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Cogging Torque reduction in Surface-Mounted Permanent Magnet Motors

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Problem description

Permanent Magnet Synchronous machines (PMSMs) are widely used in traction applications and electric vehicles, where steady-state operation, high efficiency, and torque density are needed, such as Toyota Prius, etc.

In this application note, the cogging torque of a Surface-Mounted PMSM in [1] is computed and validated using EMWorks2D software inside SOLIDWORKS. Figure 1 illustrates a 2D model of the studied motor designed using SOLIDWORKS. Then, the impact of PMs’ segmentation on the cogging torque is investigated using the “multi-configuration” feature of SOLIDWORKS.

2D cross section of the original SMPMSM using SolidWorks.

Figure 1 - 2D cross section of the original SMPMSM using SolidWorks.

Simulation and results

1- No-Load analysis of the original model

Magnetic field mappings of the original PMSM: The magnetic field mapping of the original surface mounted PMSM motor is simulated for an initial angular position of 0 degrees using EMWorks2D inside SOLIDWORKS, as presented in Figure 2.

Magnetic field of the original PMSM using EM2D
Figure 2 - Magnetic field of the original PMSM using EM2D.
 

Considering the above figures, one can notice that the magnetic field is maximum when the stator tooth is facing the totality of the pole.

Cogging torque of the original PMSM: The cogging torque versus the angle position of the angle over one period is computed using EMWorks2D inside SOLIDWORKS. This result has been validated, as shown in Figure 4.

Cogging torque of the original PMSM versus the angular position

Figure 4 - Cogging torque of the original PMSM versus the angular position.
 

The cogging torque of the original PMSM has a peak-to-peak value of 5.58N.m and an average value of -0.0185N.m.

2- Effect of magnet segmentation on the cogging torque of the PMSM

Two case studies were performed to investigate the impact of magnet segmentation on no-load torque, using EMWorks2D inside SOLIDWORKS, where the PMs are first segmented into two then three blocks each, as indicated in Figure 5.

 2D cross section of the segmented SM-PMSMs using SolidWorks. (a) Case of one segmentation, (b) Case of two segmentations
Figure 5 - 2D cross section of the segmented SM-PMSMs using SolidWorks. (a) Case of one segmentation, (b) Case of two segmentations.
 

Magnetic field mappings of the segmented PMSMs:The magnetic field mappings of the PMSMs having one and two segmentations of the PMs are illustrated in Figures 6 and 7, respectively.

Magnetic field mapping of the PMSM having 2 PMs per pole. (a) 2D model, (b) zoom

Figure 6 - Magnetic field mapping of the PMSM having 2 PMs per pole. (a) 2D model, (b) zoom.
 
 Magnetic field mapping of the PMSM having 3 PMs per pole. (a) 2D model, (b) zoom.
Figure 7 - Magnetic field mapping of the PMSM having 3 PMs per pole. (a) 2D model, (b) zoom.
 

Cogging torque of the segmented PMSMs:The cogging torque is computed using EMWorks2D for different segmentation cases, as shown in Figure 8. To better highlight, the effect of the PMs’ segmentation on the cogging torque, its peak-to-peak, and average values are given in table 1.

Cogging torque for different PMs’ segmentations

Figure 8 - Cogging torque for different PMs’ segmentations.
 
Table 1 - Cogging torque for different PMs’ segmentations.
Cogging torque 1 PM per pole 2 PMs per pole 3 PMs per pole
Peak-to-peak value (N.m) 5.4629 3.355 3.99478
Average value (N.m) 0.01486 0.011802 0.029759

Considering table 1, one can deduce that the average value of the cogging torque is almost null. The differences between the three configurations could be explained by computation errors.

Considering the second configuration with 2 PMs per pole, the peak-to-peak value of the cogging torque is significantly reduced. Whereas, it has slightly increased for the third configurations.

Summary

The magnet segmentation technique can be used to reduce the peak-to-peak value of the cogging torque, as can be seen in Table 1. However, this technique can possibly lead to its increase instead of decreasing it, if performed randomly.  Therefore, an optimization of the segmentation angle is needed to minimize the cogging torque peak-to-peak value.

References

[1] A. Frikha, “Contribution to the Multiphysics Modelling of Permanent Magnet Synchronous Machines”, Master Thesis, National Engineering School of Sfax, Tunisia, 2019.