| Summary: | In the last decades, the use of permanent magnet machine drives has experienced a sustained growth owing to their high efficiency and power density figures and due to their inherent suitability for direct-driven applications. However, and despite being highly reliable, the fact that the excitation field in a permanent magnet machine cannot be turned off at will has made engineers reluctant to employ these drives in safety critical applications in the past. Various techniques have been proposed in the related literature to grant fault-tolerance to a permanent magnet machine drive.
This thesis starts by reviewing previous work on the matter and by analyzing the different fault-tolerant approaches. After the various methods are briefly discussed, a comparison among the distinct techniques is established, from which the approach of splitting the drive in multiple independent phases emerges as one the most promising design procedures. This requires that the drive is designed to provide the maximum possible magnetic, electrical, thermal and physical isolation between phases. In order to limit the high magnitude currents arising from a short-circuit fault, a further requirement is that the permanent magnet machine is designed to have a high enough phase self-inductance. The previous requisites are naturally met in permanent magnet machines making use of fractional-slot concentrated-windings. Additionally, multiphase systems have shown to provide a number of advantages over the traditional three-phase systems; specially regarding fault-tolerance and the attainable level of performance after a fault. Owing to the aforementioned reasons, this thesis focuses on the design and analysis of fractional-slot concentrated-winding multiphase fault-tolerant permanent magnet synchronous machines.
Following the review on fault-tolerant permanent magnet drive systems, the design principles that allow to select the most appropriate winding arrangements for fractional-slot concentrated-winding multiphase machines are reviewed. From the research conducted, it is found out that the traditionally proposed rules to select the most adequate configurations are restricted to odd phase number machines or to specific winding configurations. In order to fill this gap, an analytical procedure to evaluate the merits of different winding configurations in terms of magnetic isolation and regardless of the geometry of the machine is established.
A design methodology incorporating the previous winding selection criteria is proposed. Based on this methodology, a five-phase fault-tolerant machine prototype is designed and manufactured. The design process for the prototype, including the analysis of the required specifications and design constraints, is thoroughly discussed.
Next, an analytical drive model suitable for fault analysis is developed. The model serves as a tool to predict the behavior of the designed machine under different fault conditions and to test post-fault remedial strategies. Specifically, the post-fault operation under winding open-circuit faults, terminal short-circuit faults and transistor open and short-circuit faults is investigated.
For the previous fault scenarios, modified control strategies that allow to improve the post-fault performance of AC machine drives are proposed. In particular, a unified approach to compute suitable current references for winding open-circuit and terminal short-circuit faults is derived. The method, aimed at minimizing the stator copper losses while preserving the main harmonic of the air-gap magnetomotive force, is general and valid for any phase number drive and different supply conditions. Experimental tests demonstrate the intrinsic fault-tolerant capability of the prototype machine and the adequacy of the proposed modified control strategies in reducing the parasitic effects arising from the different fault conditions. Furthermore, by adopting the proposed remedial actions, it is possible to operate the machine drive under fault scenarios for which the system previously became unstable.
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