Abstract:

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.