Design of Stator and Rotor and Pole Arcs in SR Motor

Design of Stator and Rotor and Pole Arcs in SR Motor

The torque characteristics of SRM depend on the stator–rotor pole overlap angle, pole geometry, material properties, number of poles, and number of phases (Miller 2002). The torque output and torque ripple of SRM are sensitive to stator and rotor pole arcs (Arumugam et al 1988, Faiz et al 1993). The machine needs to be designed with sufficient pole overlap between the stator and rotor to ensure enough torque during phase commutation. Because of their direct effect on inductance and torque determination the selection of optimum pole arc configuration is vital to minimize torque ripple (Zaim et al 2002). Unfortunately, in most cases, the minimization of the torque ripple implies degradation of other important features, such as the starting and the mean torques. Therefore in most of the literature average torque and torque ripple are considered as performance measures while determining optimum pole arc (Hur et al 2004, Nabeta et al 2008). However, due consideration should be given to copper loss and available window area for winding. With a wider stator pole arc copper loss is increased and the available window area for winding is reduced. Hence in this work the problem of determining optimal pole arc is formulated as a multiobjective optimization problem with maximization of average torque, minimization of torque ripple and copper loss as objectives. This chapter focuses on computational intelligence based optimization techniques combined with static finite element simulation for determination of optimal pole shapes of SRM.

In order to produce a unidirectional torque, there must be an overlap between the poles of the rotor and the poles of the excited stator phase. The overlap angle should be greater than the step angle, otherwise there will be dead zone where no torque is produced. In order to get the largest possible variation of phase inductance with rotor position, the interpolar arc of the rotor must exceed the stator pole arc. A further constraint on the pole arcs is that usually the stator pole is made slightly smaller than the rotor pole arc (Miller 1993). This permits a slight increase in the slot area, the copper winding cross section, and the aligned/unaligned inductance ratio. The constraints are set according to the rules of the feasible triangle (Lawrenson et al 1980) , which defines the range of combinations normally permissible. The variation in performance of SRM defined by different points of feasible triangle is considerable which depends on various factors, such as torque ripple, the starting torque, and the effect of saturation.