A
Report on
Under the Guidance of
Mr. Nirbhay Kumar Gupta (Maintenance Expert Group)
Submitted by ABHISHEK DAS AKSHAY NEHA SAMEEN th
B.Tech 4 year Electronics and Telecommunication Engineering
ORISSA ENGINEERING COLLEGE,BHUBANESHWAR COLLEGE,BHUBANESHWAR
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CERTIFICATE This is to certify that the project report entitled “ STUDY OF NEW
INVERTER DRIVES-PLTCM”,
being submitted by NehaSameen,
Akshay and Abhishek Das of Orissa Engineering College, Bhubaneswar to TATA STEEL, as a part of summer training course of B.Tech curriculum is a bonafide record of work carried out by them under my supervision and guidance. The sincerity and sense of dedication shown by them during the project is commendable.
Mr. Nirbhay Kumar Gupta Sr. Technologist, Technologist, Jamshedpur Maintenance Expert Group TATA STEEL, limited
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CERTIFICATE This is to certify that the project report entitled “ STUDY OF NEW
INVERTER DRIVES-PLTCM”,
being submitted by NehaSameen,
Akshay and Abhishek Das of Orissa Engineering College, Bhubaneswar to TATA STEEL, as a part of summer training course of B.Tech curriculum is a bonafide record of work carried out by them under my supervision and guidance. The sincerity and sense of dedication shown by them during the project is commendable.
Mr. Nirbhay Kumar Gupta Sr. Technologist, Technologist, Jamshedpur Maintenance Expert Group TATA STEEL, limited
2
Acknowledgment Action plan Introduction to TATA STEEL 5.
Steel making process Overview of PLTCM 5.1 Entry section 5.2 Pickle section 5.3 Tandem cold reduction section
6.
5.4 Exit inspection section Tandem cold mills
7.
Drives 7.1 Dc drives 7.2 Ac 7.2 Ac drives 7.3 Comparison
between ac and dc drives
8 . Speed and frequency control for drives 9 . Drives used in PLTCM 9.1 Cyclo converter 9.2 Voltage 9.2 Voltage source inverter inverter 9.2.1. Two level inverter 9.2.2. Three level inverter 9.3 . Difference between Cyclo converter and VSI 10 . CRM PLTCM mill stand DRIVE database 11 .Conclusion
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1.ACKNOWLEDGEMENT With a great pleasure we would like to express our deep sense of gratitude to Mr. Ajit Kar, Chief, MEG; Mr. Arghay Deb, Head, MEG and to our guide and training co-ordinator Mr. Nirbhay Kumar Gupta, Sr.Technologist, MEG, TATA STEEL Limited, for their valuable instructions, guidance and illuminating criticism throughout our project. Without their involvement and supervision we could not have been able to complete this project. We would like to express our sincere thanks to Mr. GAURAV, Sr. Engineer, in PLTCM; Mr. V.G. Rao, Consultant, MEG; Mr. O.P. Gupta, MEG and so many other countless people of TATA STEEL Limited, Jamshedpur for helping us in our project during our entire internship. We would also like to thanks Mrs. MitrabindaNayak, head of Training and Placement Department, OEC, Bhubaneswar for providing us with the opportunity of undertaking our training at TATA STEEL Limited, Jamshedpur. Last but not the least we would like to thank all of our friends and the employees of Maintenance Expert Group for their sincere co-operation and help throughout our training. Thanks to everybody and to almighty for giving us this opportunity in our lifetime.
ABHISHEK DAS (VT201301174) AKSHAY (VT201301661) NEHA SAMEEN (VT20130210) B.Tech 4th year
ORISSA ENGINEERING COLLEGE
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2.ACTION PLAN
WEEKS
WORK PLAN
WEEK 1
Study of documents on TATA STEEL and introduction of safety.
WEEK 2
Study of motor and visit of Electrical Repair shop.
WEEK 3
Study of document on drive. Visit to CRM-PLTCM.
WEEK 4
PROJECT WORK
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3.INTRODUCTION TO TATA STEEL
Type
Public
Traded as
NSE: TATA STEEL, BSE: 500470 (BSE SENSEX Constituent)
Industry
Steel
Founded
1907
Founder(s)
Dorabji Tata
Headquarters
Mumbai, Maharashtra, India
Area served
Worldwide
[1]
Cyrus Pallonji Mistry
Key people
(Chairman) Hemant M. Nerurkar (Managing Director)
Products
Steel, flat steel products, long steel products, wire products, plates
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Tata Iron and Steel Company was established by Dorabji Tata on August 26, 1907, as part of his father Jamsetji'sTata Group. By 1939 it operated the largest steel plant in the British Empire. The company launched a major modernization and expansion program in 1951. Later, the program was upgraded to 2 MTPA project. In 1990, it started expansion plan and established its subsidiary Tata Inc. in New York. The company changed its name from TISCO to TATA STEEL in 2005.
