# Research Progress of Enhanced Thermal Evacuation and Cooling Technology for High-Speed Motors
## 4 Future Development of High-Speed Motor Cooling  
Lower losses, higher efficiency, higher lifetime and less cost are the objectives of the evolution of high-speed motors. Expectations for ultra-high speeds and high power are increasing as the requirements for high-speed motor applications improve. Desired characteristics of current motors suitable for electric vehicles include high efficiency, wide speed range, high starting torque, and wide constant power range power factor. These properties are highly dependent on the motor's core length, rotor and stator slots, and air gap [83]. In addition, the high-speed motor for the Mars Exploration Rover requires a supply voltage of 40 V, an output power of 50 W, and a weight of 25 g, which means high efficiency and high-power density [84], which can be used in outer space to minimize core losses and reduce the heat generated by the motor. [PERSON] et al. found that motor power density has become a major concern in electric motor research, as shown in Figure 22. The application direction thus shifted to power systems for electric propulsion, fuel cell systems, robotic actuators and other small high-speed motors [6]. Heat dissipation and evacuation in the motor is still a challenge that needs to be improved urgently.  
For machine tools, the purpose of enhanced heat evacuation is to improve machining accuracy, and the focus is on spindle thermal elongation. In other applications, high-speed motors are required to maintain rated performance and pursue overall small temperature drop. On the basis of the evaluation of heat evacuation paths for high-speed motors, the following research directions for motor heat evacuation are summarized.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Reference**} & **Rotor Diameter** & \multirow{2}{*}{**Power (kW)**} & \multirow{2}{*}{**Rotational**} & \multirow{2}{*}{**Cooling Method**} \\  & **(mm)** & & & **Speed (rpm)** & \\ \hline  
5 & 223 & 60,000 & Air and liquid \\ \hline \hline \end{tabular}
\end{table}
Table 1: Cooling method of the motor.  
1. Reduction of heat sources: Explore ways to decrease motor losses by approaching the source of heat. Reduce heat generation by optimizing stator and rotor materials as well as slot patterns to lower motor losses during operation. Achieve the purpose of reducing the overall temperature rise of the motor.
2. Utilization of latent heat of coolant: Motor casing cooling generally uses liquid convection heat transfer. Considering the two-phase heat transfer of the coolant and fully utilizing the phase change latent heat of the coolant, the cooling capacity and heat dissipation efficiency of the motor can be improved.
3. Optimization of the design of the shell cooling channels. According to the motor's actual structure, the cooling channel design is optimized to reduce flow resistance and ensure uniformity of flow and temperature distribution.
4. Establishment of air-cooling runners. Convection heat transfer efficiency is improved by methods such as built-in fans and enlarged leaf grilles. An external circulating air system can also be installed.
5. Strengthen motorized spindle heat evacuation. Establish motorized spindle axial core cooling or heat pipe runner to reduce heat transfer thermal resistance and effectively decrease spindle temperature rise and thermal deformation.