An optimal maximum torque per ampere strategy for switched reluctance machines

Abstract

Addressing drawbacks inherent to switched reluctance machines (SRM) could allow its distinctive characteristics to broaden applications from selective, niche industrial roles to various engineering sectors. Phase winding isolation is a contributing factor of advantageous characteristics unique to SRMs when compared to the industry workhorse, pulse width modulation driven induction machines. Specifically, this increases fault tolerance, simplifies the manufacturing winding process, and allows the machine to remain in a locked rotor position safely without concern of faulting, thus contributing to a greater overall robustness. On the contrary SRMs, when compared to other electrical machine types, are most notably criticized for requiring complex control strategies to achieve optimal operation, having greater overall current requirements, operating at higher speeds, and creating greater acoustic noise and torque ripple. Cumulatively, these shortcomings alienate the SRMs commercial and industrial popularity, ultimately limiting its full potential from being exploited.Since SRM torque production is typically non-linear, various techniques have been developed in order to produce the maximum torque per given current excitation, i.e. maximum torque per ampere (MTA). The "balanced commutator" control strategy uses a look-up table to account for the non-linearity of the SRMs torque-angle characteristic, yet does not totally optimize the copper or iron losses, current requirements, or effectively mitigate the torque ripple. A stochastic search technique based on evolutionary algorithms, particle swarm optimization (PSO), allows for MTA profiles to be obtained that optimize the shortcomings inherent to the balanced commutator technique. This work presents a novel MTA SRM control strategy based on the PSO technique. The optimum phases current profiles of a 4-phase, 8/6 pole SRM are obtained such that copper losses and torque ripple are minimized while achieving the desired torque at specific rotor positions. Results are compared against the balanced commutator method.