Abstract
Accurately measuring the cogging torque, hysteresis torque, and iron losses of permanent magnet (PM) motors is a challenging task. This is especially true for sub-fractional horsepower (SFHP) variants used, e.g., in auxiliary drives for automotive fan and pump applications. Their cogging torque and hysteresis torque peak values are often in the sub-milli-Newton meter range, causing conventional measuring methods and devices to fail as especially friction impedes the measurement significantly. Moreover, their iron losses are hard to determine and usually merely estimated in the literature. This article presents an unconventional rheometer-based method to determine both 1) the cogging torque and hysteresis torque of SFHP PM motors and 2) their iron losses by evaluating the observed offset torque. Two SFHP outer-rotor PM motor topologies for fan applications are analyzed, i.e., a claw-pole motor and a salient-pole motor. The results show that, with the proposed setup, settings, and evaluation, a rheometer can successfully be used to determine the cogging torque and hysteresis torque waveforms in the sub-milli-Newton meter range and the iron losses of SFHP motors, both with excellent accuracy.
Highlights
I T IS well known that cogging torque can be a significant source of noise and vibrations [1], [2]
Cogging torque is typically studied by means of finite-element analyses (FEA) [3], and different measurement methods have been proposed [4]–[14], all with focus on integral horsepower (IHP) machines
The findings show that the rheometer can be used to determine the cogging torque and the hysteresis torque as well as to determine and separate the iron losses of sub-fractional horsepower (SFHP) permanent magnet (PM) motors
Summary
I T IS well known that cogging torque can be a significant source of noise and vibrations [1], [2]. This article presents an unconventional rheometer-based method to measure the cogging torque and hysteresis torque of SFHP PM motors in the sub-milli-Newton meter range and the iron losses, both with excellent accuracy. This article extends the results presented in [37], including the analysis of another PM motor topology for comparison which has a lamination stack, separating the hysteresis and cogging torque components from the measured torque waveforms, and discussing the assumptions made, the measurement uncertainty, and the run-out accuracy. As opposed to integral and fractional horsepower motors, measuring the cogging torque of SFHP motors (e.g., motors in the 1-W range, which have cogging torque peak values in the sub-milli-Newton meter range) is most challenging because the hysteresis torque and especially the bearing friction of either the MUT or the measuring device itself can significantly influence the results.
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