Masters Theses

Brian Lain

Date of Award

12-1996

Degree Type

Thesis

Degree Name

Master of Science

Major

Electrical Engineering

Major Professor

Jack S. Lawler

Committee Members

J. M. Bailey, F. W. Symonds

Abstract

In this study the post-failure torque production of three-phase and five-phase brushless dc motors is compared for inverter single phase failures. Brushless dc motors are conventionally configured as three-phase motors, however a five-phase brushless dc motor can produce more torque after a single phase failure. To allow equitable comparison of these two systems an "equivalent" basis is established. The "equivalent" systems will be equal with respect to physical size, dc supply voltage, dc supply current, rated power, and rated torque. After a failure in the inverter the three-phase and five-phase systems have different abilities to produce torque. Two types of failures were chosen to represent typical failures and to provide a basis to compare the post-failure torque production of the systems. Specifically the failures used for comparison are one transistor open and two transistors open in one leg of the inverter. For nominal conditions the equivalent five-phase inverter requires a lower transistor current rating. It is found that the nominal equivalent five-phase inverter has a volt-ampere product that is five-sixths the value required of the equivalent three-phase system. After a failure of one or two transistors in one phase, currents of the five-phase system exceed nominal values. An increased transistor current rating or a limiting of the phase currents is required for the five-phase system to operate safely. Although the transistor currents exceed nominal values, the five-phase motor can develop full torque even after two transistors fail open in one phase of the inverter circuit. With increased transistor current ratings the five-phase motor can safely produce full torque even after a failure of one complete phase. The three-phase system can only produce 33.33% of the average nominal torque for one transistor failed open and only 66.67% of the average nominal torque for two transistors failed open in one phase. If the transistor current rating of the five-phase inverter could not be increased then the five-phase system could be operated with the phase currents limited to the maximum allowed limit. The limiting of the phase currents would reduce the torque capability of the five-phase system. With the current limited the five-phase system could produce 80% and 60% of average nominal torque with one and two transistors failed open, respectively. The post-failure starting torque capabilities of the equivalent three-phase and five-phase systems are significantly different. The three-phase system can only produce torque for two-thirds and one-third of the rotor positions for one and two transistors failed, respectively. Therefore, the three-phase motor can not start after a failure for many of the rotor positions. After a failure the five-phase system with the current limited can produce at least 50% of rated torque for all rotor positions. The five-phase system with adequate transistor current ratings can produce full rated torque for all rotor positions after either one or two transistors have failed open in one leg of the inverter.

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