This paper presents a method for reducing the cogging torque and ripple torque of interior permanent magnet synchronous motor (IPMSM) based on the magnetic field harmonic offset method. This method establishes the internal correlation between cogging torque harmonics and ripple torque harmonics. The suppression or cancellation of magnetic field harmonics in the rotor pole is utilized as transmission link to simultaneously weaken or eliminate lower order harmonics of cogging torque and ripple torque, which can improve operating quality of the IPMSM and obtain an acceptable total average torque. A mathematical and physical model of harmonic offset method for cogging torque is established, the distribution characteristics of permeability harmonics and field harmonics that affect cogging torque are analyzed, the analytical expression for the electromagnetic torque of the IPMSM including reluctance torque is derived, and the collaborative suppression mechanism of cogging torque and ripple torque, as well as common solutions, are studied. Finally, the suppression law of cogging torque and operating ripple torque is verified by the finite element simulation, and the compromise selection principle of permanent magnetic pole is summarized. Due to the absence of the average torque of motor in the offset method, the effects of effective pole arc of the combined rotor on the torque ripple and torque-speed characteristic curve of the IPMSM are compared and evaluated.

In order to improve the power density of the permanent‐magnet synchronous machine and reduce the cost of effective materials, this paper investigated approaches of power‐density improvement and discussed the influence of flux‐weakening level and salient ratio parameters on the characteristics of torque and power for the interior permanent magnet synchronous machine (IPMSM) based on the permanent magnet (PM) minimisation. Firstly, the model of PM minimisation can be established based on the principle of equal area of stator and rotor magnetomotive force. According to the minimisation model, a formula is derived for calculation of structural parameters of the permanent magnets in the IPMSM. Besides, the relation among the interior multilayer permanent magnets is established. Then, comparing and analysing the extremum characteristics of the maximum torque output and maximum power output of the motor under different parameters, the regular influence on the IPMSM can be summarised between the parameter and the internal power factor angle. So, it can be proposed that the corresponding internal power factor angle with respect to the golden section ratio of the maximum torque output is around −35°. Finally, combined with the results of the finite element and analytical calculation, a 45 kW PM motor for electric vehicles was manufactured and tested in no‐load, load and thermal experiment, respectively. The peak torque of the prototype lasted for 94 s and reached 2.3 times the rated torque. Compared with the conventional PM motor used for the electric vehicle, the power density was increased by more than 0.5 kW/kg which can verify the correctness of the theoretical analysis.