In August 2004, TATA STEEL agreed to acquire the steelmaking operations of the Singapore-based NatSteel for S$486.4 million in cash. The acquisition was completed in February 2005. In 2005, TATA STEEL acquired a 40% stake in the Thailand-based steelmaker Millennium Steel for $130 million from Siam Cement. On 20 October 2006, TATA STEEL signed a deal with Anglo-Dutch company, Corus. On 19 November 2006, the Brazilian steel company CompanhiaSiderúrgicaNacional (CSN) launched a counter offer for Corus at 475 pence per share, valuing it at £4.5 billion. On 11 December 2006, Tata preemptively upped its offer to 500 pence per share, which was within hours trumped by CSN's offer of 515 pence per share, valuing the deal at £4.9 billion. The Corus board promptly recommended both the revised offers to its shareholders. On 31 January 2007 TATA STEEL won their bid for Corus after offering 608 pence per share, valuing Corus at £6.7 billion. In 2007 TATA STEEL through its wholly owned Singapore subsidiary, NatSteel Asia Pte Ltd acquired controlling stake in two rolling mills: SSE Steel Ltd, Vinausteel Ltd located in Vietna. The TataGroup of Companies has business operations (114 companies and subsidiaries) in seven defined sectors – Materials, Engineering, Information Technology and Communications, Energy, Services, Consumer Products and Chemicals.TATA STEEL with its acquisition of Corus has secured a place among the top ten steel manufacturers in the world and it is the Tata Group’s flagship Company. Other Group Companies in the different sectors are – Tata Motors, Tata Consultancy Services(TCS),Tata Communications,Tata Power, Indian Hotels, Tata Global Beverages and Tata Chemicals.
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Tata Motors is India’s largest automobile company by revenue and is among the top five commercial vehicle manufacturers in the world. Jaguar andLandrover are now part of Tata Motor’ s portfolio. Tata Consultancy Services) is an integrated software solutions provider with delivery centres in more than 18 countries. It ranked fifth overall, and topped the list for IT services, in Bloomberg Business week's 12th annual 'Tech 100', a ranking of the world's best performing tech companies. Tata Power has pioneered hydro-power generation in India and is the largest power generator(production capacity of 2300 MW) in India in the private sector. Indian Hotels Company(Taj Hotels, resorts and palaces) happens to be the leading chain of hotels in India and one of the largest hospitality groups in Asia. It has a presence in 12 countries in 5 continents. Tata Global Beverages (formerly Tata tea) its major acquisitions like Tetley and Good. Earth is at present the second largest global branded tea operation.
TATA STEEL is headquartered in Mumbai, Maharashtra, India and has its marketing headquarters at the Tata Centre in Kolkata, West Bengal It has a presence in around 50 countries with manufacturing ope rations in 26 countries including: India, Malaysia, Vietnam Thailand, Dubai, Daggaron, Ivory Coast, Mozambique, South Africa, Australia, United Kingdom The Netherlands, France and Canada. TATA STEEL primarily serves customers in the automotive, construction, consumer goods engineering, packaging, lifting and excavating, energy and power, aerospace, shipbuilding, rail and defense and security sectors TATA STEEL has set a target of achieving an annual production capacity of 100 million tons by 2015; ; it is planning for capacity expansion to be balanced roughly 50:50 between greenfield developments and acquisitions. Overseas acquisitions have already added an additional 21.4 million tons of capacity, including Corus (18.2 million tons), NatSteel (2 million tons) and 8
Millennium Steel (1.2 million tons). Tata plans to add another 29 million tons of capacity through acquisitions. Major greenfield steel plant expansion projects planned by TATA STEEL include:-
A 6 million ton per annum capacity plant in Kalinganagar, Odisha, India An expansion of the capacity of its plant in Jharkhand, India from 6.8 to 10 million tons per annum. A 5 million ton per annum capacity plant in Chhattisgarh, India (TATA STEEL signed a memorandum of understanding with the Chhattisgarh government in 2005; the plant is facing strong protest from tribal people).
A 3 million ton per annum capacity plant in Iran;
A 2.4 million ton per annum capacity plant in Bangladesh;
A 10.5 million ton per annum capacity plant in Vietnam (feasibility studies are underway); A 6 million ton per annum capacity plant in Haveri, Karnataka
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4.STEEL MAKING PROCESS
Iron ore fines are not suitable for use in the Blast Furnace. Hence, the iron ore fines areagglomerated into larger porous lumps, which is suitable for use in the Blast Furnace. A greenmix of carefully proportioned iron ore fines, fluxes and coke breeze is prepared in granular formin Mixers. Heat generated through combustion
within the mass itself produces large lumps ofhot Sinter. This Sinter is cooled, sized and stored for use in the blast furnace.Iron ore fines are recycled to make sinter, to help produce hot metal of predictable andstandard quality in the Blast Furnace. Naturally found coal contains Fixed Carbon (FC), Volatile matter (VM), Ash, Moisture and otherimpurities. Its poor crushing strength and the volatile matter content makes it unsuitable foruse in Blast Furnace. Hence, naturally found coal is converted into coke in the coke oven for usein Blast Furnaces.Heating coal in the absence of air carbonizing it to form a hard porous mass, devoid of volatilematter produces coke. Coal after carbonization, which gives blast furnace quality coke, is called metallurgical coal. Coal is graded as prime, medium and bendable, based on its cokingproperties. Blending of different grades of coal is necessary in order to conserve metallurgicalcoal, yet ensure uniform coking properties. Currently, there are 6 batteries of coke ovensupplying coke to the blast furnaces. The coke plant blends coal from different sources, converts coal to coke and cuts to the correctsize for use in the blast furnace.
