Effects of nonuniform airgap and hysteresis on the performance of hybrid stepper motor

This paper presents the results of experimental investigations carried out on the effects of airgap nonuniformity, and also the hysteresis of the soft magnetic material used in stator stack, on the static torque and detent torque profiles of a hybrid stepper motor. It is observed that the nonuniformity of the airgap causes changes in static and detent torque patterns of the hybrid stepper motor. It introduces a position dependent fundamental detent torque along with the conventional fourth harmonic detent torque, thereby changing the detent torque profile, and stable equilibrium points of the motor. The changed detent torque produces undesirable effect on the static torque profile, making it position dependent, and different for different phases of the motor. This effect has been analysed using the permeance functions of the motor under uniform and nonuniform airgap conditions. The stator stack, made out of soft magnetic material having comparatively larger hysteresis loops, retain a small amount of magnetism, even when the stator excitation is withdrawn, and delivers a predominantly fundamental detent torque, similar to the developed torque with reduced stator excitation. As both the airgap nonuniformity, and the hysteresis property of the soft magnetic material, cause for the presence of a fundamental detent torque, analysis of the measured detent torque pattern requires the history of the magnetic condition of the motor, at the time of testing as pre or postexcitation, to identify the respective torque components.