Ground fault detection and protection method for a variable speed ac

Abstract

A method and apparatus for detecting the presence of a ground leakage path in an electric motor system including an alternating current (ac) electric motor coupled for receiving electric power from a switching inverter of the type having, for each phase of the motor, a pair of serially connected electronic switching devices connected between relatively positive and negative direct current (dc) voltage buses with the switching devices being alternately gated into and out of conduction for providing switched electric power to the motor. The inverter includes a current detector for detecting current in the relatively negative voltage bus. The method comprises initially applying a gating pulse each time the motor is energized to the switching device of each phase connected to the negative voltage bus and identifying a ground leakage path when such gating pulse produces current at the current detector. Preferably, the first gating pulse is of very short time duration and the process of applying gating pulses is repeated for each of a plurality of continuously increasing pulse widths until a selected maximum width is attained without generating current. Thereafter, the motor is started in a conventional manner.