How to judge whether the motor heat dissipation is normal

Temperature rise depends on the heat generation and heat dissipation during the operation of the motor. The temperature rise is often used to judge whether the heat dissipation of the motor is normal. Temperature rise is an important performance indicator of the motor. So what factors are related to the temperature rise of a motor? Let's learn about it together.

How to judge whether the motor heat dissipation is normal

Temperature rise refers to the temperature of various parts of the motor above the ambient temperature. The rated temperature rise of the motor refers to the maximum allowable temperature rise of the motor winding under the specified ambient temperature (40°C) in the design, which depends on the insulation grade of the winding.

1. Motor insulation grade

The insulation grade of a motor refers to the heat resistance grade of the insulating material used, which is divided into A, E, B, F, and H. The allowable temperature rise refers to the limit of the motor's temperature increase compared to the ambient temperature.

The temperature grades of insulation are A, E, B, F and H.

Maximum allowable temperature (℃) 105 120 130 155 180 Winding temperature rise limit (K) 60 75 80 100 125.

Performance reference temperature (℃) 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 accelerates aging and damage. Different insulation materials have different heat resistance properties, and electrical equipment using different insulation materials has different high temperature tolerance capabilities. Therefore, general electrical equipment has a maximum operating temperature specified.

The allowable limit temperature refers to the maximum allowable operating temperature of the motor insulation material, which reflects the heat resistance of the insulation material.

2. Motor temperature rise

Temperature rise is the temperature difference between the motor and the environment, which is caused by the heat of the motor. The iron core of the running motor will produce iron loss in the alternating magnetic field, and the winding will produce copper loss after power is turned on, as well as other stray losses. These will increase the temperature of the motor.

In addition, the motor will also dissipate heat. When the heat generation and heat dissipation are equal, the equilibrium state is reached, and the temperature no longer rises but stabilizes at a level. When the heat generation increases or the heat dissipation decreases, the balance will be destroyed, causing the temperature to continue to rise and the temperature difference to expand. In this case, the 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 compared to 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 means that the motor is faulty, or the air duct is blocked, or the load is too heavy, or the winding is burned, etc.

3. Temperature limits of 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 insulation of the winding in contact (resistance method), that is, 60°C for Class A, 75°C for Class E, 80°C for Class B, 105°C for Class F, and 125°C for Class H.

(2) The temperature of rolling bearings should not exceed 95°C, and the temperature of sliding bearings should not exceed 80°C. Too high a temperature will change the oil quality and damage the oil film.

(3) In practice, the casing temperature is usually not too 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 not endangering the adjacent insulation. It can be estimated by applying irreversible color-changing paint in advance.

4. Relationship between temperature rise and other factors such as temperature

For a normally operating motor, theoretically its temperature rise under rated load should be independent of the ambient temperature, but in reality it is still affected by factors such as the 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 drop in temperature, the resistance decreases by about 0.4%.

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

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

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