is moved perpendicular to the poles, the voltages that are induced in the rotor neutralize each other and produce no voltage at the brushes. No armature current flows and no torque is produced. However, when the brushes are anywhere between the horizontal and vertical positions, there is a resultant voltage and current flow in the armature. This current flow in the armature creates a field that produces north and south poles on the rotor. These poles are displaced by the same angle as the brush location. Interaction of the fields creates a magnetic repulsion force (torque) that turns the rotor in the direction of the brushshift, as shown in Figure 28.

Repulsion Motor with Rotation Figure 28


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Capacitor Star t Motors Capacitor start motors are very similar to split-phase motors. In the capacitor start motor, the addition of the capacitor adds to the motor ability to simulate a rotating magnetic field. The following topics will be covered pertinent to capacitor start motors: •

Major Components

•


Major Components

The capacitor-type motor is a modified form of split-phase motor. Figure 29 is a diagram of a capacitor-start motor. In the capacitor start motor, the following components are the same as in a split-phase motor: the main winding, the starting winding, the rotor, and the centrifugal switch. The additional major components of a capacitor start motor are the starting capacitor in series with the starting winding and the centrifugal switch.


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Capacitor-Start Motor Figure 29


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The operating principles of a capacitor start motor also are very similar to the split-phase motor. The capacitor will create a larger electrical phase separation between the starting winding and main winding. The addition of the capacitor also will lower the overall impedance of the circuit, thereby allowing a larger current to flow, which produces a greater magnetic field. The interactions of the fields and torque production are the same as previously discussed. Universal M otors The universal motor is very unique in that this motor can be operated on either AC or DC input. The ability to use either an AC or DC input allows a great amount of flexibility in the use of the universal motor. The following topics will be covered pertinent to the universal motor: •

Major Components

•


Major Components

A universal motor consists of a series DC motor that on either DC or single-phase AC, is designed to operate at approximately the same speed and output, within a specified frequency range, and at the same root-mean-square voltage. The major components of the universal motor will be the same as the major components of the series DC motor. Figure 30 shows the universal motor's physical and electrical arrangement. In the universal motor, the series field and the armature are connected in series. The main difference in the construction between the universal motor and the series DC motors is the insulation of the motor. The universal motor is designed for the same root-mean square voltage in AC and DC operation, but the insulation must be able to withstand peak voltage of the sine wave in AC operation.


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Universal Motor Diagram Figure 30 Operating Principles

The operating principles of the universal motor are the same as the operating principles of a series DC motor. Because it is difficult to obtain similar performance on AC and DC at low speeds, most universal motors are designed for high speed operation.


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GLOSSARY alternating curr ent (AC)

A periodic current, the average value of which is zero over a period of time.

ar matur e coil

A unit of the armature winding that is composed of one or more insulated conductors.

brush

A conductor, usually composed in part of some form of the element carbon, that maintains an electric connection between stationary and moving parts of a machine or apparatus.

Counterelectromotive for ce (CEM F)

Within a system, the effective electromotive force that opposes the passage of current in a specific direction.

centr ifugal switch

A centrifugally-operated automatic mechanism that is used to perform a circuit changing function in the primary winding of a single-phase induction motor after the rotor has attained a pre-determined speed. Also used to perform the reverse circuit changing operation prior to the time the rotor comes to rest.

commutator

An assembly of conducting members that are insulated from one another in the radial-axial plane and against which brushes bear, this assembly is used to enable current to flow from one part of a circuit to another through a sliding contact.

direct curr ent (DC)

A unidirectional current in which the changes in value are either zero or so small that they may be neglected.

electromagnet

A device, consisting of a ferromagnetic core and a coil, that produces appreciable magnetic effects only when an electric current exists in the coil.

field p ole

A structure of magnetic material on which a field coil may be mounted.

hystersis

The loss of energy that occurs in a material that is magnetized from an alternating current when the elementary magnets that are within the magnetic field seek to align themselves when the magnetic field reverses.

magnetic field

A region in which a moving charged body is subject to a force in proportion to its charge and to its velocity.


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Electrical motor

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