Hybrid stepper motors are used as actuators for equipment where position detection accuracy is required, such as the joints of robots or rotary tables for machine tools. The rotor has a construction that sandwiches a magnet that is magnetized in the axial direction between two rotor cores that have serrated teeth to create salient poles, and the tips of the stator core’s teeth are shaped like gears as well. Because the rotation resolution is determined by the number of rotor teeth and the number of phases for the drive coil, the design uses large number of teeth, such as 50 or 100, so the angle resolution can be increased .The most important characteristics for a stepper motor are the controllability, the detent torque, which is a non-excitation holding torque, and the stiffness torque, which is an excitation holding torque, and not the motor’s output.
The two-plated rotor core of a stepper motor has an N pole on one side and an S pole on the other, so a multipole magnet is achieved by deviating the saliency of the gear condition by 1/2 pitch. Consequently, the magnetic circuit is 3D. There are also times when the division pitch geometry of the teeth is complicated, so it is necessary to carry out a 3D electromagnetic field analysis using the finite element method (FEM) to proceed with an accurate preliminary study.
This document introduces how the detent torque and stiffness torque can be calculated for a hybrid stepper motor.