About motor temperature rise
The temperature rise is the difference between the temperature of the heating part of the motor and the ambient temperature, and the temperature rise of the sub-core and the winding is usually specified. When the motor is running, the core is in the alternating magnetic field, causing hysteresis and eddy current loss, current flowing through the winding to generate copper loss, wind and wear, mechanical consumption, and the magnetic field gap magnetic field pulse wave generated by the stator and rotor slotting. The stray loss caused by the intrinsic non-working harmonics of the windings, etc., will eventually be reflected in the form of heat, which will increase the temperature of the motor.
In addition, corresponding to the heating factor, there are the following heat dissipation factors:
● There is a temperature difference between the motor surface and the surrounding environment, and there is heat transfer and heat radiation.
● There is heat exchange between the internal heating element of the motor and the internal air, between the internal air of the motor and the motor housing, and between the motor housing and the wind blown over the surface of the motor.
When the two factors, that is, the heating factor and the heat dissipation factor reach equilibrium, the temperature does not rise and stabilizes at a level. If there is disturbance from external factors, such as load increase or sudden short circuit failure, the balance state will be destroyed, the motor temperature will continue to rise, and the temperature difference will continue to expand until the new heat dissipation is equal to the new heat generation. A new equilibrium is reached at high temperatures, but the temperature difference at this time, that is, the temperature rise has increased.
One of the most important tasks of motor design is to study the qualitative or quantitative relationship between the ventilation conditions or factors and the amount of heat generated to ensure that the actual temperature rise of the motor fluctuates within a reasonable range. In actual operation, if the temperature rise of the motor suddenly increases, it indicates that the motor has an electrical fault such as a turn-to-turn short circuit or an abnormal situation such as air duct obstruction and sudden load increase.
Factors affecting temperature rise
For a normally operating motor, the temperature rise under theoretical load should be independent of the ambient temperature, but it is actually affected by factors such as ambient temperature.
● When the temperature drops, the temperature rise of the normal motor drops slightly. (The relationship between the resistance of copper and temperature)
● For self-cooling motors, the temperature rise increases when the ambient temperature rises.
● When the air humidity increases, the heat conduction effect is improved, and the temperature rise can be slightly lowered.
● The altitude is 1000m. When the altitude is high, the temperature rise will be slightly increased due to the relatively poor heat dissipation effect.
The motor temperature rise during the type test does not necessarily guarantee the safety of the motor operation. Temperature rise is the difference between the temperature and the actual operating environment temperature of the motor. When the ambient temperature is high, the actual temperature of the motor will also increase, which involves the matching of the heat resistance level and the actual working temperature. relationship.
At the same time, it is directly related to the highest heat resistant temperature of the relevant components, such as the operating temperature control requirements of the bearing. If the rolling bearing temperature should not exceed 95 °C, the temperature of the sliding bearing should not exceed 80 °C. If the temperature is too high, the oil quality will change and the oil film will be destroyed. The direct consequence of this problem is that the motor bearing system will heat up due to bearing failure, and the motor winding temperature will rise rapidly. At the same time, the motor will stagnate and hold the shaft, which will cause the motor to burn.
A motor manufacturer produces a screw air compressor motor. When the type test is carried out, the temperature rise of the motor is about 70K. However, after the air compressor of the motor is put on the market, the damaging motor faults such as overheating of the winding and the bearing loose frame occur irregularly in batches. Statistics show that the end customers of such faulty motors are in the south during the hot season.
Expectations and actual total deviations, many motor failures that seem to be not supposed to be in fact are also a good thing. First, it provides an excellent reference for motor designers, and product improvement has a strong target. Second, warning motor manufacturers, especially northern enterprises, that the in-plant test verification does not mean that the product development is successful, and the matching relationship between the temperature rise of the motor test and the actual operating conditions must be fully demonstrated. Third, the match between the test value and the design value is only a necessary condition. It is the best touchstone for reliable and good customer reputation.
