Download - New Model Variable Frequency Transformer (NMVFT) – A Technology for V/f Control of Induction Motors

ACEEE Int. J. on Electrical and Power Engineering, Vol. 02, No. 01, Feb 2011

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New Model Variable Frequency Transformer(NMVFT) – A Technology for V/f Control of

Induction MotorsFarhad Ilahi Bakhsh, Mohammad Irshad, and Shirazul Islam

Department of Electrical Engineering, A.M.U, Aligarh, IndiaEmail: [email protected]

Email: [email protected]: [email protected]

Abstract— Variable frequency transformer (VFT) is used as acontrollable bidirectional transmission device that can transferpower between asynchronous networks and functionally issimilar to back-to-back HVDC. This paper describes the basicconcept of a New Model Variable Frequency Transformer(NMVFT). NMVFT is a new technology which is used for v/fcontrol of induction motors. A digital simulation model ofNMVFT and its control system are developed using MATLAB.The out power thus generated in v/f mode has been practicallyverified for the speed control of a three-phase induction motor.Thus constant speed-torque characteristics were achieved.

Index Terms—Variable frequency transformer (VFT),Asynchronous, New Model Variable Frequency Transformer(NMVFT), V/f control, MATLAB.

I. INTRODUCTION

Variable frequency transformer (VFT) is a controllablebidirectional transmission device that can transfer powerbetween asynchronous networks. The construction of VFTis similar to conventional asynchronous machines, wherethe two separate electrical networks are connected to thestator and rotor respectively. One power system is connectedwith the rotor side of the VFT via the electrical brush ringsand step-up transformers. And another power system isconnected with the stator side of the VFT directly via a step-up transformer. Electrical power is exchanged between thetwo networks by magnetic coupling through the air gap ofthe VFT. A motor and drive system are used to adjust therotational position of the rotor relative to the stator, therebycontrolling the magnitude and direction of the power flowingthrough the VFT [1, 2].

Both the winding currents of the stator and rotorinduce a rotary magnetic field Fstator and Frotor respectively.In the steady state, the two rotary magnetic fields are rotatingwith the same angular speed i.e. Fstator is standstill to Frotor.The composite magnetic fields Fstator_rotor will rotate with aspeed of ωsystem-stator, cutting the stator coils with the samespeed, and cutting the rotor coils with a speed of ωsystem_rotor.And the angle frequency of the resulting inductive potentialat stator and rotor windings is synchronous with their currentsrespectively. A stable power exchange between the twoasynchronous systems is possible. The transferred power andits direction are controlled by the torque applied to the rotor,

which is supplied by the dc drive motor. If torque is appliedin one direction, then power flows from the stator windingto the rotor winding. If torque is applied in the oppositedirection, then power flows from the rotor winding to thestator winding. If no torque is applied, then no power flowsthrough the rotary transformer [3, 4].

The world’s first VFT, which was manufactured byGE, installed and commissioned in Hydro-Quebec’s Langloissubstation, where it will be used to exchange power up to100 MW between the asynchronous power grids of Quebec(Canada) and New York (USA). Figure 1 shows a simplifiedone-line diagram of the Langlois VFT, which is comprisedof the following: a rotary transformer for power exchange, adrive motor to control the movement of the rotor and totransfer power, a collector to connect the rotor windings withthe outside system via electric brushes [5 - 7].

Figure 1. Photograph of Langlois 100 MW VFT

II. NMVFT MODEL AND SYSTEM DESCRIPTION

A. NMVFT ModelIn the model, NMVFT is a three phase singly fed

slip ring type induction machine. The stator winding is en-ergized with three phase ac source and the rotor winding iskept open. The rotor is coupled to a controllable constantspeed drive i.e. dc shunt motor via mechanical coupler and


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scope is connected to measure the magnitude of voltage andfrequency of voltage across the rotor winding. Figure 2 showsthe circuit diagram of NMVFT.

