A new thyristor inverter without external commutation for AC motor control

The paper presents a new thyristorised inverter without the necessity of external commutation circuits for 3 phase induction motor control. The DC input is fed to the inverter through a fast acting load-commutated DC to DC thyristor chopper without any filter in between. The chopper output consists of a series of high frequency (1 to 3 kHz) saw-tooth shaped voltage pulses which are appropriately transferred to the three phase load by the inverter. Barring the high frequency capacitor used in the chopper, there are no other force-commutation elements. Only one snubber circuit at the input end of the inverter is sufficient. The turn-on and turn-off losses are small since the switchings are mostly soft. The paper explains the operation of the inverter with its control circuits and presents experimental results regarding voltage and current waveforms when used to drive a laboratory size induction motor