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The Blast Furnace is a ceramic refractory lined tall reactor, used for the production of liquidiron called Hot Metal. Iron oxide, present in the iron bearing raw materials, is reduced insidethe reactor by coke and carbon monoxide. Coke is used for combustion to attain the high temperatures required for reduction. Coke oncombustion generates carbon monoxide, which acts as the reducing agent and converts theiron oxides into molten iron. Fluxes are used to make low melting slag and control the quality ofHot Metal. Hot Metal and Slag are collected in the hearth and tapped periodically. Blast Furnaces A, B, C, D,E and F together produce 2.8 million tons of hot metal, annually. G-Blast Furnace produces 1.30million tons of hot metal annually. Blast Furnace F has been rebuilt in 2002 to enlarge itscapacity to 1 million tons.Blast Furnaces are used for producing Hot Metal. Hot Liquid Iron (commonly called Hot Metal in TATA STEEL) is converted to Steel in the SteelMelting Shops. Hot Metal from the Blast Furnace is stored in Mixers in LD#1 shop. The HotMetal is converted to Steel in the LD converters by removing its carbon, silicon, sulphur andphosphorous contents. The liquid steel from the converter is converted to billets using Continuous Casting machine. Asmall portion of steel is teemed into ingots through Bogie Bottom Poring process using cast ironmoulds. The liquid steel is treated in On Line purging, Ladle Refining Furnace or Argon Rinsingstation before continuous casting. Special grades of steel, which are cast as ingots, areprocessed in On-line purging, followed by the Vacuum Arc degassing & refining unit.The Steel Melting Shop requires an Oxygen Plant to cater to the requirement of oxygen for steelmaking. The Lime Calcining Plant and the Tar Dolo Plant are auxiliary units required for themanufacture of Steel. Hot metal is converted to Steel and cast into Billets. The LD#2 Shop has three Converters of 140 tons capacity each, producing 2.6 million tons ofcrude steel per annum. Hot Metal is brought from A, D, E, F and G Blast Furnaces in Torpedoladles. The metal from the Torpedo ladle is taken into the Hot Metal for Desulphurization. It isthen charged into the vessel.Primary refining of steel is done in the Ladle Furnace (LF) and RH Degasser (RH) to make cleanersteel of different value added grades.LD 2 makes superior & cleaner grades of steel required to process Flat products of world-classstandards.
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5.Overview of PLTCM Cold rolling mills are used for deforming various metals by passing them through rollers at a temperature below its re-crystallization temperature. This process of rolling increases the yield strength and hardness of a metal by introducing defects into the metal's crystal structure. These defects prevent further slip and also reduces the grain size of the metal.Cold Rolling mills are used to obtain desired level of thickness reduction. Basically Pickle Line Tandem Cold Mill consists of four sections: 1.Entry Section 2.Pickle(cleaning) Section 3.Tandem Cold Reduction Section 4.Exit Section 5.1Entry Section: Its basic operation is divided into two parts: 1.CALL TRANSFER :-This system optimizes logistics and transport from the hot strip mill,reducing the potential for damage to the bands that can occur prior totheir arrival at the PLTCM. Additionally, a horizontal storage positionallows for better heat
transfer for faster processing time. 2.COIL TRACKING SYSTEM:-A sophisticated tracking system uses ―smart‖ cranes to track themovement of the steel between the hot strip mill and the PLTCM.Automatic band identification is made based on weight, width and barcodeto guarantee that the correct coils are delivered to the customer. There are four parts in this section:-
Automated Coil Preparation Station:- The automated strap removal and crop shearprocess not only assures operator safety, butoptimizes product yield by cutting off only theportions determined by the band quality data. SMS Siemag Coil Handling:-Built by SMS Siemag, this system uses automatedmotion to move and lift the product,reducing the risk of product mixing and damage.
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Miebach HSL 19 Laser Welder:- In order to roll AHSS grades, this laser weldercan join high-alloy coils with absolutely minimalstrip breaks. The result is superior weld qualityand increased reliability and on-time delivery. Tension Leveler in Pickle Section:-The tension leveler removes scale to provideuniform surface quality regardless of producttype and flattens the strip. 5.2Pickle(Cleaning) Section:-The PLTCM uses a four-tank, cascading type hydrochloric acid pickling section with surface agitation to remove iron fines. This, coupled with afour-stage rinse section utilizing a demineralized water final rinse spray before drying, ensures a perfectly clean and
dry strip.
It basically has two parts:1. Mitsubishi-Hitachi Immersion BoxTurbulent Pickling:-This system uses strip motion to achieveoptimum fluid turbulence and minimized pickling time. No moving parts assure effective scale removal and guarantee superior reliability.
2. Automated Turret Side Trimmer:-This fully automated system provides precisewidths and ideal trim quality. Optional automatic set-up for gap and overlap leadsto improved edge trim quality while allowingfor optimized product yield.
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5.3 Tandem Cold Reduction Section:It basically consists of two parts:1. Independent Main Mill Motots:-Five independent motors, inconjunction with two tension reelmotors, provide the capability to processa full range of current and future productrequirements, and allow maximum coldreductioncapability. 2. Automated Roll change equipment:- All work rolls and intermediate rolls can bechanged without stopping the strip in the pickleline, optimizing surface quality. Rolls aredelivered directly into the roll shop to bereconditioned for further use.
Apart from that it has Five stand, six high universal crown mill.
By incorporating smaller diameter work rolls, the millcan realize heavy reduction rolling and provide a widerrange of product thickness, width and hardness. Thisallows us to produce products from ultra low carbonto advanced high strength steel. Since the mill utilizesa straight roll at the number five stand, we are able toprovide significant improvement in shape control.The five-stand, six-high universal crown mill is theperfect partner to the pickle line. Together, they areredefining the shape of today’s steel solutions.The Universal Crown Mill’s cylindricalrolls create a more consistent relationship between strip speed and roll speed, improving texture consistency across the width. 5.4 Exit Inspection Section:This section has three parts:-
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1. Carousel Tension Reel:-The twin-exit reel system insures a constantthread position and single mode of operationfor greater consistency and uninterruptedproduction from coil to coil.
