Rheometer-based Cogging Torque Measurement for Sub-Fractional HP Permanent Magnet Motors

The accurate measurement of the cogging torque of permanent magnet (PM)motors is per se a challenging task. This is especially true for sub-fractional hp PM motors (used, e.g., in auxiliary drives for automotive fan and pump applications), where the cogging torque peak values are often in the sub-milli-Newton meter range, causing conventional measuring methods and devices to fail. This paper presents an unconventional rheometer-based method to measure the cogging torque of these motors. Therefore, the rheometer's ability to measure the reaction of a sample in response to applied forces is investigated regarding the required settings to measure cogging torque waveforms of sub-fractional hp PM motors. The measured torque waveforms are compared with the simulated cogging torque waveform obtained from finite element analysis. The results show that, with the proposed setup and settings, a rheometer can successfully be used to measure cogging torque waveforms in the sub-milli-Newton meter range with excellent accuracy. In addition, as identified in this paper, investigating the measured offset torque for different rotational speeds may enable an iron loss determination and separation of the motor under test.