Matrix Converter Based Open-end Winding Drives

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Matrix Converter Based Open-end Winding Drives

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2015-08

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A significant portion of all electric power generated is consumed by electric motors employed in commercial, industrial, and transportation sectors. Variable frequency drives (VFDs) are desirable, and in many cases necessary, for superior control performance and efficiency. At present, most VFDs use a `line frequency AC' -> `DC-link' -> `variable frequency AC' architecture, where the front-end converter may or may not support bidirectional power flow and input power factor control. In these drives, the load-end converter is almost always a two- or multi-level voltage source inverter (VSI). The front-end converter may be another VSI, or a line-commutated rectifier. This is a robust architecture that has benefited from extensive use; consequently, the inverter design and control methods are quite standard, and the behavior of all components in the drive system is generally understood. VSI based drives need large capacitors to support the DC-link voltage and significant reactance at the drive input to limit the harmonic current drawn from the grid. The switching common-mode output voltage generated by these drives causes bearing currents and ultimately motor failure. Therefore, even though the drive topology is quite robust, the system suffers from downtime and high maintenance costs because of the unreliable capacitors and bearing failure; and large volume because of the large capacitor and line reactor requirement. Matrix converters offer `line frequency AC' -> `variable frequency AC' conversion without an intermediate DC-link, although an actual or implied soft link may exist. These converters use reactive components only for filtering the harmonics of the PWM frequency. Furthermore, when modulated using rotating vectors, the output common-mode voltage is ideally equal to zero. A major limitation of this modulation technique is a poor voltage transfer ratio of 0.50, and therefore modulation using stationary vectors has received more attention, even though the latter generates switching common-mode voltage at the output, and allows input power factor control only at the expense of the voltage transfer ratio. The output common-mode voltage can be eliminated while maintaining a good voltage transfer ratio using a direct matrix converter based open-end winding drive reported in 2010. This drive topology is also capable of input power factor control; and is expected to have significantly lower reactive element requirements compared to VSI based drives. Indirect topologies for matrix converter based open-end winding drives are also possible. These topologies utilize a three-level inverter structure and employ three converters: the front-end converter converts the input voltages to ordered three-level link voltages. The two load-end converters convert the link voltages to variable frequency voltages to be applied at the two sets of motor terminals. The additional advantages of the indirect approach are a more mature structure, clamp circuit elimination, robust and efficient commutation, lower voltage stress on the switches, and lower losses. The indirect topologies also lend themselves to low-voltage-ride-through without any additional switches. This dissertation presents experimental results from two distinct indirect matrix converter based open-end winding drives. The results demonstrate good common-mode performance, high voltage transfer ratio, and input power factor control. Having established the feasibility of the indirect approach for matrix converter based open-end winding drives, the two indirect drives reported here and the direct drive reported in literature are compared on semiconductor requirements, semiconductor losses, and input/output harmonic content. The most promising matrix converter based open-end winding drive is then compared with state-of-the-art systems on the same criteria, as well as on passive elements, control, and instrumentation requirements. To this end, a new filter design procedure with optimal damping for matrix converter applications is also developed in this dissertation. A comparison of the reactive components used by this filter to the reactive components used in back-to-back VSI systems shows that the matrix converters' passive element requirements are in fact lower than the back-to-back VSI based systems. In summary, this dissertation demonstrates the feasibility of two distinct drive topologies with significant advantages using experimental results. The practical questions pertinent to any new design are answered, and the conclusions have been used to identify the best matrix converter based open-end winding drive topology. Qualitative and quantitative comparison with the state-of-the-art systems reveal a clear advantage in the common-mode voltage related effects and the passive components' sizing.

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University of Minnesota Ph.D. dissertation. August 2015. Major: Electrical/Computer Engineering. Advisor: Ned Mohan. 1 computer file (PDF); xiii, 172 pages.

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Tewari, Saurabh. (2015). Matrix Converter Based Open-end Winding Drives. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/191431.

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