Q&A on motor temperature issues

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The insulation class of a motor refers to the heat resistance level of the insulating materials used, and is classified into grades A, E, B, F, and H. The allowable temperature rise denotes the maximum increase in the motor’s temperature compared to the surrounding ambient temperature. Temperature rise refers to the difference between the temperature of the stator windings and the ambient temperature when the motor operates under rated conditions. The ambient temperature is defined as 35°C or below; if no specific value is indicated on the nameplate, it is considered to be 40°C. Insulation temperature classes: Class A, Class E, Class B, Class F, Class H. Maximum allowable temperatures (°C): 105, 120, 130, 155, 180 respectively. Limits on temperature rise of windings (K): 60, 75, 80, 100, 125 respectively. Reference operating temperatures (°C): 80, 95, 100, 120, 145 respectively. In electrical equipment such as generators, insulating materials represent the weakest link. These materials are particularly susceptible to accelerated aging and damage due to high temperatures. Different insulating materials exhibit varying levels of heat resistance; consequently, electrical devices made from different insulating materials have differing capabilities to withstand high temperatures. Therefore, a maximum operating temperature is specified for most electrical equipment. Based on the ability of different insulating materials to withstand high temperatures, 7 maximum allowable temperatures have been specified for them. Arranged in order of increasing temperature, these are: Y, A, E, B, F, H, and C. Their respective allowable operating temperatures are: 90, 105, 120, 130, 155, 180, and above 180°C. Therefore, Class B insulation indicates that the heat resistance temperature of the insulation used in this generator is 130°C. Users must ensure that the insulation material of the generator does not exceed this temperature while it is in operation, in order to maintain its proper functioning. Insulation materials with insulation class B are mainly made from mica, asbestos, and glass fibers that have been bonded or impregnated with organic adhesives. Question: At what temperature can a regular motor operate normally? What is the maximum temperature that the motor can withstand? Answer: If the temperature measured on the motor cover exceeds the ambient temperature by more than 25 degrees, it indicates that the motor’s temperature rise has gone beyond the normal range. Generally, the temperature rise of a motor should be less than 20 degrees. Typically, motor coils are wound using enameled wire, and at temperatures above around 150 degrees, the enamel coating on this wire peels off due to the high temperature, resulting in a short circuit in the coil. When the coil temperature is above 150 degrees, the temperature of the motor housing is around 100 degrees; therefore, if the temperature of the housing is used as a reference, the maximum temperature experienced by the motor is 100 degrees. Question: The temperature of the motor should be below 20 degrees Celsius; that is, the temperature of the motor’s end cover should be less than 20 degrees Celsius compared to the ambient temperature. But what is the reason why the motor heats up to over 20 degrees Celsius? Answer: The direct cause of motor overheating is high current. It is usually caused by coil short circuits or open circuits, demagnetization of the magnets, or low motor efficiency; under normal circumstances, it results from the motor operating at high current for a long period of time. Question: What causes a motor to heat up? What is this process like? Answer: When a motor operates under load, there is power loss within the motor, which is ultimately converted into heat. This causes the motor’s temperature to rise above that of the surrounding environment. The value by which the motor temperature is higher than the ambient temperature is called the temperature rise. Once there is a temperature rise, the motor must dissipate heat to its surroundings ； The higher the temperature, the faster the heat dissipation. When the amount of heat generated by the motor per unit time equals the amount of heat dissipated, the motor temperature ceases to rise and remains at a constant level; that is, it reaches a state of equilibrium between heat generation and heat dissipation. Question: What is the typical allowable temperature rise per click? Which part of the motor is most affected by the motor’s temperature rise? How is it defined? Answer: When an electric motor is operating under load, in order to make the best use of its capabilities, the load it carries—or in other words, the output power—should be as large as possible (provided that mechanical strength is not a concern). However, the greater the output power, the greater the loss power, and the higher the temperature. We know that the part in a motor that is most vulnerable to high temperatures is the insulating material, such as enameled wire. There is a limit to the temperature resistance of insulating materials. Within this limit, their physical, chemical, mechanical, and electrical properties remain stable; their service life is generally around 20 years. Beyond this limit, the lifespan of the insulating material is drastically reduced; it may even burn out. This temperature limit is known as the allowable temperature of insulating materials. The allowable temperature of the insulating material is the allowable temperature of the motor ； The lifespan of the insulating material is generally the lifespan of the motor.