Figure 2. NMVFT circuit diagram

B. NMVFT OperationThe three phase ac supply is applied to the stator

winding of the three phase four poles singly fed slip ringtype induction machine. The rotor is coupled with the dcshunt motor through a mechanical coupler. When the 220Vdc supply is applied to the dc shunt motor the NMVFT comesin operation. The induced emf ‘Er’ in the rotor circuit is givenby Er = sV (1)where, V = Supply voltage per phase, s = Slip of Induction Motor and Er = Emf induced in the rotor circuit per phaseAnd the frequency of the induced voltage in the rotor circuitis given by fr = sf (2)where, f = Supply frequency, s = Slip of Induction Motor and fr = frequency of the induced voltage in the rotor circuitThe operation of NMVFT includes the following: i) If the rotational speed of dc shunt motor is zero then thevalue of slip is equal to one as a result the emf induced acrossthe rotor winding is the rated rotor terminal voltage and thefrequency of voltage is the rated frequency. ii) If the dc shunt motor rotates in

a)The direction of the rotating air gap flux then therelative motion between them decreases results in decreasein slip as a result the emf induced across the rotor windingas well as its frequency decreases. When the speed of rotationof dc shunt motor is equal to synchronous speed then theslip becomes zero as a result the emf induced a)across the rotor winding as well as its frequency becomeszero.

b)The direction opposite to the rotating air gap fluxthen the relative motion between them increases results inincrease in slip as a result the emf induced across the rotorwinding as well as its frequency increases. When the speedof rotation of dc shunt motor is equal to synchronous speedthen the slip becomes double as a result the emf induced

across the rotor winding as well as its frequency becomesdouble the rated value. Thus a constant V/f characteristics is achieved. Thisoperation of NMVFT is verified digitally by MATLABSimulation and practically.

III. DIGITAL SIMULATION OF VFT

A. MATLAB SimulationIn the view of MATLAB simulink, NMVFT is a

type of machine which can be simulated with theasynchronous machine SI units. The asynchronous machineSI units having a three-phase excitation system on stator side.The constant speed achieved from dc shunt motor issimulated by using a constant block. And then we could usethis simulated model, as shown as Figure 3, to solve electricsystem of NMVFT.

Figure 3. MATLAB Simulation diagram of NMVFT

B. Simulation Figures and ResultsWhen the rotational speed of dc shunt motor is

zero.


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a) The direction of rotation of rotor is same asthat of the air gap field:-

When the rotational speed of dc shunt motor is equal tothe synchronous speed i.e. relative speed is zero.

0 0.5 1 1.5 2 2.5 3x 104

-300

-200

-100

0

100

200

Time (second)

Vol

tage

(vo

lts)

In this way we can control output voltage from zerovolts to rated voltage and frequency from zero Hertz torated frequency i.e. 50 Hz in India. The emf inducedacross the rotor winding versus its frequency graphachieved is shown as:

b) The direction of rotation of rotor is opposite tothe air gap field:-

When the rotational speed of dc shunt motor is equalto the synchronous speed i.e. relative speed is double ofsynchronous speed.

0 0.5 1 1.5 2 2.5 3 3.5

x 104

-1000

-500

0

500

1000

Time (seconds)

Vol

tage

(vo

lts)

-50*pi

In this way we can control output voltage from ratedvoltage to twice of rated voltage and frequency from ratedfrequency to twice the rated frequency i.e. 50 –100 Hz inIndia. The emf induced across the rotor winding versus itsfrequency graph achieved is shown as:

Figure 4. MATLAB Simulation results showing variation of rotor circuitvoltages with time and voltage versus frequency graphs.


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IV. PRACTICAL ANALYSIS

Figure 5. Mechanical and electrical parameters used during experiment.

