Reliable Multiphase Induction Motor Drives

Abstract

A motor is said to be reliable if it can run at its rated operating condition for a specified period of time. With the widespread use of electric motors in newer applications, reliability is a major concern in terms of safety as well as revenue. About 30-40% of reported failures in induction motors are due to stator faults. It is well known that a stator fault starts as an inter-turn fault within a phase and then propagates into phase-to-phase and phase-to-ground faults that can then lead to complete shutdown of the motor. Two approaches have been taken in this dissertation to make an induction motor drive system more tolerant to stator faults; integration of an inter-turn fault detection method into a five-phase induction motor drive and design of fault-tolerant induction motors.

The phase redundancy of five-phase motors makes it possible to achieve continued operation of the motor with an open phase. However, for true fault tolerance the drive must be able to detect an incipient fault and then transition to post fault operation. A low-cost diagnostic method based on DC voltage injection has been developed for detection of inter-turn faults in five-phase induction motor drive systems. It has been shown that difference in DC current response to an injected voltage before and after an inter-turn fault serves as a reliable fault indicator. The diagnostic is non-intrusive, requires no additional hardware and effectively integrates both fault detection and fault-tolerant control into the motor controller. The method has been successfully implemented and tested on low-cost microcontroller.

The propagation of a stator inter-turn fault into a phase-to-phase fault is worsened in distributed winding induction motors where the different phase windings overlap each other at the end connections. Tooth wound or fractional slot concentrated winding (FSCW) stators have non-overlapping end connections and hence more physical and thermal isolation between the phases as compared to distributed winding stators. While FSCW configurations have been widely used for permanent magnet motors, their adoption for induction motors is a challenge. An FSCW configuration has been designed for outer rotor induction motors by using a dual slot layer stator structure and multilayer windings. Comparison with a conventional induction motor shows an 11% reduction in the copper usage in addition to having non-overlapping phase windings.