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Compressor overheat protection is a mechanism designed to prevent the compressor motor from being damaged. When the temperature of the motor or the compressor itself exceeds a certain threshold, built-in or external protection devices are activated to cut off the power supply to the compressor, thereby protecting it. The refrigeration compressor draws in the low-temperature, low-pressure gaseous refrigerant from the system and sends it directly into the compressor’s chamber; the refrigerant drawn in first cools the motor before being compressed. Therefore, the degree of superheat of the inhaled refrigerant vapor is an important factor determining whether the compressor overheats. The main reasons for excessive exhaust temperature are as follows: high return air temperature, high motor heating, high compression ratio, high condensing pressure, and improper selection of refrigerant. 1. High return air temperature. The level of the return air temperature is relative to the evaporation temperature. To prevent backflow, the return air circuit generally requires a return air superheat of 20°C. If the return air pipeline is not properly insulated, the superheat will far exceed 20°C. The higher the return air temperature, the higher the intake and exhaust temperatures in the cylinder. For every 1°C increase in the return air temperature, the exhaust air temperature will rise by 1–1.3°C. 2. Motor heating: In recirculating air-cooled compressors, the refrigerant vapor is heated by the motor as it passes through the motor chamber, which raises the suction temperature in the cylinders once again. The heat generation of a motor is influenced by power and efficiency, while the power consumption is closely related to displacement, volumetric efficiency, operating conditions, frictional losses, and so on. For the back-air cooling type semi-hermetic compressor, the temperature rise of the refrigerant in the motor chamber is generally between 15 and 45°C. In air-cooled compressors, the refrigerant does not pass through the windings, so there is no issue of motor heating. 3. Excessively high compression ratio: The exhaust temperature is greatly influenced by the compression ratio; the higher the compression ratio, the higher the exhaust temperature. Reducing the compression ratio can significantly lower the exhaust temperature; methods to achieve this include increasing the intake pressure and reducing the exhaust pressure. The suction pressure is determined by the evaporation pressure and the resistance in the suction line. Increasing the evaporation temperature can effectively raise the suction pressure, rapidly reduce the compression ratio, and thereby lower the exhaust temperature. Some users mistakenly believe that the lower the evaporation temperature, the faster the cooling rate; this idea actually has many problems. Although lowering the evaporation temperature can increase the freezing temperature difference, the cooling capacity of the compressor decreases, so the freezing speed is not necessarily faster. Moreover, the lower the evaporation temperature, the lower the coefficient of performance, while the load increases, the operating time lengthens, and power consumption rises. Reducing the resistance in the return air circuit can also increase the return air pressure; specific methods include replacing the dirty and clogged return air filter in a timely manner, and minimizing the length of the evaporation tube and the return air circuit. Furthermore, a lack of refrigerant is also a factor contributing to low suction pressure. The refrigerant should be replenished promptly in case of leakage. Practice has shown that reducing the exhaust temperature by increasing the intake pressure is simpler and more effective than other methods. The main reason for excessive exhaust pressure is too high condensation pressure. Insufficient cooling surface area of the condenser, fouling, inadequate volume of cooling air or water, and excessively high temperatures of the cooling water or air can all lead to high condensation pressure. It is very important to select an appropriate condensation area and maintain a sufficient flow rate of the cooling medium. At high temperatures, the operating compression ratio of air-conditioning compressors is relatively low; after cooling, this compression ratio increases significantly, resulting in very high exhaust temperatures. The cooling system is not able to keep up, leading to overheating. It is necessary to avoid excessive use of the compressor and to operate it at the lowest possible pressure ratio. In some low-temperature systems, superheating is the primary cause of compressor failures. 4. Anti-expansion and gas mixing: Once the intake stroke begins, the high-pressure gas remaining in the cylinder clearance undergoes an anti-expansion process. After anti-expansion, the gas pressure returns to the intake pressure, and the energy consumed in compressing this gas is lost during the anti-expansion process. The smaller the clearance, the lower the power consumption resulting from back expansion on one hand, and the greater the amount of air drawn in, which in turn **increases** the compressor’s efficiency ratio. During the reverse expansion process, the gas absorbs heat by coming into contact with the high-temperature surfaces of the valve plate, the top of the piston, and the top of the cylinder; as a result, the gas temperature does not drop to the intake temperature at the end of the reverse expansion. Only after the anti-expansion is complete does the actual inhalation process begin. Once the gas enters the cylinder, it mixes with the counter-expanding gas, causing the temperature to rise ； On the other hand, the mixed gas absorbs heat from the wall surface and heats up. Therefore, the gas temperature at the beginning of the compression process is higher than the intake temperature. However, since the anti-expansion process and the suction process are extremely brief, the actual temperature rise is very limited—generally less than 5°C. Anti-expansion is caused by the clearance in the cylinder, and it is a drawback that traditional piston compressors cannot avoid. If the gas in the valve plate’s exhaust holes cannot be discharged, reverse expansion will occur. 5. Compression temperature rise and type of refrigerant: Different refrigerants have different thermophysical properties, resulting in varying increases in exhaust temperature after undergoing the same compression process. Therefore, different refrigerants should be selected for different cooling temperatures. Conclusions and recommendations: Within its operating range, a compressor should operate normally without any overheating phenomena such as excessively high motor temperature or discharge gas temperature. Compressor overheating is an important fault indicator, suggesting that there are relatively serious problems with the refrigeration system, or that the compressor is being used and maintained improperly. If the cause of compressor overheating lies in the refrigeration system, the problem can only be resolved by improving the design and maintenance of the refrigeration system. Replacing it with a new compressor cannot fundamentally eliminate the overheating problem.