94 – Analysis of Detent Torque of a PM Stepper Motor

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Application Note / Model Data

Overview

PM stepper motors are commonly used for positioning of moving parts in small devices such as printers and video equipment. In order for its drive to function with an open loop, the most important characteristics for a stepper motor are controllability and holding torque, and not the motor’s output. Therefore, the desired characteristics are detent torque, which is a non-excitation holding torque, and stiffness torque, which is an excitation holding torque.
A PM stepper motor is made up of a multi-pole magnetized rotor and offset inductors for each phase. In order to reduce their size and number of parts, claw pole inductors are made from folded steel sheet. Because of this, the flow of magnetic flux is three dimensional, 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 Application Note describes how the detent torque can be calculated for a PM stepper motor.

Detent Torque/Magnetic Flux Density Distribution

The detent torque is shown in fig. 1, and the magnetic flux density distribution in the yoke at a rotation of 7.5 deg is shown in fig. 2.
From the geometry, the detent torque seems to have a period of 7.5 deg, but due to the three dimensional flow of magnetic flux between the upper inductor and the lower inductor, the period is 15 deg.

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