A Cycloconverter - Synchronous Motor Drive for Traction Applications

An electric drive suitable for traction is described consisting of a cycloconverter and a synchronous motor. Essentially, the same system can be used with either single-phase or three-phase input with frequencies around 50-100 Hz. The motor speed range is not limited by the input frequency. The cycloconverter thyristors are commutated by the source voltage at low motor speed and by the motor electromotive force (EMF) at higher speed. The transition between the two regions is smooth and automatic. Only 16 low- switching speed thyristors are required for the single-phase input system. The short time overload capability of this drive at low speed is excellent, and full range regenerative, as well as dynamic braking, is readily accomplished. Compared to dc motors occupying the same space, three-phase ac motors, both synchronous and induction, offer higher output power, higher efficiency, and the elimination of brush and commutator maintenance. Compared to inverters, cycloconverters have higher efficiency and lower cost and weight.