Contents
1. Introduction
2. Water-cooled PMSM
3. Loss analysis
4. Thermal analysis
5. Comparison with measurement
6. Calculation cost
7. Summary
8. References

1. Introduction

In the development of high-performance motors, effective temperature management is critical to maximizing their performance. Therefore, a machine design that simultaneously considers both magnetic properties and cooling performance is essential. However, conventional design methods often perform magnetic and thermal analyses separately, which frequently leads to design rework.
Furthermore, while thermal equivalent circuits are commonly used for thermal design, they often require calibration of thermal resistance values with physical prototypes, making thermal analysis difficult in the design phase when a physical model is not yet available. A simulation-based approach using Computational Fluid Dynamics (CFD) is also an option, but its high computational cost makes it impractical for use during the design stage. Consequently, a key challenge in the design phase is the ability to predict motor temperature with high speed and without the need for physical prototypes.
To solve this problem, this paper adopts a method that combines the Finite Element Analysis (FEA) and a thermal equivalent circuit. By calculating thermal resistances and heat transfer based solely on motor and cooling specifications, this approach enables a purely simulation-based temperature prediction. We applied this method to a permanent magnet synchronous motor (PMSM) that is water-cooled by a cooling jacket and validated its predictive accuracy by comparing it with measured data.

2. Water-cooled PMSM

Fig. 1 and Table 1 show the motor to be analyzed and its specifications. The motor is cooled by a cooling jacket built into the housing. Tables 2, 3, and 4 show the specifications of the cooling jacket, winding, and the gaps between components.
Temperature measurements were taken at two operating points with different rotational speeds. The rotational speed and current command values for the operating points are shown in Table 2. The carrier frequency was set to 6 kHz for both.

Fig. 1 Motor Assembly
The left image shows the motor with its case, and the right image is a cross-section of the motor. In the right image, the upper and lower pipes serve as the inlet and outlet of the cooling jacket, where the coolant flows along the outer diameter of the stator core.