**CIRCUIT DIAGRAM**

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

**PROCEDURE:**

- Apply
gate signals to the IGBTs T
_{1}-T_{6} from SPWM generator. The
gate signals for T_{1}, T_{4} are generated by comparing the
high frequency (f_{car}) triangular carrier signal v_{car} with
sine control signal v_{c_R} of frequency f_{o}. Similarly, the
gate signals for T_{3}, T_{6} and T_{5}, T_{2} are
generated by comparing the same v_{car} with sine control signals v_{c_Y}
and v_{c}__{B} of frequency f_{o}. The peak magnitude
of control sine signal controls the modulation index m_{a} which in
turn controls the rms output voltage. The output frequency is controlled by
setting control signals frequency (f_{o}).
- 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 (N
_{r}^{*}).
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.