Study of Magneto-Thermal Problems in Low-Speed High-Torque Direct Drive PMSM Based on Demagnetization Detection and Loss Optimization of Permanent Magnets

Abstract

Low-speed high-torque direct drive permanent magnet synchronous motor (LHDD-PMSM) can suffer from a typical magneto-thermal coupling problem during long periods of continuous load operation, resulting in temperature rise and leading to reversible demagnetization of the permanent magnets (PMs), thus affecting the motor’s operational performance. In this paper, we experimentally verify the existence of the magneto-thermal problem and investigate methods to reduce the magneto-thermal coupling problem. The paper first researches the on-line detection method of reversible demagnetization. As reversible demagnetization in motors is difficult to identify, we propose a method by measuring the transient PM back EMF of the motor to detect the magnetic properties of PMs in the field. And furtherly, an on-line tester is developed to detect the reversible demagnetization of the LHDD-PMSM. Secondly, this paper analyses the main sources of PM losses and proposes an electromagnetic optimized model combining mixed structure magnetic slot wedge and rotor magnetic insulation slot, so that the eddy current loss of PMs can be greatly weakened. Finally, a bi-directional coupled magneto-thermal simulation model is established with the comparison to a temperature rise experiments, and the model can be used to illustrate the improvement of the magneto-thermal problem of LHDD-PMSM by the loss reduction method proposed in the paper.

Keywords

• No-load back EMF
• permanent magnet eddy current loss
• reversible demagnetization
• slot wedge
• tooth harmonics