PM synchronous motors are used in HEV drives, air conditioner compressors, and various other industries. Especially in case of PM synchronous motors for HEV drives, high-speed rotation is advancing faster than ever to achieve a small size with high output density. The iron core of a PM synchronous motor is composed of magnetic steel sheets, and the iron loss generated in the magnetic steel sheets have generally been evaluated in the post processing of a magnetic field analysis, In this case, the effects of iron loss on the magnetic field analysis were not accounted for. However, the effects of iron loss on the magnetic field can no longer be ignored as rotation speeds are further increased, which causes the risk of not being able to accurately analyze the torque output by the motor when evaluating iron loss in post-processing.
Here we discuss the steps for accounting for eddy current loss occurring in the magnetic steel sheets during a magnetic field analysis, and then analyze the effects on torque.
Torque characteristics
Fig. 1 compares torques between one that does not account for eddy current inside the magnetic steel sheet during magnetic field analysis, and one that does. It shows that transient phenomena occur in the early stages of analysis when accounting for eddy current. When the revolution speed is high like in this motor, the torque is overestimated when eddy current is not accounted for in the magnetic steel sheet.
Line voltage
Fig. 2 shows the comparison between U-V line voltage when the eddy current inside the magnetic steel sheet is accounted for in the magnetic field analysis and when it is not. Like it is with torque, if eddy current inside the magnetic steel sheet is not accounted for, voltage will be overestimated. Fig. 2 shows that accounting for eddy current controls the harmonic component of voltage. This is because changes in the magnetic flux of magnets and current is controlled by the eddy current.
Iron Loss, Efficiency
Fig.3 shows the eddy current distribution (average value between electric angle of 180 to 360deg) obtained in the analysis accounting for eddy current of magnetic steel sheets. Large eddy currents occur in the tip of the teeth part and rotor surface of the stator. Next, the iron loss of analysis accounting for eddy current of magnetic steel sheets and one that is not will be summarized in fig 4. Iron losses are sum of the hysteresis loss and eddy current loss of magnetic steel sheet and eddy current loss of the magnet. However, even in an analysis accounting for eddy current, the hysteresis loss is calculated with post-processing of the magnetic field analysis.
The efficiency calculated from copper loss, iron loss and the output will be shown in fig 5. This table shows that there is when eddy current of magnetic steel sheets is not accounted for in the magnetic field analysis, efficiency is also overestimated by about 1%.
