Unifying approach to the static torque of stepping-motor structures

Important stepping-motor structures are compared and their logical relationship to a basic doubly-salient machine is demonstrated. This leads to a single expression for holding torque, applicable to the majority of motors, comprising a product of the following terms: (a) a theoretical limiting mean torque, (b) a factor allowing for finite excitation, (c) the fraction of rotor volume effectively utilised on any one step, (d) a shape factor for the torque/angle curve. The analysis is cast in dimensionless form, both in respect of component terms in the torque expression and of motor variables. In particular, a critical value of m.m.f., calculable from gap geometry, is shown to be the significant unit of applied excitation. Advantages offered by this approach are quick calculation, a ready means of comparing different motors and insight into physical behaviour. Sample calculations are presented for a variety of motors and several individual problems discussed. Support for the theory is also provided by a range of experimental work and an appraisal of commercial data.