A Standstill Method to Measure Electromagnetically Induced Torque Ripple of Permanent Magnet Synchronous Machines

The accurate measurement of the electromagnetically induced torque ripple of the permanent magnet synchronous machines (PMSMs) is essential to experimentally validate the effects of various minimization techniques applied during design optimization. The conventional torque-ripple measurement methods often fail to isolate the electromagnetically induced ripple of the test machine. The components of a conventional test setup, such as the inverter and controller, load machine, bearings, coupling systems, and mechanical resonances, all have the potential to contribute to the overall ripples measured using existing test procedures. Utmost care must be taken to isolate the test machine induced torque ripple from the other components. This article proposes a simple, standstill test method to measure the ripple of the air-gap torque of a PMSM with high accuracy. In the proposed method, at incremental rotor positions, the rotor is blocked, and the stator windings are supplied with the current that corresponds to an operating condition at which the torque ripples need to be measured. Due to the standstill nature of the proposed method, many uncertainties related to the rotation are eliminated. A theoretical analysis was carried out to show that the induced ripples of the air-gap torque of PMSMs are independent of speed and supply frequency, and hence, the measurement of these ripples in a standstill is possible. The proposed method was experimentally validated using two PMSMs with different torque profiles. The validation of the results demonstrates an excellent agreement between the measured torque ripple and those obtained from the FEA calculation.