Talking about the thermal stress in the starting process of high-power motors

Update:04 Jun 2021

For high-power motors that require forward and reverse rotation, if the full-speed motor is braked in the reverse direction and rotates in the reverse direction, the energy consumed by the rotor at this time is much larger than the energy consumed during normal starting, about 3 times the energy.

In the process of starting a high-power motor, due to the relatively short time, the temperature of the stator and rotor cores is relatively low, and the temperature of the energized conductor rises rapidly during this period. The original integrated layout is due to the varying heating environment. Due to the inconsistency and the difference in the thermal expansion coefficient of the material, the conductive cage bars in the rotor will be thermally stressed due to the limitation of free expansion in the physical space.

Thermal stress, also known as variable temperature stress, refers to the stress that occurs when an object cannot be completely free to expand and contract due to external constraints and mutual constraints between internal departments when the temperature changes.

When the thermal stress is higher than the yield strength or fatigue limit, the guide bar will have cracks; when the deep groove rotor motor is started, due to the squeezing effect of the motor, the temperature of the upper and lower parts of the rotor guide bar will be greatly different, and the difference will occur. This phenomenon is more serious for the non-forging process of the guide bar rotor. In severe cases, the rotor guide bar may be broken.

At the same time, the stator winding current is very large during the starting process, and instantaneous high temperature will occur. The motor will crawl for a long time at low speed and start frequently with heavy loads. The city will cause premature aging of the stator winding insulation, or the motor will be caused by excessive temperature The insulation and the magnet wire paint film soften and break down, resulting in inter-turn obstruction of the winding.

From the above clarification, we can draw a conclusion that a good high-power motor must have a good starting performance, otherwise the electrical timing will end prematurely due to starting.

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