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8 : V/F Control Of VSI Fed Three-Phase Induction Motor



Fig. 8 (f). Circuit diagram of V/f control of VSI fed Induction Motor drive.


  • Apply gate signals to the IGBTs T1-T6 from SPWM generator. The gate signals for T1, T4 are generated by comparing the high frequency (fcar) triangular carrier signal vcar with sine control signal vc_R of frequency fo. Similarly, the gate signals for T3, T6 and T5, T2 are generated by comparing the same vcar with sine control signals vc_Y and vc_B of frequency fo. The peak magnitude of control sine signal controls the modulation index ma which in turn controls the rms output voltage. The output frequency is controlled by setting control signals frequency (fo).
  • Observe the waveforms of PWM pulses and across each element of the power circuit with three-phase induction motor. 
  • The output rotor speed, rms voltage, rms currents and their instantaneous waveforms are to be observed for different values of reference speed (Nr*). From reference speed the reference frequency (f*) and reference voltage (V*) commands are calculated such that V/f ratio maintained to be constant. The same can be repeated also for different values of load torque.








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