2. Strip-Surface Inspection Station:-This station provides an optimal view of thestrip-surface quality and enables comprehensivesurface inspection under tension withoutdisrupting processing.
3. SMS Siemag Exit Coil Handling System:- Utilizing a system very similar to the entrycoil handling system, this automated exit coilhandling system allows for the automaticdelivery of finished product. After weighingand strapping to safely restrain the coil, thesystem automatically marks, and applies barcode tags to the finished coils.
6.Tandem Cold Mills The main advantages of Tandem Cold Mills are:-
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Reduced material loss can be attributed to thefollowing: Thick ends eliminated by continuous operation Coil handling damage reduced due to less handling Minimized off gauge lengths as a result of constantprocess conditions. Yield increased by 1.5 – 2.0% Improved product quality is attained due to thefollowing criteria: Constant process conditions result in higher quality and more consistent products Reduced strip-edge damage due to less coil handling Higher proportion of coil with stable operatingconditions Surface finish flexibility Flatness tolerance: below 15 I-Units (thickness- andwidth-dependent) Gauge tolerance: standard tolerance (DIN standard) Surface quality: roughness, cleanness improvements Reduced operating cost is reached due to: Continuous, compact plant requires fewer operators 40% longer roll service life because of less damage dueto reduced threading and tailing outReduction in energy consumption of 10% by anoptimized process due to fewer acceleration anddeceleration operations Reduced roll oil consumption by 32 – 40% Reduced maintenance costs due to constant operating conditions and less threading and tailing-out operations
Mill Power:-Since mill speed had to be increased to meet the capacitytargets, a power upgrade of the existing mill drives from 10,800 kW to 22,100 kW was necessary. The optimizationof the speed cone required an adaptation of the gearratios and hence new gears. Mill Stand:-The 5th stand uses smaller work-rolls than stand no. 1 – 4which is required to obtain the aimed maximum reduction of approximately 6% in particular for rolling of stronghardening, high strength materials down to the minimumthickness of 0.3 mm. Since the maximum total reductionfrom hot gauge to cold gauge can reach 84%, the materialhardening is significantly high. The required reductionson the last stand could therefore not be performed withconventional work-roll diameters. To compensate forthe decrease in flexural stiffness of the work-/back-uproll assembly due to the smaller work-roll diameters, thediameter of the back-up rolls was increased accordingly .
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Working Principle of Tandem Cold Mill:-
Application of the UCM-MILL makes possible to roll all products with straight rolls, which straight rolls, which allow to get rapid start up. The operator derives most stable rolling and shape controllability. Thanks to horizontal rigidity created by shifting of intermediate roll, the user can control strip thickness deviation without jeopardizing the shape of the strip. The mill is equipped with high response Hydraulic Roll Position Device (HYROP-F).Laser Doppler type speed measuring device and X-ray type thickness gauge meters. By using these equipment, high performance up-to-date Automatic Gauge Control (AGC) system, which includes the mass flow control, can obtain the higher standard of the finished thickness. At the exit of the mill , a modern designed shape measuring sensor and Automatic Shape Control (ASC) system is installed. They can guarantee the required flatness of the finished coil and the qualified products can be produced.
Technical specification of Motors Used in Tandem Mills:-
Stand #No 1.
2.
Type
Power
Synchronous 2300KW
Induction
Current
Voltage
Speed
Stator Rotor Insulation Insulation
1565A
900V
132/600RPM
CLASS F CLASS H
3000V
440/1200RPM
5000KW
3.
Synchronous 3500KW
1551A
1350V
201/600RPM
CLASS F CLASS H
4.
Synchronous 3500KW
1551A
1350V
201/600RPM
CLASS F CLASS H
5.
Synchronous 3500KW
1551A
1350V
201/600RPM
CLASS F CLASS H
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7.DRIVES Whenever the term electric motor or generator is used, we tend to think that the speed of rotation of these machines are totally controlled only by the applied voltage and frequency of the source current. But the speed of rotation of an electrical machine can be controlled precisely also by implementing the concept of drive. The main advantage of this concept is, the motion control is easily optimized with the help of drive. In very simple words, the systems which controls the motion of the electrical machines, are known as electrical drives. A typical drive system is assembled with a electric motor (may be several) and a sophisticated control system that controls the rotation of the motor shaft. Now a days, this control can be be done easily with the help of software. So, the controlling becomes more and more accurate and this concept of drive also provides the ease of use. This drive system is widely used in large number of industrial and domestic applications like factories, transportation systems, textile mills, fans, pumps, motors, robots etc. Drives are employed as prime movers for diesel or petrol engines, gas or steam turbines, hydraulic motors and electric motors.
Types of Drives: DC Drives AC Drives
7.1 DC DRIVE CONTROL SYSTEM: A basic DC drive control system generally contains a drive controller and DC motor as shown inthe figure given below: The controls allow the operator to start, stop, and change direction and speed of the motor byturning potentiometers or other operator devices. These controls may be an integral part of thecontroller or may be remotely mounted.The drive controller converts a 3-phase AC voltage to an adjustable DC voltage, which is thenapplied to a DC motor armature.
