Improved Reliability in Solid-State AC Drives by Means of Multiple Independent Phase Drive Units

Abstract

Phase redundancy is a proposed technique for improving the reliability of large solid-state adjustable-frequency ac drives. Based on the more general concept of parallel redundancy, this new technique relies heavily on the inherent capability of a general n-phase ac motor to continue operation with (n -1) or less of its stator phases excited. Motor performance degradation which inevitably accompanies such operation is sensitive to a variety of system parameters including the number of stator phases and the type of excitation source. A thorough evaluation of (n - 1)-phase excitation performance characteristics of an n-phase squirrel-cage induction motor is presented in this paper. Under favorable conditions, a large percentage of the motor's balanced excitation rating can be retained during postfault conditions of steady-state (n -1)-phase excitation. The n-phase-drive units must be conservatively designed to withstand transient stresses associated with fault-induced transitions from balanced n-phase to (n-1)-phase excitation. Key analytical results have been experimentally verified by tests conducted on a 5-hp six-phase squirrel-cage induction motor. Results of this investigation support the basic soundness of the phase-redundancy concept.