Extending the low frequency operation of load commutated inverters with torque control techniques

The authors propose extending the operating range of load commutated inverter (LCI) based synchronous machine drive systems from zero speed to full speed by applying concepts originally developed for soft switching power converters. Improved performance is achieved by adding small capacitors on the load side and applying a novel switching strategy. Thyristor commutation is assured and current is well-controlled by the switching strategy. Device stresses under these conditions are within rated values. Through the use of torque control techniques, at rated current from standstill to approximately 0.4 per unit speed, a controlled torque is achieved in a synchronous machine fed by an LCI. Experimental verification on a small LCI is presented