180 – Analysis of SR Motor Dynamic Characteristics

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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 re-examined.
SR motors create their excitation state by alternating between opening and closing switches in accordance with the position of the rotor’s rotation, however the timing of the alternating causes major changes in the torque properties. Also, it is important not only to increase the torque average and torque constant, but also to consider the optimum switch timing to control vibration and noise.
This example presents how to carry out an analysis with different switch timings to evaluate torque and current in SR motors.

Torque Waveform

The torque waveform for each voltage application width θw with changing the voltage application start angle θs are shown in fig .1 and fig .2.
By changing θs and θw, we understand that the torque waveform has changed.

Current Waveform

The current waveform of the A-phase coil for each voltage application width θw with changing the voltage application start angle θs are shown in fig. 3 and fig. 4.
It is apparent that if the θw is 30 deg, it takes a longer time for the current to decrease to 0A. This is because the rotor tooth is nearing the stator and inductance has increased.

Copper Loss Waveform

The copper loss waveform for each voltage application width θw with changing the voltage application start angle θs are shown in fig. 5 and fig. 6.
It is apparent that depending on the voltage application width, there is a large difference in the waveform. We can understand that if θw is 30 deg, copper loss is bigger in the range of multiple phase currents flowing simultaneously.

Switching Properties

The average torque Tave(θs), torque ripple Tr(θs), torque ripple rate Tr(θs)/Tave(θs), and torque constant Kt for each voltage application width θw with changing the voltage application start angles are shown in table 1 and table 2.
In this case, average torque and torque constant are at a maximum and the torque ripple rate is at a minimum at θw = 30 deg, θs = 5 deg. It is possible to consider optimum switch timing in accordance with required specifications.

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