178 – Analysis of SR Motor I-Psi 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 of flux linkage, inductance and torque for each rotor position when the flowing current value is changed.

I-Psi Characteristics

Fig. 1 shows the SR motor’s I-Psi characteristics, fig. 2 shows the current value 2 A and the magnetic flux density distribution for 10 A.
When the tooth is not facing to each other and is at 0 deg, because magnetic resistance is dominant in the main circuit, there is an almost linear increase in magnetic flux linkage in relation to the current. Also, we understand that particularly when the tooth is facing to each other, the more the current is increased, the more the amount of magnetic flux decreases. This is caused by influence from magnetic saturation of the core, as shown in fig. 2. In addition, the bigger the difference in the flux leakage (inductance) when the tooth is facing to each other and when it is not, increases torque so it is also possible to consider how this could be used.

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