Starting system for chopper controlled motor-commutated thyristor

Abstract

If the thyristor switching devices of a chopper controlled inverter are normally motor-commutated during running operation, a starting system is needed to initially rotate the motor fast enough to develop a sufficient back EMF in the motor for motor commutation to occur. This is achieved by regulating the operation of the chopper to produce bus current pulses and by gating the thyristors on in predetermined sets and in a prescribed sequence to current pulse energize the motor to effect step-by-step rotation. After a set of conducting thryistors have supplied a bus current pulse to the motor, the reactive energy that builds up and becomes locked or stored in the inductances, through which the motor current flows, must be dissipated in order to commutate the conducting thyristors off and reduce the motor current to zero before the next set of thyristors are turned on. The required energy dissipation is accomplished by a circuit path which is effective during the intervals between the bus current pulses from the chopper. A d-c voltage source, in the circuit path causes the reactive energy to be rapidly absorbed, to permit faster turn on of the next set of thyristors and consequently faster starting. Once the motor has reached a speed sufficiently high to permit motor commutation to occur, and SCR is gated on to by-pass the d-c voltage source and provide a low loss free-wheeling path for reactive energy during running operation of the system.