Finite elements applied to synchronous and switched reluctance motors

Abstract

This presentation covers two examples of finite-element analysis applied to practical design problems: one in relation to a synchronous motor, and the other in relation to a switched reluctance motor. The emphasis is on speed of calculation and therefore the finite-element process is restricted to a 2-dimensional one, even though three-dimensional effects are important in both examples. The first example is the calculation of cogging torque in synchronous motors, especially brushless motors as used for servos and many similar applications where torque ripple has to be maintained at a low level; automotive electric power steering is another important example. The effect of skew is included. The second example is the determination of the set of magnetization curves for a switched reluctance motor. The objective is to produce an acceptable lamination geometry in a reasonably short time without resorting to 3-dimensional computation or scaling from previous designs. In both cases we are concerned with reluctance torques. In the synchronous motor we are trying to minimize harmonic reluctance torques associated with stator slotting, while maximizing the reluctance torque produced by the interaction of the rotor saliency with the fundamental space-harmonic of the stator MMF distribution. In the switched reluctance motor we usually contemplate only a single torque component, the total reluctance torque of one phase, and the objective is to maximize it. Both examples use the flux-MMF diagram technique: in one case at the level of individual mesh elements, and in the other at the level of integrals over the entire domain. (4 pages)