Chen Yang

To improve the power density of permanent magnet synchronous motors (PMSM) for electric vehicles, round wire winding is gradually replaced by flat wire winding. The large cross-sectional area of the flat wire winding causes significant AC loss and temperature. AC loss includes the eddy current loss and DC loss. The eddy current loss in the high-frequency region needs to be suppressed, and the slot fill rate needs to be guaranteed to suppress the DC loss in the high-torque region. The loss distribution of different layer windings is affected by the distribution of magnetic leakage in the slot. The magnetic leakage is generated by the joint action of armature current and the permanent magnet. Based on the analysis of the causes of AC loss in different layers under various working conditions, this paper proposed the layered loss suppression method (LLSM). The novel hybrid-cross-section flat wire winding (HCSW) is proposed based on the LLSM and verified by experiments. Results show that the loss of HCSW is reduced by 21.4% in peak speed working condition and the slot fill rate is guaranteed. The area of the high efficiency region is increased by 11% compared with the original model.

The heat dissipation of the permanent magnet synchronous motor (PMSM) of an electric vehicle is always worthy of attention, and the design of the cooling system of the PMSM is very important. The rotor rotational oil-spray-cooling structure is widely used in PMSM with hairpin winding. The oil enters from one side of the hollow shaft and is sprayed from the outlets of the front and rear, respectively, which realizes the cooling of the winding at both ends. However, the flow difference on both sides of the shaft will cause the problem of uneven cooling of the winding and the issue of local overheating of the motor. Therefore, the hollow shaft structure needs to be optimized. A 120 kW oil-cooled motor is analyzed as an ex-ample in this paper. The flow field distribution during the rotation of the rotor and the reasons for the uneven shunt are analyzed. The structural parameters of the hollow shaft are optimized by using the Taguchi method and the response surface method. It was found that the new shaft structure can better realize the shunt and reduce the winding and rotor temperature. It provides a guide for the design of the rotational oil-spray-cooling structure of the oil-cooling PMSM.

In the oil-cooling PMSM, the oil that drips on the windings and motor cavity has a risk of entering the air gap when the motor rotates at a high speed. Since the oil viscosity is higher than the air viscosity, the mechanical friction loss increases when oil enters the air gap, resulting in a sharp rise in the rotor temperature. Therefore, based on experiments and simulation, the paths that oil enters air gap are analyzed in this study. And the CFD model of the oil-cooling motor is built to analyze the fluid field at various times. Friction loss and temperature are calculated at various speeds and oil volume ratios. The result shows that the process of oil entering the air gap changes the fluid form and increases the friction loss.

The oil cooling method has been widely used in the permanent magnet synchronous motor with hairpin winding. Because of the irregular shape of the hairpin end winding, there are complex oil circuits in the fluid domain, resulting in a large number of grids and a high computational cost. It is still a challenge to calculate the oil‐cooling performance of the hairpin end winding. Therefore, the porous medium model (PMM) is first proposed to replace the real hairpin end winding to analyse the oil‐cooling performance. By comparing oil volume fraction and velocity at different oil‐supplied conditions using three methods: experiments, real model (the non‐equivalent fluid domain model based on the real hairpin end winding) and PMM, the feasibility of using the PMM to calculate the oil‐cooling performance on the end winding is verified. The oil distribution of three methods is the same. The use of the PMM saves 80% of the number of grids, which improves the simulation efficiency. Relationships between the porosity, permeability and resistance coefficient and the geometry parameters of windings are determined. The results show that the flow field changes greatly with changes in porosity, permeability and resistance coefficient.