# Research Progress of Enhanced Thermal Evacuation and Cooling Technology for High-Speed Motors
## 2 Different Application and Cooling Requirements for High-Speed Motors
### Common Cooling Methods and Coolants  
In current applications of motor cooling, liquid cooling through the housing and forced ventilation cooling in the axial gap are two common cooling methods; sometimes both are used.  
For the rotor with small thermal loads, gas cooling is a direct, effective, and economical cooling method. Gaseous coolants are usually used for convection-enhanced cooling in the gap, mostly using air. [PERSON] et al. [50] created an air-cooled structure for a 600-kW motor, as presented in Figure 18. The motor had a radial air inlet on one side and an axial air outlet on the other side. This allowed airflow to circulate inside the motor for forced convection cooling.  
In motors that generate a large internal heat load, liquid cooling is required for heat evacuation and cooling [51]. The liquid coolant normally used for convection cooling of the housing is applied with oil and other refrigerants in addition to the commonly used water. Compared with air-cooling, its cooling effect improves, but it is necessary to design airtight water-cooled housing. The water-cooled motor is shown in Figure 19, with cooling water channels and water inlet and outlet openings in the motor housing [52].  
Figure 19: Liquid-cooled construction motor [52].  
Figure 17: Schematic of the loop rotating heat pipe [49].  
Figure 18: Single air-cooled structure motor [50].  
When the motor power and speed requirements are higher, a combination of the air and liquid cooling methods need to be considered. [PERSON] et al. [53] investigated high-speed motors for compressors with a combined air-liquid cooling form, as illustrated in Figure 20. Cooling air is drawn into the motor from the left side, flows into the gap, and subsequently exits on the right side. Cooling water is then convected into the motor housing for heat evacuation.  
Based on the requirements of the application environment., other coolants have also been applied. [PERSON] et al. [54] studied a 1 kW high-speed motor cooled using passing liquid nitrogen. This is a motor used for low-temperature superconductivity to increase power for aircraft propulsion. [PERSON] et al. [55] studied the use of R245 as a refrigerant for cooling high-speed motors, as shown in Figure 21. [PERSON] et al. [56] demonstrated a cooling system for unmanned aircraft drive motors. The system utilizes a spiral grooved self-cooling enclosure and internal oil cooling to reduce maximum temperature of the motor spindle by approximately 30 \({}^{\circ}\)C.  
Most motor cooling forms are air-cooled, liquid-cooled, or a combination of both. Table 1 lists the different forms of motor cooling published in the literature. Air-cooled motors have relatively large axis diameters and relatively low rotational speeds. Liquid-cooled motors tend to rotate at higher speeds than air-cooled motors. When ultra-high speeds are demanded, motors are typically equipped with air-liquid composite cooling.  
Figure 21: Motor cooling systems with R245 [55].  
Figure 20: Air–liquid composite motor cooling structure [53].