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The DC motor converts power from the adjustable DC voltage source to rotating mechanicalforce. Motor shaft rotation and direction are proportional to the magnitude and polarity of theDC voltage applied to the motorThe tachometer (feedback device) shown in Figure converts actual speed to an electrical signalthat is summed with the desired reference signal. The output of the summing junction providesan error signal to the controller and a speed correction is made.
FEATURES: • Field orientation via mechanical commutator. • Controlling variables are Armature Current and Field Current, measured DIRECTLY from themotor. • Torque control is direct. OPERATION: The magnetic field is created by the current through the field winding in the stator. This field is always at right angles to the field created by the armature winding. This condition, known asfield orientation, is needed to generate maximum torque. The commutator-brush assemblyensures this condition is maintained regardless of the rotor position.Once field orientation is achieved, the DC motor’s torque is easily controlled by varying thearmature current and by keeping the magnetizing current constant. Torque is the inner control loop and speed is the outer control loop TYPES OF DC DRIVES: Armature and Field Controlled DC Drives The motor is armature voltage controlled for constant torque-variable HP operation up to basespeed. Above base speed the motor is transferred to field current control for constant HPreduced torque operation up to maximum speed.
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APPLICATIONS: DC drives were used for VARIABLE SPEED CONTROL DRIVES because they could easily achieve a good torque and speed response with high accuracy. Variable Speed Control Drive: Basic function of a variable speed drive (VSD) is to control the flow of energy from the mains tothe process. Energy is supplied to the process through the motor shaft. Two physical quantitiesdescribe the state of the shaft: torque and speed. To control the flow of energy we musttherefore, ultimately, control these quantities. In practice, either one of them is controlled andwe speak of ―torque control‖ or ―speed control‖. When the VSD operates in torque controlmode, the speed is determined by the load. Likewise, when operated in speed control, thetorque is determined by the load. ADVANTAGES: • Accurate and fast torque control • High dynamic speed response • Simple to control DRAWBACKS: • Reduced motor reliability: the fact that brushes and commutators wear downand needs regular servicing. • Regular maintenance • Motor costly to purchase •Needs encoder for feedback .
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7.2 AC DRIVES AC drives provide a very efficient and direct method of controlling the speed of the most rugged and reliable of prime movers, the squirrel cage motor. AC drives provide many economic and performance advantages in a wide variety of adjustable speed drive applications. The following are some of the benefits provided: 1. High efficiency and low operating cost. 2. Minimal motor maintenance. 3. Controlled linear acceleration and deceleration provide soft. 4. Starting and stopping and smooth speed changes. 5. Multiple motor operations are easily accomplished. 6. Current limit provides for quick and accurate torque control. 7. Adjustable speed operation can be accomplished with existing AC motors. 8. Improved speed regulation can be accomplished by slip compensation. 9. AC motors are available in a wide variety of mechanical configurations. 10. Flexibility of machine design due to the light weight and compact size of AC motors. 11. IR compensation provides high starting torque easily and economically. 12. AC motors are available in enclosures suitable for hazardous or corrosive environments. 13. Fewer spare motors are required since the same motor can be used for both adjustablespeed and constant speed operations. 14. Cutler-Hammer rugged and reliable designs ensure minimum downtime expense. 15. High speed operation can be economically accomplished using extended frequencyoperation. 16. Reverse operation is accomplished electronically without the need for a reversingstarter. ADJUSTABLE FREQUENCY AC DRIVE SYSTEM INTRODUCTION An adjustable frequency AC drive system consists of an ordinary three-phase induction motor,an adjustable frequency drive to control the speed of the motor and an operator's controlstation.The most common motor used with an AF drive system is a standard NEMA design B squirrelcage induction motor, rated for 230 or 460 volt, 3-phase, 60 Hz operation. The adjustablefrequency controller is a solid-state power conversion unit. It receives 240 or 480 volt, 3-phase,60 Hz power and converts it to a variable frequency supply which can be steplessly adjustedbetween 0 and 60 Hz. The controller also adjusts the output voltage in proportion to thefrequency to provide a nominally constant ratio of voltage to frequency as required by thecharacteristics of the motor. The operator's station provides the operator with the necessary controls for starting and stopping the motor and varying the motor speed. These functions canalso be performed by a wide variety of automatic control systems. There are severalclassifications of adjustable frequency AC drives. Some common types of drives are VariableInput (VVI) sometimes called Six Step drives, current source input (CSI), pulse widthmodulated (PWM) drives, Sensorless Vector drives, Field Oriented drives and Closed LoopVector drives.
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PRINCIPLES OF ADJUSTABLE FREQUENCY MOTOR OPERATION The operating speed of an AC induction motor can be determined by the frequency of theapplied power and the number of poles created by the stator windings. Synchronous speed isthe speed of the magnetic field created in the stator windings. It is given by: N = 120f /P Where: n = speed in RPM f = operating frequency P = number of poles When the frequency is changed, the voltage must also be changed, based on the formula forreactance and Ohm’s Law. XL = 2πfL
Where L = inductance XL = reactance V = voltage Im = magnetizing current Im = V/XL Combining the above equations yields: Im = (V/f).(1/2πfL) For steady-state operation, a constant volts per hertz ratio must be maintained. This is equal tothe motor rated voltage divided by the rated frequency. For the magnetizing current to remain constant, the V/f ratio, or the volts per hertz ratio, mustremain constant. Therefore, the voltage must increase and decrease as the frequency increasesand decreases.
