The Temperature Rise of Motor
1, What does the temperature rise mean to motor?
The temperature rise is the biggest difference between the motor parts and ambient when the motor works at rated power/wattage.
Due to different motors use different materials with different temperature limitation, the motors are classified into different insulation classes. Normally there are A (105℃), E (130℃), B (130℃), F (155℃), H (180℃), the temperature means the motor must works under the temperature limitation at rated power/wattage. There are also Y and C classes, Y is below 90℃, C is above 180℃.
When we test the motor temperature, we normally assume the ambient is 40 ℃, plus the temperature rise and 5-15℃ as margin, so the temperature of the motor we test is “ambient + T.R + buffer”.
2, What is the damage to a motor if the motor is overheat?
1, If the motor is overheat, the materials used on the motor may be damaged and can weaken or lost the insulation function, the dielectric test may be failed or even short the circuit inside the motor if the temperature is too high.
2, When the motor works under the temperature limitation, the motor can get a proper motor life. According to some studies, the motor is sensitive to the temperature, if the temperature is 10 ℃ above the temperature limitation, the motor life can be half, higher temperature of the motor, shorter motor life.
3, When the motor temperature goes up, the wire resistance will be higher, but the magnetic field density will be lower, this will reduce the performance/efficiency of the motor.
4, The mechanical strength may be changed when the motor temperature is high, the lubrication system will not function well as normal, there will be big friction and resistance between the shaft and the bearing, will cause bigger noise, bigger load, more power consumption, this will definitely shorten the motor life too.
3, What is the main sources of motor heat?
1, Copper loss: this is easy to understand, when there is current on the wires, there will be heat according to Ohm’s law (P = I2R).
2, Magnetic hysteresis loss (iron loss): the motor must create an alternating magnetic field to push the rotor to rotate, during the process, there is some energy lost, this is named as magnetic hysteresis loss.
3, The friction between the bearing and the shaft, or brush (for some motor with brush).
4, How to avoid the motor overheat?
Though the heat is not good for motor’s performance, or motor life, we can’t eliminate the heat, we can only reduce the heat and lower the motor temperature.
1, We can use big motor for small power consumption condition, but this is not economic, we should choose the right motor type and size for specified purpose, this can ensure the motor life and meet the application.
2, We should use proper wire and stator material to reduce the copper loss and iron loss. Of course, less copper or iron loss materials will be higher cost. this is another factor we should consider during designing the motor.
3, Design the good tolerance for motor assembly, especially for rotation parts (shaft, bearing), choose the proper lubricant. Less resistance and friction, less heat created.
4, Design proper ventilation, cooling construction, this will dramatically lower the motor temperature.
Heat is uneliminated for motor, but we can design the motor properly to meet our application by balancing the performance, life and cost. 😊
