The motor starts running at room temperature (the temperature of each part is the same as the ambient temperature), and the temperature continues to rise. When it is higher than the ambient temperature, it continues to absorb heat and slowly heats up. On the other hand, it begins to radiate heat to the surroundings. When the motor is in a thermal equilibrium state and the temperature no longer rises, the difference between the motor temperature and the ambient temperature is called the motor temperature rise. That is: temperature rise = motor temperature – ambient temperature, in units of K.

The maximum allowable temperature of the motor is the maximum temperature that the winding can withstand. When used for a long time at this temperature, the physical, mechanical, chemical and electrical properties of the insulating material do not undergo significant malignant changes. If this temperature is exceeded, the performance of the insulating material will undergo qualitative changes or cause rapid aging. Therefore, the maximum allowable operating temperature of the insulating material is determined based on its economic service life.

The maximum allowable temperature of the motor is determined, and the limit of temperature rise at this time depends on the temperature of the cooling medium. The cooling medium in general motors is air, and its temperature varies with regions and seasons. In order to manufacture motors that can be used throughout the country throughout the year and clarify unified inspection standards, the national standard stipulates that the temperature of the cooling air is set at 40°C.

When the motor is running, the greater the output power, the greater the current and loss, and the higher the temperature, but the maximum temperature must not exceed the maximum allowable temperature of the insulation. Therefore, the long-term maximum output power allowed by the motor (that is, the capacity or rated power of the motor) is limited by the maximum allowable temperature of the insulation, or the capacity is determined by the maximum allowable temperature of the insulation. The rated power indicated on the motor nameplate refers to the maximum output power when the temperature does not exceed the maximum allowable temperature of the insulation under the standard ambient temperature (40°C in my country) and the specified working mode.

The temperature rise limit basically depends on the grade of the insulation material, but it is also related to the temperature measurement method, the heat transfer and heat dissipation conditions of the measured part, and the maximum allowable operating temperature of the insulation material. When the maximum temperature of the surrounding cooling medium (such as air) is determined, the temperature rise limit of the motor components can be specified according to the maximum allowable operating temperature of the insulation material.

The allowable temperature rise refers to the allowable limit of the motor temperature compared with the ambient temperature, also called the winding temperature rise limit. During the operation of the motor, the temperature rise of the motor windings and the core parts is not completely equal, but there is a small difference, which is mainly determined by process factors and ventilation conditions. (In order to avoid permanent damage to the motor caused by differences caused by such factors, a 5°C margin is generally left in the measurement.)

Then there is the measurement error. In the temperature measurement, the more accurate method is the resistance method, which requires the resistance value of the winding to be measured quickly and accurately after the motor stops running, and then converted into temperature. This method also requires a 5°C margin. The second is the temperature measurement component embedding method. The error of this measurement method can be smaller, but the embedded component is not necessarily the highest temperature point, so there is also an error, and generally a 5°C margin is also left.

The allowable temperature rise of the motor is determined by the insulation level of the motor. Different insulation levels have different allowable temperature rises. The insulation level of the motor refers to the heat resistance level of the insulating material used, which is divided into A, E, B, F, and H levels.

For example: if the ambient temperature is 30℃ and the motor temperature is 80℃, the temperature rise is 50K. The temperature rise on the motor nameplate refers to the allowable temperature rise of the winding at the specified ambient temperature (generally set at 35℃). For another example: if the temperature rise on the nameplate is 60K, it means that when the ambient temperature is 35℃, the temperature rise of the winding shall not exceed 60K, that is, the temperature of the winding shall not exceed 95℃.