6 – Analysis of the SR Motor Torque Ripple

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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 reexamined.
SR motors operate using the nonlinear region of a magnetic steel sheet, so the inductance displays nonlinear behavior that distorts the excitation current waveform a great deal, making it 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 as well as transient currents.
This Application Note explains how to carry out a torque analysis that changes the switch conversion timing and evaluate both the torque ripples and average torque in an SR motor.

Torque Waveform

Fig. 1 shows the torque waveform when voltage is applied to U-phase at 50 deg and turned off at 80 deg.
It is apparent that torque ripple peaks coincide with declines in torque that occur when the phase torque switches.

Torque Ripple Comparison

Fig. 2 shows a graph comparing a load analysis at voltage switching angles of 55, 56, and 57 deg in the U-phase with the torque waveform of a switching angle of 50 deg. A comparison of each of the average torques and the torque ripples at a voltage switching angle of 50 deg is listed in Table 1.
Fig. 2 shows that the switch timing can significantly reduce the torque ripple.
Comparing a 56 deg voltage switching angle with one of 50 deg, the torque ripple has been contained by approximately 20%, but at the same time there has been a decline in the average torque, so further studies are necessary in order to improve the torque ripple without allowing the average torque to decline, such as considering increases in the overlap of the torque in each phase by changing either the number of phases or the width of the salient poles.

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