Switched reluctance motor drives with fully pitched windings

Abstract

Switched reluctance motors with fully pitched windings are a relatively recent advancementin motor technology having only been in existences ince the early 1990's. They have been shown previously to offer greater torque per unit copper loss, and hence higher torque density, than conventional switched reluctance machines with short pitched windings. Early work by Mecrow and Barrass has demonstrated operation of prototype machines, developed and assessedv arious methods of control strategy, and made some comparisons of machine efficiency and inverter rating. The results presented here build on this early work by, in essence, examining the aspects of machine design, control strategy and inverter topology that affect drive performance and cost. Detailed comparisons of inverter rating and machine efficiency are made under equal conditions with the various methods of excitation that are possible. This is achieved with results from a test rig, including temperature rise tests, and the use of accurate dynamic simulation. The latter is developed to accurately model the motor with its strong mutual coupling between phases, various inverter topologies and the details of the controller such as digital PWM. As a result comparisons between simulated and measured results are shown to be very good. The fundamentals of machine design are examined with a view to optimising the machine for fully pitched windings. Previous work has indicated that good results are achieved when a conventional machine is simply rewound, however it is shown that further improvements can be made. Proposals are made to improve the drive in terms of both machine performance and power electronic rating. A search method is proposed that optimises current waveshape for either maximum torque per unit copper loss, or smooth torque for lowest loss. The method works over the entire speed range, as the rate of change of flux linkage is taken into account. Three alternative power electronic converters are developed, one of which is also particularly suitable for the short pitched winding machine. Aspects of silicon rating, current controllability, and current sensor requirements are discussed.