Performance capabilities of two types of variable-reluctance stepping motors

Abstract

The question considered in this paper is the ultimate performance limitations of variable-reluctance stepping motors. It has been shown that solving the problem of matching a stepping motor to a load reveals two key parameters needed to characterize its performance for point-to-point positional control. These parameters are the work per step and the upper limit on stepping frequency when the motor runs free of its load. These parameters apply when certain operating conditions, termed “Class I operation”, are satisfied. The electromagnetically limited performance capability of two types of stepping motors is examined in terms of the above two parameters. The types considered are machines with armature motion parallel to the air gap and those with armature motion normal to the air gap. Expressions are derived that relate the key performance parameters to the physical dimensions of the machines and to basic properties of materials. It is concluded that the machines with motion normal to the air gap potentially offer superior performance to those with parallel motion, at least over size ranges of greatest interest. However the full realization of this superior performance will not be possible until the mechanical problem of converting small linear displacements into shaft rotation is solved in an efficient manner.