179 – Analysis of SR Motor Static Characteristics

Application Note / Model Data

Overview

With the skyrocketing prices of rare earth magnets, expectations have been rising for SR (switched reluctance) motors because they have a motor format that does not use permanent magnets. SR motors have a simple structure that can achieve solid performance at a low price. However, torque generation depends only upon the saliency between the stator and rotor, so torque variations are extremely large and cause vibration and noise, meaning that the use applications are limited. On the other hand, because of the skyrocketing prices of rare earth metals, the improvement in current control technology, the possibility of optimized designs thanks to magnetic field analysis, and the rising ability to reduce challenges, SR motors are being re-examined.
SR motors operate using the nonlinear region of a magnetic steel sheet, so because the inductance displays nonlinear behavior, it is impossible to carry out advanced projections that are accurate with calculation methods that follow linear formulas. Consequently, it becomes necessary to use the finite element method (FEM), which can handle nonlinear magnetic properties in material and minute geometry.
This example presents an evaluation for each rotor position of the effect on flux linkage (shown as I-Psi characteristics below) when excitation current is changed.

Flux linkage waveform / Inductance waveform / Torque waveform

For each rotor’s position change, the SR motor’s flux linage waveform is indicated in fig. 1, the inductance waveform in fig. 2, and the torque waveform in fig. 3.
From fig. 1 we understand that in the range of 45 deg when the tooth is facing to each other, the more the current is increased, the more the amount of magnetic flux decreases. As a result, as the currency value increases, inductance decreases, which we also understand from fig. 2. These are due to the nonlinear properties of the core material.
In fig. 3 it is possible to estimate the approximate average torque for each current value. Also, it is possible to consider geometry in accordance with setting specifications, such as peaks and reductions in the torque.

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