INDUCTION MOTOR SPEED CONTROL Standard induction motors (NEMA design B) have approximately 3% slip at full load.If the drive only controls the output frequency, the motor speed will deviate from the set speeddue to slip. For many fan and pump applications, precise speed control is not needed. Vector controlled drives need speed feedback of the rotor. For Sensorless Vector, the rotorspeed is calculated based on a model of the motor stored in the drive. For Closed Loop Vector,a digital encoder is added to the motor to provide actual rotor speed.
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VOLTS PER HERTZ REGULATION In order to operate the motor with the desired speed/torque curve, we must apply the propervoltage to the motor at each frequency. As we have already seen, it is necessary to regulatemotor voltage in proportion to the frequency at a constant ratio. In reality, this requirement forconstant volts/hertz does not apply to the motor terminals, but to a hypothetical point insidethe motor. The voltage at this point is called the air gap voltage. The difference between air gapvoltage and motor terminal voltage is the IR voltage drop. AC DRIVE APPLICATIONS MATCHING THE AC DRIVE TO THE MOTOR PWM and Vector AC Drives are designed for use with any standard squirrel cage motor. Sizingthe drive is a simple matter of matching the drive output voltage, frequency and current ratingsto the motor ratings.
OUTPUT VOLTAGE AND FREQUENCY Most modern AC Drives are designed for use with various voltages and frequencies. Byadjusting the V/Hz properly, almost any 3-phase motor can be used. OUTPUT CURRENT AC drive full load currents are matched to typical full load motor current ratings. Usually an ACdrive can be matched to an AC motor by their hp ratings; however, actual motor currentrequired under operating conditions is the determining factor. If the motor will be run at fullload, the drive current rating must be at least as high as the motor current rating. If the drive isto be used with multiple motors, the sum of all the full load current ratings must be used, andadding up the hp ratings of the motors will usually not provide an accurate estimate of thedrive needed. MOTOR PROTECTION Motor overload protection must be provided as required by the applicable codes. Motorprotection is not automatically provided as part of all AC drives. It may be provided as astandard feature on one model or it may be an optional feature on another. The best means ofmotor protection is a direct winding over temperature protection such as an over temperatureswitch imbedded in the motor windings. Direct over temperature protection is preferredbecause overheating can occur at normal operating currents at low speeds. Most AC drives areequipped with electronic overcurrent protection, such as I2t protection, similar to a conventionoverload. In multiple motor applications, individual motor overload protection must be provided evenwhere electronic protection is provided by the drive. In some cases, short circuit protectionmay be required. MOTOR WINDING DAMAGE The voltage output of AC drives contains voltage steps. In modern PWM drives, the dV/dt of amotor causes can cause very large voltage spikes. Voltage spikes of - 23 -
1500 volts or more aretypical for a 460 volt motor. This can cause the end windings of a Non-Inverter Duty or standardinduction motor to fail. This problem gets worse as the cable length from the drive to the motorgets longer. Corrective action is normally required for cables longer than 150 feet. Load sidereactors, installed at the drive output terminals, will reduce the voltage spikes at the motorterminals. Most drive manufacturers have load side reactors available as an option.
7.3 COMPARISON BETWEEN AC DRIVES AND DC DRIVES
1. The Dc motor is complicated and requires a lot of maintenance, which makes it expensiveto run; it also has a lower degree of protection. The AC motor, on the other hand, is simpleand sturdy, does not need much maintenance, is therefore less expensive, and possesses ahigher degree of protection into the bargain . 2. In contrast to the AC standard motor with fixed basic speeds (synchronous speeds of 3000/1500/1000/... rpm at 50 Hz), the DC motor's basic speed can be designed from approx. 300 rpmto about 4000 rpm for each working point. 3. Power limitation is caused by the breakdown torque of AC motor decreasing as the square ofspeed (1/n2). Power limitation is caused by the commutation of DC motor.
8. Speed and Frequency control for Drives:Both voltage and frequency reference are fed into a modulatorwhich simulates an AC sine wave and feeds this to the motor’sstator windings. This technique is called Pulse WidthModulation (PWM) and utilises the fact that there is a
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dioderectifier towards the mains and the intermediate DC voltageis kept constant. The inverter controls the motor in the formof a PWM pulse train dictating both the voltage and frequency. Significantly, this method does not use a feedback devicewhich takes speed or position measurements from the motor’sshaft and feeds these back into the control loop.Such an arrangement, without a feedback device, is calledan ―open-loop drive‖. To emulate the magnetic operating conditions of a DC motor,i.e. to perform the field orientation process, the flux-vectordrive needs to know the spatial angular position of the rotorflux inside the AC induction motor.With flux vector PWM drives, field orientation is achieved byelectronic means rather than the mechanical commutator/brush assembly of the DC motor.Firstly, information about the rotor status is obtained by feedingback rotor speed and angular position relative to the statorfield by means of a pulse encoder. A drive that uses speedencoders is referred to as a ―closed-loop drive‖. Also the mo tor’s electrical characteristics are mathematicallymodelled with microprocessors used to process the data.The electronic controller of a flux-vector drive creates electricalquantities such as voltage, current and frequency, which arethe controlling variables, and feeds these through a modulatorto the AC induction motor. Torque, therefore, is controlled indirectly.
