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UBC Theses and Dissertations

Average-value modeling of brushless dc motors with 120-degree voltage source inverter Han, Qiang


Average-value modeling is indispensable for large and small-signal analysis of electromechanical systems with power electronic drives. Analytical development of accurate average-value models for the brushless dc motor with 120-degree inverter systems is particularly challenging due to the complicated commutation-conduction patterns of the stator currents. This thesis is comprised of several manuscripts that present the research results in this area. First manuscript compares the brushless dc motors with 180 and 120-degree inverters, and the average-value modeling of these systems. In 180-degree inverters, each phase is always connected to the dc source and the average voltages are readily known. In 120-degree inverters, each phase is open-circuited for a fraction of a revolution resulting in discontinuous phase current and different commutation-conduction patterns, which makes it challenging to obtain an accurate average-value model. The second manuscript extends a recently proposed parametric approach to numerical averaging of power electronic converters to construct an average-value model for a brushless dc motor with a 120-degree inverter system. In the proposed model, a proper qd model of the permanent magnet synchronous machine is used, and the inverter dynamics are represented by nonlinear algebraic functions that relate the averaged terminal voltages and currents of the inverter switching-cell. These parametric functions are obtained numerically using the detailed model. The third manuscript presents a different parametric approach to construct a new average-value model that possesses greater accuracy. This new model also uses proper qd model of the permanent magnet synchronous machine but is based on appropriately averaging the transformed stator voltages in both commutation and conduction subintervals. In this approach, the required commutation angle is represented as a nonlinear algebraic function, which is obtained from the detailed simulation. The conducted case studies are based on two typical industrial motors with different electrical time constants, and include hardware measurements, detailed simulation, and comparison with the best-known-to-day published average-value model. The proposed models are shown to be more accurate in a large- and small-signal sense than the previous published model and applicable to motors with a large range of parameters including electrical time constant.

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