Static torque profiles of a hybrid stepper motor having relative eccentricity between stator and rotor axes

A smooth and controlled driving load for a hybrid stepper motor requires detailed knowledge of the motor’s static and detent torque profiles. Uncertainties to measured torque profiles may be due to rotor/stator concentric misalignment as one possible factor. This article presents the results of a detailed experimental analysis on a stepper motor’s relative eccentricity between the rotor and stator axes. The nonuniform airgap not only introduces a position-dependent fundamental detent torque along with the conventional fourth harmonic detent torque, but also changes the static torque profile. The sum effect on static torque is a changed torque equilibrium point, dependent both on rotor position and motor phase. Detent torque harmonic component analysis of a set of 0.5° hybrid stepper motors, with rotor-to-stator eccentricity as the control variable, showed how increased eccentricity exceeding a given limit will cause a loss of stable equilibrium of the detent torque at each step position