9. Drives used in PLTCM In PLTCM basically two types of drives are used,they are cycloconverter and Hitachi VSI drives. 9.1 CYCLO CONVERTER:-Cycloconverter isnothing but basically a Variable Speed drive.This type of VSD makes a direct conversion from constant frequency, constant voltage to variable frequency, variablevoltage in one stage, without resorting to an intermediate DC linkwith energy storage. By supplying each phase of the motor winding from a reversibleconverter, a low frequency AC drive system is formed, as shown in Figure 1.16. Although thisVSD is complex, cycloconveters use a large number of thyristor switches but they do notrequire forced commutation circuits and thus can use relatively inexpensive, converter-gradethyristors. The generated structure of cycloconverters presented in Figure 1.16a, shows a largenumber of power switches and the need for a special three-phase secondary transformer.
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A cycloconverter is a device that converts alternating current, or AC, power at one frequency into AC power of an adjustable but lower frequency without any direct current, or DC, stage in between. It can also be considered as a static frequency changer and typically contains silicon-controlled rectifiers. The device consists of an array containing back-to-back, parallel, connected switches, which are used to fabricate the desired output AC waveforms. It's possible to control the frequency of these output AC waveforms by opening and closing the switches in a controlled fashion. This converter converts single-phase or three-phase AC power to single-phase or three-phase power having a variable frequency and magnitude. Typically, the output frequency of the AC power is lower than the input frequency. A cycloconverter has the capacity to operate with loads of variable power factors and also allows bidirectional power flow. They can be broadly classified into two types — phase-controlled cycloconverters and envelope cycloconverters. In the former, control of the firing angle is accomplished through adjustable gate impulses, while in the latter, the switches remain in an on state and conduct in consecutive half cycles. They are mostly used to control the speed of drives and for converting variable input frequency power into constant frequency output, such as in very high-power applications, including driving synchronous motors and induction motors. Some of the places where cycloconverters are employed include cement mill drives, mine winders, and ore grinding mills. They are also utilized in ship propulsion drives, scherbius drives, and rolling mill drives. Offering many advantages, a cycloconverter can be used in quite a few low-speed applications and is also a compact system. Its ability to directly affect the frequency conversion of power without any intermediate stage involving DC power is another huge advantage. If the cycloconverter experiences a commutation failure, the results are minimal, such as the blowing off of individual fuses. It also has the capacity of regeneration, covering the total range of speeds. Another huge advantage of the cycloconverter is its ability to deliver a sinusoidal waveform at a lower output frequency. This advantage comes from its ability synthesize the output waveform using a large number of segments of the input waveform. This technology does have some disadvantages, though. Firstly, the frequency of the output power is around one third or less of the input frequency. It's possible to improve the quality of the output waveform if a larger number of switching devices are employed. A cycloconverter requires quite a complex control mechanism and also uses a large quantity of thyristors. Its use is also limited by severe harmonics and the low-output frequency range.
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Cycloconverters are used for high power machines (above 1MW) with low frequency operation(e.g. rolling mills, cement kilns). The output frequency is typically below 25Hz since thequality of the voltage waveforms degrades as the output frequency increases. They can operatedown to zero speed and they can be used both with induction and synchronous motors. Themain disadvantages are the complex circuit design and the low power factor at low speed. 9.2 VSI DRIVES:-With fast switching semiconductor devices like the insulated gate bipolar transistors (IGBT) it is possible to build inverter based high voltage power supplies for electrostatic precipitators. Comparing conventional SCR (Silicon controlled rectifier) based technology the average corona power can be increased significantly to improve the precipitator efficiency. Additionally, during flashovers the fast current control of IGBT power inverters improves the precipitator performance due to fast voltage recovery resulting in further increasing of the peak and average precipitator voltage. In a new approach, the advantages of higher distances up to 400 mm between the discharge and collecting electrodes could be addressed by a voltage up to 150 kV applied to the precipitator. Due to the exact voltage control of the IGBT inverter a smooth DC voltage can be generated and therefore, the overvoltage capability of the system is much lower than it would have to be with a conventional thyristor based high voltage generation system. Thus, the IGBT inverter solution becomes more economical or less expensive to operate than the conventional supply. With the availability of the latest generation of integrated IGBT modules a very compact IGBT inverter has been developed to meet the design requirements by operating at a frequency up to 10 kHz. The new IGBT types have lower saturation voltages than the previous modules resulting in lower power losses. The HV-transformer has been designed with the required rating and stray inductance.
The three-phase voltage source inverter (VSI) is used to control AC-motors in the lower andmedium power ranges, from small high dynamic performance servo drives with speed andposition control capability (<10kW) to most auxiliary drives in industry, ranging up to several hundred kW. The VSI is suitable for supplying induction, as well as synchronous motors. Figure 1.10 shows a simplified diagram of the basic three-phase voltage source inverter. Theinput rectifier serves to produce a DC supply, and the relatively large electrolytic capacitor isinserted to filter ("stiffen") the DC voltage which feeds the inverter. Typically, the capacitor of2 to 20 milifarads, is a mayor cost item in the system. Additionally, it is usual to insert areactance between the rectifier and the AC supply to limit the fault current and to reduce theharmonic distortion produced by the rectifier. The inverter module converts the DC voltage to avariable frequency, variable voltage output. The variable voltage inverter (VVI) uses an SCR converterbridge to convert the incoming AC voltage into DC. The SCRsprovide a means of controlling the value of the rectified DC voltage from 0 to approximately 600 VDC. The L1 choke And C1 capacitor(s) make up the DC link section and smooththe converted DC voltage. The inverter section consists ofsix switching devices. Various devices - 27 -
can be used such as thyristors, bipolar transistors, MOSFETS, and IGBTs. Thefollowing schematic shows an inverter that utilizes bipolartransistors. Control logic (not shown) uses a microprocessorto switch the transistors on and off providing a variable voltageand frequency to the motor.
