Experimental Study on Behavior of Coolants, Particularly the Oil-Cooling Method, in Electric Vehicle Motors Using Hairpin Winding

Abstract

This paper analyzes the characteristics of oil behavior in the oil-cooling of motors with hairpin winding to understand how to maximize cooling performance. The oil cooling is performed by directly spraying oil onto the motor components. The results show that as the temperature of the oil increases, the viscosity decreases, and the oil film is formed more evenly; however, oil splashing also increases. Similarly, as the flow rate increases, oil splashing also increases, but the amount of oil forming the oil film increases. However, the oil film is not affected by the rotor’s rotation. In contrast, the immersed oil is found to be closely related to the rotor’s rotation. As the rotational speed increases, the immersion oil is mixed with the air, and oil churning occurs. The mixing phenomenon increases as the temperature and flow rate of the oil increases. The higher the oil level, the greater the oil churning. As the oil is mixed with air, the heat transfer coefficient decreases, which adversely affects the thermal management of the motor. As a result, when considering the oil film and the immersion oil, the optimal oil temperature, flow rate, and oil level are at 60 °C, 0.140 kg/s, and 85 mm, respectively. The results of this paper give important information about EV motor cooling and can contribute to the development of high-performance motors.