Doctoral Dissertations

Date of Award

12-1993

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Electrical Engineering

Major Professor

B.K. Bose

Committee Members

J.M. Bailey, F.W. Symonds, P.W. Schaefer

Abstract

The reliability of power electronics systems is of paramount importance in industrial, commercial, aerospace and military applications. The work presented in this thesis aims at improving the reliability of voltage-fed inverter induction motor drives. Redundancy technique, which is the traditional approach of improving reliability is expensive. Therefore, a new approach based on fault tolerant control strategy is proposed in this thesis. In the beginning, a number of possible failure modes of the converter are identified, analyzed and simulated. Simulation results clearly demonstrates that following certain faults, the drive can continue operation in a degraded mode within a restricted zone of the torque-speed plane. The simulation study also provided important clues for fault detection and fault isolation. Next, fault tolerant control strategy was developed for two commonly occurring inverter faults namely transistor base drive open fault and transistor short circuit fault. Formulating an acceptable post fault steady state operating strategy is by far the most important consideration in the development of fault tolerant control. During the simulation study it was established that the drive can continue operation in single phase mode following these two inverter faults. Operating characteristics of the three phase drive in single phase mode has been extensively investigated. The safe operating area of the single phase drive has been established. In the single phase mode of operation, large low frequency pulsating torque causes large speed ripple and may cause mechanical resonance. It has been established that a load dependant flux program that minimizes input current for a given load torque also reduces the amplitude of the pulsating torque compared to rated three phase flux operation. To reduce the effect of pulsating torque even further a novel torque harmonic elimination algorithm has been developed. The algorithm proposes to inject odd harmonic voltages at appropriate phase angle to neutralize the low frequency pulsating torque. Operating characteristics of the drive have been verified by experiment. In the experimental study a commercial IGBT inverter controlled from a TMS320C30 based DSP board was utilized. The torque harmonic elimination algorithm was implemented by two methods. In the first method, the magnitudes and phase angles of the harmonic voltages were obtained from theoretically determined characteristics where as in the second method, on line search technique was used to arrive at these values. Results from both these methods are presented here. It was observed that either of these two methods yields superior steady state performance in the single phase mode of operation compared to uncompensated and rated three phase flux operation. During analysis of the converter faults it was concluded that some faults do not allow continued post-fault operation. Short circuit of the dc link electrolytic capacitor is one such fault. Electrolytic capacitor are inherently unreliable. To enhance converter reliability, a high frequency active filter replacement of the electrolytic capacitor has been proposed. Analysis, design and simulation of this active filter system is also presented in this thesis. The study indicates that it is entirely possible to eliminate the electrolytic capacitor by high frequency active filter.

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