The insulation class of an electric motor refers to the heat resistance class of the insulation material used, which is divided into classes A, E, B, F, and H. The allowable temperature rise refers to the limit to which the temperature of the electric motor can rise compared to the ambient temperature.
Temperature rise refers to the difference between the temperature of the stator windings of an electric motor and the ambient temperature when the motor is operating at its rated temperature (the ambient temperature is specified as 35℃ or below 40℃; if no specific value is indicated on the nameplate, then it is 40℃).
Insulation temperature class: A, E, B, F, H
Maximum permissible temperature (°C): 105, 120, 130, 155, 180
Winding temperature rise limit (K) 60 75 80 100 125
Performance reference temperature (°C): 80 95 100 120 145
In electrical equipment such as generators , insulation materials are the weakest link. Insulation materials are particularly susceptible to high temperatures, which accelerate aging and damage. Different insulation materials have different heat resistance properties, and electrical equipment using different insulation materials has different high-temperature resistance capabilities. Therefore, the maximum operating temperature of general electrical equipment is specified.
Based on the high-temperature resistance of different insulation materials, seven permissible maximum temperatures have been defined, arranged in descending order of temperature: Y, A, E, B, F, H, and C. Their permissible operating temperatures are 90, 105, 120, 130, 155, 180, and above 180℃, respectively. Therefore, Class B insulation indicates that the insulation used in this generator has a heat resistance temperature of 130℃. Users should ensure that the generator's insulation material does not exceed this temperature during generator operation to guarantee normal generator function.
Insulation materials with an insulation class of B are mainly composed of mica, asbestos, and glass fiber bonded or impregnated with organic adhesives.
●At what temperature can a motor operate normally? What is the maximum temperature a motor can withstand?
A: If the temperature of the motor cover exceeds the ambient temperature by more than 25 degrees Celsius, it indicates that the motor's temperature rise has exceeded the normal range. Generally, the motor temperature rise should be below 20 degrees Celsius. Motor coils are typically made of enameled wire, and when the temperature exceeds approximately 150 degrees Celsius, the enamel coating will peel off due to the excessive heat, causing a short circuit in the coil. When the coil temperature is above 150 degrees Celsius, the temperature exhibited by the motor casing is around 100 degrees Celsius. Therefore, based on the casing temperature, the maximum temperature the motor can withstand is 100 degrees Celsius.
● The temperature of the motor should be below 20 degrees Celsius, that is, the temperature of the motor end cover should be less than 20 degrees Celsius above the ambient temperature. But what are the reasons for the motor to overheat by more than 20 degrees Celsius?
A: The direct cause of motor overheating is high current. This is usually caused by a short circuit or open circuit in the coil, demagnetization of the magnets, or low motor efficiency. Under normal circumstances, it is caused by the motor running at high current for an extended period of time.
●What causes the motor to heat up? What is this process?
A: When a motor is under load, there is power loss within the motor, which is ultimately converted into heat energy. This causes the motor temperature to rise, exceeding the ambient temperature. The difference between the motor temperature and the ambient temperature is called temperature rise. Once the temperature rises, the motor must dissipate heat to the surroundings; the higher the temperature, the faster the heat dissipation. When the heat generated by the motor per unit time equals the heat dissipated, the motor temperature no longer increases and remains at a stable temperature, that is, it is in a state of equilibrium between heat generation and heat dissipation.
●What is the typical allowable temperature rise for a motor? Which part of the motor is most affected by the temperature rise? How is it defined?
A: When a motor is under load, to maximize its performance, the higher the load it carries, i.e., the greater the output power (if mechanical strength is not considered), the better. However, higher output power means higher power loss and higher temperature. We know that the weakest point in a motor's temperature resistance is the insulation material, such as the enameled wire. Insulation materials have a temperature limit. Within this limit, the physical, chemical, mechanical, and electrical properties of the insulation material are very stable, and its service life is generally about 20 years. Exceeding this limit, the lifespan of the insulation material is drastically shortened, and it may even burn out. This temperature limit is called the allowable temperature of the insulation material. The allowable temperature of the insulation material is the allowable temperature of the motor; the lifespan of the insulation material is generally the lifespan of the motor.
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