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

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Variable frequency transformer (VFT) is used as a controllable bidirectional transmission device that can transfer power between asynchronous networks and functionally is similar to back-to-back HVDC. This paper describes the basic concept of a New Model Variable Frequency Transformer (NMVFT). NMVFT is a new technology which is used for v/f control of induction motors. A digital simulation model of NMVFT and its control system are developed using MATLAB. The out power thus generated in v/f mode has been practically verified for the speed control of a three-phase induction motor. Thus constant speed-torque characteristics were achieved.
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  • 1. ACEEE Int. J. on Electrical and Power Engineering, Vol. 02, No. 01, Feb 2011New Model Variable Frequency Transformer(NMVFT) A Technology for V/f Control of Induction Motors Farhad Ilahi Bakhsh, Mohammad Irshad, and Shirazul Islam Department of Electrical Engineering, A.M.U, Aligarh, IndiaEmail: [email protected]: [email protected]: [email protected] Variable frequency transformer (VFT) is used as awhich is supplied by the dc drive motor. If torque is appliedcontrollable bidirectional transmission device that can transfer in one direction, then power flows from the stator windingpower between asynchronous networks and functionally isto the rotor winding. If torque is applied in the oppositesimilar to back-to-back HVDC. This paper describes the basic direction, then power flows from the rotor winding to theconcept of a New Model Variable Frequency Transformerstator winding. If no torque is applied, then no power flows(NMVFT). NMVFT is a new technology which is used for v/f through the rotary transformer [3, 4].control of induction motors. A digital simulation model ofNMVFT and its control system are developed using MATLAB. The worlds first VFT, which was manufactured byThe out power thus generated in v/f mode has been practicallyGE, installed and commissioned in Hydro-Quebecs Langloisverified for the speed control of a three-phase induction motor. substation, where it will be used to exchange power up toThus constant speed-torque characteristics were achieved.100 MW between the asynchronous power grids of Quebec (Canada) and New York (USA). Figure 1 shows a simplifiedIndex TermsVariable frequency transformer (VFT),one-line diagram of the Langlois VFT, which is comprisedAsynchronous, New Model Variable Frequency Transformer of the following: a rotary transformer for power exchange, a(NMVFT), V/f control, MATLAB.drive motor to control the movement of the rotor and to transfer power, a collector to connect the rotor windings with I. INTRODUCTION the outside system via electric brushes [5 - 7].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 a Figure 1. Photograph of Langlois 100 MW VFTspeed of system-stator, cutting the stator coils with the sameII. NMVFT MODEL AND SYSTEM DESCRIPTIONspeed, and cutting the rotor coils with a speed of system_rotor. A. NMVFT ModelAnd the angle frequency of the resulting inductive potentialat stator and rotor windings is synchronous with their currentsIn the model, NMVFT is a three phase singly fedrespectively. A stable power exchange between the twoslip ring type induction machine. The stator winding is en-asynchronous systems is possible. The transferred power andergized with three phase ac source and the rotor winding isits direction are controlled by the torque applied to the rotor, kept open. The rotor is coupled to a controllable constant speed drive i.e. dc shunt motor via mechanical coupler and 2011 ACEEE11DOI: 01.IJEPE.02.01.50

2. ACEEE Int. J. on Electrical and Power Engineering, Vol. 02, No. 01, Feb 2011scope is connected to measure the magnitude of voltage andacross the rotor winding as well as its frequency becomesfrequency of voltage across the rotor winding. Figure 2 shows double the rated value.the circuit diagram of NMVFT. Thus a constant V/f characteristics is achieved. Thisoperation of NMVFT is verified digitally by MATLABSimulation and practically. III. DIGITAL SIMULATION OF VFTA. MATLAB SimulationIn the view of MATLAB simulink, NMVFT is atype 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 is Figure 2. NMVFT circuit diagramsimulated by using a constant block. And then we could usethis simulated model, as shown as Figure 3, to solve electric B. NMVFT Operationsystem of NMVFT. The three phase ac supply is applied to the statorwinding 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 givenbyEr = 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 andFigure 3. MATLAB Simulation diagram of NMVFT fr = frequency of the induced voltage in the rotor circuitThe operation of NMVFT includes the following:B. Simulation Figures and Results i) If the rotational speed of dc shunt motor is zero then theWhen the rotational speed of dc shunt motor isvalue of slip is equal to one as a result the emf induced acrosszero.the 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 2011 ACEEE 12DOI: 01.IJEPE.02.01.50 3. ACEEE Int. J. on Electrical and Power Engineering, Vol. 02, No. 01, Feb 2011 b) The direction of rotation of rotor is opposite to a)The direction of rotation of rotor is same asthe air gap field:- that of the air gap field:- When the rotational speed of dc shunt motor is equalto the synchronous speed i.e. relative speed is double ofsynchronous speed.-50*pi 1000500 Voltage (volts)0When the rotational speed of dc shunt motor is equal tothe synchronous speed i.e. relative speed is zero.-500 -10000 0.5 1 1.52 2.5 33.5Time (seconds)4 x 10 200 100In this way we can control output voltage from ratedvoltage to twice of rated voltage and frequency from ratedV o ltage (vo lts) 0frequency to twice the rated frequency i.e. 50 100 Hz in -100India. The emf induced across the rotor winding versus its -200frequency graph achieved is shown as: -300 0 0.5 11.52 2.5 3 Time (second) x 104In 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:Figure 4. MATLAB Simulation results showing variation of rotor circuitvoltages with time and voltage versus frequency graphs. 2011 ACEEE 13DOI: 01.IJEPE.02.01.50 4. ACEEE Int. J. on Electrical and Power Engineering, Vol. 02, No. 01, Feb 2011IV. PRACTICAL ANALYSISFigure 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 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 the Figure 7 shows the variation of rotor voltage with itsflux of dc shunt winding varies, resulting in variation of speedfrequency of table II.of 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:- 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, 40

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