Date of Award

Spring 2008

Document Type

Dissertation - Restricted

Degree Name

Doctor of Philosophy (PhD)


Electrical and Computer Engineering

First Advisor

Demerdash, Nabeel A. O.

Second Advisor

Yaz, Edwin E.

Third Advisor

Brown, Ronald H.


This dissertation presents fault-tolerant/"limp-home" strategies of ac motor soft starters and adjustable-speed drives (ASDs) when experiencing a power switch open-circuit or short-circuit fault. The present low-cost fault mitigation solutions can be retrofitted into the existing off-the-shelf soft starters and ASDs to enhance their reliability and fault tolerant capability, with only minimum hardware modifications. The conceived fault-tolerant soft starters are capable of operating in a two-phase mode in the event of a thyristor/SCR open-circuit or short-circuit switch-fault in any one of the phases using a novel resilient closed-loop control scheme. The performance resulting from using the conceived soft starter fault-tolerant control has demonstrated reduced starting motor torque pulsations and reduced inrush current magnitudes. Small-signal model representation of the motor-soft starter controller system is also developed here in order to design the closed-loop regulators of the control system at a desired bandwidth to render a good dynamic and fast transient response. In addition, the transient motor performance under these types of faults is investigated using analytical closed-form solutions, the results of which are in good agreement with both the detailed simulation and experimental test results of the actual hardware. As for ASDs, a low-cost fault mitigation strategy, based on a quasi-cycloconverter-based topology and control, for low-speed applications such as "self-healing/limp-home" needs for vehicles and propulsion systems is developed. The present approach offers the potential of mitigating both transistor open-circuit and short-circuit switch faults, as well as other drive-related faults such as faults occurring in the rectifier bridge or dc-link capacitor. Furthermore, some of the drawbacks associated with previously known fault mitigation techniques such as the need for accessibility to a motor neutral, the need for larger size dc-link capacitors, or higher dc-bus voltage, are overcome here using the present approach. Due to its unique control algorithm, torque pulsations are introduced as a result of the non-sinusoidal current waveforms. Simulation and experimental work have been performed to demonstrate the efficacy and validity of the conceived fault-tolerant solutions for induction motor fault mitigation applications.



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