129 – Analysis of a PM Stepper Motor Accounting for Magnetization

Application Note / Model Data

Overview

Stepper motors are commonly used for positioning in printers and digital cameras. In PM stepper motors, the motor’s characteristics are determined through design of the magnetization state of the permanent magnet used. It is necessary to accurately evaluate this magnetization state in order to improve the precision of estimations of a PM stepper motor’s characteristics.
Detailed measurements of the magnetization distribution inside a magnet are difficult to make. However, it is possible to accurately find the magnet’s magnetization because it can be obtained through analysis of the magnetization field from the magnetization device, using the finite element method (FEM).
In this analysis, a magnetization device model is created and a magnet is magnetized. The magnetization distribution and surface magnetic flux density of the magnetized magnet, and the induced voltage of a PM stepper motor with the magnetized magnet integrated into it, are then obtained.

Magnetization Distribution and Surface Magnetic Flux Density of the Magnet

The magnetization distribution of the magnet magnetized by the magnetization device is shown in fig. 1, and fig. 2 shows the radial direction component of a two-pole span of the surface magnetic flux density at 0.2 mm from the magnet surface.
It can be seen from fig. 1 that magnetization is strong in all parts of the magnet under complete magnetization, and magnetization is weak toward the inner side of the magnet under incomplete magnetization. This is why in fig. 2, the maximum surface magnetic flux density value under complete magnetization is around 0.8 T, and is smaller at around 0.3 T under incomplete magnetization. This shows that it is necessary to consider the magnetization ratio and recoil relative permeability in order to grasp the magnetization state.

Induced Voltage Waveform

The induced voltage waveform of the stepper motor rotating at 1,000 r/min is shown in fig. 3. The figure makes it clear that the maximum induced voltage value goes down from around 7.5 V under complete magnetization to around 5.0 V under incomplete magnetization due to the difference in the strength of the magnet’s magnetization, and the PM stepper motor’s characteristics are different depending on whether the magnetization ratio and recoil relative permeability are accounted for or not.

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