Temperature rise is a very important performance indicator of a motor. It refers to the value by which the winding temperature of the motor is higher than the ambient temperature under rated operating conditions. For a motor, is the temperature rise related to other factors in the motor's operation?

About motor insulation level

Insulating materials are divided into 7 grades according to their heat resistance: Y, A, E, B, F, H and C. Their ultimate operating temperatures are 90°C, 105°C, 120°C, 130°C, 155°C, 180°C and Above 180℃. 

The so-called ultimate operating temperature of the insulation material refers to the temperature value corresponding to the hottest point in the winding insulation during operation of the motor within the expected design life.

According to experience, the service life of Class A materials at 105°C and Class B materials at 130°C can reach 10 years. However, in actual situations, neither the ambient temperature nor the temperature rise will reach the design value for a long time, so the general service life is 15 to 20 years. Year. If the operating temperature exceeds the ultimate operating temperature of the material for a long time, the aging of the insulation will be accelerated and the service life will be greatly shortened. Therefore, when the motor is running, the ambient temperature is one of the main factors affecting the life of the motor.

About motor temperature rise

Temperature rise is the temperature difference between the motor and the environment, which is caused by the heat generated by the motor. The core of a running motor is in an alternating magnetic field and will produce iron losses. When the winding is energized, copper losses will occur, as well as other stray losses. These will increase the temperature of the motor.

On the other hand, the motor also dissipates heat. When heat generation and heat dissipation are equal, a state of equilibrium is reached, and the temperature no longer rises but stabilizes at a level. When heat generation increases or heat dissipation decreases, the balance will be destroyed, causing the temperature to continue to rise and the temperature difference to expand. Then heat dissipation should be increased to reach a new balance at another higher temperature. However, the temperature difference at this time, that is, the temperature rise, has increased than before. Therefore, the temperature rise is an important indicator in the design and operation of the motor, indicating the degree of heat generation of the motor.

During the operation of the motor, if the temperature rise suddenly increases, it indicates that the motor is faulty, or the air duct is blocked, or the load is too heavy, or the windings are burned out, etc.

The relationship between temperature rise and temperature and other factors

For a normally operating motor, theoretically the temperature rise should have nothing to do with the ambient temperature under rated load, but in fact it is still affected by factors such as ambient temperature.

(1) When the temperature drops, the temperature rise of a normal motor will decrease slightly. This is because the winding resistance decreases and the copper loss decreases. For every 1°C temperature drop, the resistance drops by approximately 0.4%.

(2) For self-cooling motors, for every 10°C increase in ambient temperature, the temperature rise increases by 1.5~3°C. This is because winding copper losses increase as the temperature rises. Therefore, temperature changes have a greater impact on large motors and enclosed motors.

(3) For every 10% increase in air humidity, due to improved thermal conductivity, the temperature rise can drop by 0.07~0.38℃, with an average of about 0.2℃.

(4) The altitude is based on 1000m. For every 100m of elevation, the temperature rise increases by 1% of the temperature rise limit.

Temperature limits for various parts of the motor

(1) The temperature rise of the core in contact with the winding (thermometer method) should not exceed the temperature rise limit of the winding insulation in contact (resistance method), that is, Class A is 60℃, Class E is 75℃, and Class B is 80℃. Class F is 105℃ and Class H is 125℃.

(2) The temperature of rolling bearings should not exceed 95°C, and the temperature of sliding bearings should not exceed 80°C. Because the temperature is too high, the oil quality will change and the oil film will be destroyed.

(3) In practice, the temperature of the casing is often based on the temperature that it is not hot to the touch.

(4) The stray loss on the surface of the squirrel cage rotor is very large and the temperature is relatively high, which is generally limited to the extent that it does not endanger the adjacent insulation. It can be estimated by applying irreversible color-changing paint in advance.

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