Maximum Pulling Force Calculation of Permanent Magnet Tractor Motors in Electric Vehicle Applications

In electric vehicle applications, the majority of the traction motors can be categorized as Permanent Magnet (PM) motors due to their outstanding performance. As indicated in the name, there are strong permanent magnets used inside the rotor of the motor, which interacts with the stator and causes strong magnetic pulling force during the assembly process. How to estimate this magnetic pulling force can be critical for manufacturing safety and efficiency. In this paper, a full 3D magnetostatic model has been proposed to calculate the baseline force using a dummy non-slotted cylinder stator and a simplified rotor for less meshing elements. Then, the full 360 deg model is simplified to a half-pole model based on motor symmetry to save the simulation time from 2 days to 2 hours. A rotor position sweep was conducted to find the maximum pulling force position. The result shows that the max pulling force happens when the rotor is 1% overlapping with the stator core. The impact of asymmetric air gap as well as temperature is analyzed for comprehensiveness. It shows that the asymmetric air gap can cause about 2.7% higher force. Finally, the dummy non-slotted cylinder stator is swapped with a slotted stator for better model fidelity. FEA shows that the slotted stator can have 21% lower force than a dummy cylinder stator. Although the method and conclusions proposed in this paper are based on a case study of one motor, they are still valid and applicable to other traction motors in a more generalized sense and can serve as a good reference for the EV motor industry for a safer and more efficient manufacturing assembly.