This type of switching is often referred to as six-step becauseit takes six 60° steps to complete one 360° cycle. Although themotor prefers a smooth sine wave, a six-step output can besatisfactorily used. The main disadvantage is torque pulsationwhich occurs each time a switching device, such as a bipolartransistor, is switched. The pulsations can be noticeable at lowspeeds as speed variations in the motor. These speed variationsare sometimes referred to as cogging. The non-sinusoidalcurrent waveform causes extra heating in the motor requiring amotor derating.
Classification of Voltage Source Inverter:-
Voltage source inverters can be classified according to different criterions. They can be classified according to number of phases they output. Accordingly there are single-phase or three-phase inverters depending on whether they output single or three-phase voltages. It is also possible to have inverters with two or five or any other number of output phases. Inverters can also be classified according to their ability in controlling the magnitude of output parameters like, frequency, voltage, harmonic content etc. Some inverters can output only fixed magnitude (though variable frequency) voltages whereas some others are capable of both variable voltage, variable frequency (VVVF) output. Output of some voltage source inverters is corrupted by significant amount of rd
th
th
th
th
many low order harmonics like 3 , 5 , 7 , 11 , 13 order of the desired (fundamental) frequency voltage. Some other inverters may be free from low order harmonics but may still be corrupted by some high order harmonics. Inverters used for ac motor drive applications are expected to have less of low order harmonics in the output voltage waveform, even if it is at the cost of increased high order harmonics. Higher order harmonic
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voltage distortions are, in most ac motor loads, filtered away by the inductive nature of the load itself. Inverters may also be classified according to their topologies. Some inverter topologies are suitable for low and medium voltage ratings whereas some others are more suitable for higher voltage applications. The inverters shown in Figs. 33.3(c), 33.4(a) and 33.4(b) are two level inverters as the pole voltages may acquire either positive dc bus or negative dc bus potential. For higher voltage applications it may not be uncommon to have three level or five level inverters. In VSI drives Very High Voltage control and very Fast Switching Device is required ,henceIGBT is used in VSI Drives. Insulated Gate Bipolar Transistor (IGBT):-
It is a Power transisitor employed in the power conversion master circuit.This system uses 33.3kV,1200 A IGBTdevices.IGBT has the characteristics of both bipolar transisitor and MOSFET.In other words, it is characterized with comparatively high withstand voltage and high current capacity by means of bipolar action, high-speed switching by MOS gate, and easy gate drive at low ON voltage as ideal requirements. Since this device provides a self arc extinguishing function, it does not require any forced commutation circuit to ensure compact design of device.
Block Diagram of Hitachi VSI :There are two types of inverter main circuit used. They are: 3 level type circuit 2 level type circuit
9.2.1--2 Level type Circuit :-The IGBT drive employs a 2-level system conversion circuit.This conversion circuit connects 2 serial switching devices between P bus and N bus. - 29 -
9.2.2--3 level type Circuit:-The IGBT converter employs a 3-level system conversion circuit. This 3-level system conversion circuit divides the DC power voltage into 2 parts via the neutral point,and connects serial switching devices to the positive bus-neutral point and the neutral point-N bus as a feature.
From the figure when switch QP and QPC is ON the the output voltage is +Edc,when the switch QPC is ON but QP is OFF then the output voltage is zero. Similarly when the switch QNC and QN is ON then the output voltage is – Edc, when the switch QN is OFF but QNC is ON then the output voltage is zero.
In this circuit when the switch QP is ON then the output voltage is +E/2 and when the switch QN is ON then the outut voltage is – E/2.
9.3 Difference between CYCLOCONVERTER and VSI CYCLOCONVERTER
VSI
1. It uses tyristor switch.
1. It uses IGBT switches.
2. It converts constant ac to variable ac directly.
2. It converter contact ac to dc then to variable ac.
3. It is complex device used large number of components.
3. It is less complex. 4. Power factor is nearly unity.
4. Power factor is less then unity. 5. It operates in frequency of less than 25HZ. 6. It is used to operate motors of higher power range.
5. It can be operate up to frequency of 200HZ. 6. It is used to operate motors of lower and medium power ranges.
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MAKE
MODEL NO.
QTY.
AC/DC
ANALOGUE/
KVA
VOLTAGE
APPLICATION
YEAR OF INSTALLING
DIGITAL
HITACHI
HIVECTOL
1
AC
DIGITAL
800
-VSI-M
DC
MILL
1200V
ENTRY #4
1999
BRIDLE ROLL
HITACHI
HIVECTOL
1
AC
DIGITAL
200
DC 600V
MILL
1999
ENTRY
-VSI-M
BRIDLE ROLL
HITACHI
HIVECTOL
2
AC
DIGITAL
400
DC 600V
MILL
1999
ENTRY
-VSI-M
BRIDLE ROLL
HITACHI
HIVECTOL
1
AC
DIGITAL
75
DC 600V
MILL EXIT
1999
PINCH
-VSI-M
ROLL
HITACHI
HIVECTOL
1
AC
DIGITAL
AC1074
-VSI-S
HITACHI
HIVECTOL
TOSVERT
1999
STAND
4
AC
DIGITAL
AC 1645
-CYC
TOSHIB
#1 MILL
#2~5 MILL
1999
STAND
5
AC
DIGITAL
A
- 31 -
AC 415
X-RAY
1999
