Consequently, in order to accurately evaluate a motor that has skew applied, one needs a magnetic field analysis simulation that uses the finite element method (FEM), which can account for a detailed 3D geometry, instead of studies that use the magnetic circuit method or a 2D magnetic field analysis.
This Application Note presents the use of magnetic field analysis to evaluate the magnetic flux density distribution and cogging torque in each part of an SPM motor with a step skewed magnet.
Magnetic Flux Density Distribution
Fig. 1 shows the magnetic flux density distribution at the rotation angle of 60 degrees, and fig. 2 shows the frequency component of the magnetic flux density waveform in the gap. The upper limit of the frequency component is 1440 Hz, and the primary component of the frequency is 120 Hz because there are 4 poles and the rotation speed is 30 rps.
As shown in fig. 1, the magnetic circuit is changed by the application of the step skew. As shown in fig. 2, the ratio of the frequency component at 360 Hz, which is the fundamental frequency of the cogging torque, is 13 % when the magnet is not step skewed and 4 % when the magnet is step skewed, so the cogging torque may be reduced by applying the step skew.