While performing the experiment the rotor of dc shuntmotor is mechanically coupled to the rotor of the inductionmotor. The three phase ac supply is given to the statorwinding of three phase four poles slip ring type inductionmotor through an auto transformer. The rotor windings arekept open circuited and voltmeter is connected across therotor winding. The 220V dc is applied to the dc shunt motorthrough a rheostat. A tachometer is used to measure the speedof the dc shunt motor. With the help of auto transformer theinput voltage of induction motor is maintained constant andthrough rheostat the voltage of the dc shunt motor is variedwhich varies the current in shunt winding and as a result theflux of dc shunt winding varies, resulting in variation of speedof dc shunt motor. Since the rotor of dc shunt motor ismechanically coupled with the rotor of induction motor, thusthe speed of the induction motor varies accordingly. Thevoltage induced across the rotor winding and its frequencyis given in the table: a) The direction of rotation of rotor is same as that of theair gap field:-

Figure 6 showing the variation of rotor voltage with itsfrequency of table I.

Figure 6. Voltage versus Frequency curve (table I)

b) The direction of rotation of rotor is opposite to theair gap field:-

Figure 7 shows the variation of rotor voltage with itsfrequency of table II.

V. INDUCTION MOTOR V/F CONTROL

The output power of the NMVFT is applied to three phaseinduction motor of 1HP, 420V, 2A, 50Hz, 1430rpm. A lineartorque-speed characteristics is achieved. The torque-speedcharacteristics for different magnitude of voltage anddifferent frequency of voltage i.e. 50Hz, 40Hz and 30Hz,with and without boost up voltage, are shown as:


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Figure 9. Torque versus Speed characteristics (with boost-up voltage)

Figure 11. D.C Shunt motor mechanically coupled with Induction motor

CONCLUSIONS

Using NMVFT we can vary voltage from zero voltsto twice the rated voltage and frequency from zero hertz totwice the rated frequency. Thus a constant V/f is achievedby NMVFT and is harmonics free. This is a new alternativefor v/f control of Induction Motor. It eliminates the use ofconventional v/f power supply using a MOSFET or IGBTbased inverter. Therefore the reliability of the system is highand the proposed system can be used at MW level of power.MATLAB based simulation is done for the overallperformance of the NMVFT system. Moreover, a satisfactoryperformance of a three-phase induction motor is alsoobtained, when energized form the NMVFT supply. Itshowed almost linear torque-speed characteristics.

REFERENCES

[1] E.Larsen, R.Piwko, D.McLaren, D.McNabb, M.Granger,M.Dusseault,L-P.Rollin, J.Primeau, “Variable-FrequencyTransformer - A New Alternative for Asynchronous PowerTransfer,” Canada Power, Toronto,Ontario, Canada,September 28-30,2004.

[2] P.Doyon, D.McLaren, M.White, Y.Li, P.Truman, E.Larsen,C.Wegner, E.Pratico, R.Piwko, “Development of a 100 MWVariable Frequency Transformer,” Canada Power, Toronto,Ontario, Canada, September 28-30, 2004.

[3] M. Dusseault, J.M.Gagnon, D.Galibois, M.Granger,D.McNabb, D.Nadeau, J. Primeau, S.Fiset, E.Larsen,G.Drobniak, I.McIntyre, E.Pratico, C.Wegner, “First VFTApplication and Commissioning,” Canada Power, Toronto,Ontario, CANADA, September 28-30, 2004.

[4] D. McLaren, J. Michalec, “The Variable Frequency Transformer(VFT) A Rotating Machine”. GE Energy and American Electric Power(AEP) Doble, 2006.

[5] A. Merkhouf, S. Upadhyay and P. Doyon, “Variable frequencytransformer - an overview”, in Proc. of the 2006 IEEE PowerEngineering Society General Meeting, June 18-22, 2006 pp.

[6] Gesong Chen, and Xiaoxin Zhou, “Digital Simulation ofVariable Frequency Transformers For AsynchronousInterconnection in Power System,” 2005.

[7] Arezki Merkhouf, Pierre Doyon and Sanjoy Upadhyay,“Variable Frequency Transformer—Concept andElectromagnetic Design Evaluation,” IEEE Transactions onEnergy Conversion, vol. 23, no. 4, December 2008.