Study of magnet asymmetry for reduction of cogging torque in permanent magnet motors

In order to reduce cogging torque of permanent magnet (PM) motor, the lower harmonics of cogging torque should be reduced. It is found that if the slot number per pole is not an integer, new lower harmonics of cogging torque may be introduced when the method of asymmetry magnet is adopted. To reduce cogging torque, both the lower original and newly introduced harmonics of cogging torque should be reduced. In this paper, the analytical expression of cogging torque with asymmetry magnet is got. Based on the distribution of the square of residual flux density of PM, the analytical expression of its Fourier coefficient can be got. If the Fourier coefficient of the square of residual flux density of PM is reduced by asymmetry magnet, the corresponding harmonics of cogging torque can be reduced. So in order to eliminate the lower harmonics of cogging torque, we can let Fourier coefficient (which is corresponding to the lower harmonics of cogging torque) to be zero. By solving the system of equations, we can get the shifting angles of magnets. It is verified at last that the cogging torque can be greatly reduced by the shifting angles got in this paper.