In this paper a three phase voltage regulator (thyristor based) with RL load and induction motor load is simulated for fixed firing angle first. As was seen from the results obtained that the induction motor response is highly impaired with this fixed firing angle starting. The transient response as well as the steady state response is highly oscillatory in nature. Then a suitable firing scheme was developed to vary the firing angles of each thyristor in reference to the zero crossing of the respective phase voltages. The control circuit was studied for R-L load first and then the induction motor was simulated with this firing angle control scheme. The basic objective of this work is to improve the transient response of the voltage regulator fed induction motor. One basic requirement for the induction motor to have an improved transient response is that the applied voltage to the motor must gradually increase. The same was achieved with the proposed control logic. Next the fault mode (short circuit and open circuit switch fault) analysis of the induction motor was taken up. Till now not much work has been done on this fault tolerant induction motor drive. Here a 2-phase close loop control was adopted to improve response of the induction motor in fault mode. We used both the voltage control loop as well as the current control loop to do so. Unfortunately not much can be done on this fault tolerant operation as the time was very short. Till the speed and torque pulsation during a short circuited switch fault was greatly improved by adopting this two phase control strategy.
Simulation of Three Phase Voltage Controlled Soft Switching Start of Induction Motor Drive