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.

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.