Optimum tooth-geometry for hybrid stepper motor using finite element analysis

Hybrid stepper motors are used in space, military and industrial applications. The design of the hybrid stepper motor for achieving a specific performance necessitates the choice of appropriate tooth-geometry. In this paper, a detailed account of the results of 2-D and 3-D finite element (FE) analyses, conducted with different tooth-width/tooth-pitch ratios (t/λ) and also with various tooth/slot shapes such as rectangular, trapezoidal, triangular and circular, is presented. The study has shown that the rectangular tooth/slot shape is better with respect to torque density. The best results are obtained when t/λ lies between 0.38 and 0.4. The use of equal tooth-pitch on stator and rotor structures provide nearly 10% more torque than the case with unequal tooth-pitch, but with corresponding increase in detent torque. For the requirements of maximum torque density, high detent torque and better positional accuracy, equal pitch slotting with equal tooth widths has to be adopted. Instead, for maximum torque density, less detent torque, better positional accuracy and smooth static torque profile, different pitch slotting with equal tooth width has to be provided. The above results are verified for two stepper motors, one having equal t/λ, of 0.5 and the other having different t/λ ratios of 0.38 and 0.4 in stator and rotor respectively. The results of this work will be useful to the designer for selecting an appropriate tooth-geometry for specific performance requirements.