Reduction of torque ripple induced acoustic emissions in permanent magnet synchronous motors

Torque ripple exists at the output of permanent magnet synchronous motors (PMSMs) as a result of current measurement error, non-sinusoidal back-electromotive force (EMF), and cogging torque. Acoustic emissions caused by torque ripple coupling through the mechanical system and interacting with the motor housing limit the applications available to PMSMs. Since part of the cogging torque component is based on manufacturing error, motor design alone may not eliminate all torque ripple. One possible method for the reduction of torque ripple is to use the inverse of a torque ripple estimate to adjust the output torque via the motor controller. This digest uses a coherence calculation to show that acoustic emissions are a direct result of torque ripple and outlines a control method that uses a microphone to sample the acoustic emissions and determine an estimate of, and then to reduce, the torque ripple for a number of orders simultaneously. The transfer function between the torque reference and the sensor is also determined as part of this method. A comparison of results between a high quality microphone and low cost electret microphone is also made. Experimental results show that there is good coherence between torque ripple and acoustic emissions at the orders associated with torque ripple. The proposed method was shown to be effective in significantly reducing the acoustic emissions caused by torque ripple, using both the high quality microphone and electret microphone, in some instances by more than 23dBA. After reduction, the magnitude of the acoustic emissions were similar to that of the background noise at other frequencies.