1. Harmonic electromagnetic noise and vibration

When the ordinary asynchronous motor is supplied by frequency converter, the vibration and noise caused by electromagnetic, mechanical, ventilation and other factors will become more complex. The time harmonics contained in the frequency conversion power supply interfere with the inherent space harmonics of the electromagnetic part of the motor, forming various electromagnetic excitation forces. When the frequency of electromagnetic force wave is consistent with or close to the natural vibration frequency of motor body, resonance phenomenon will occur, thus increasing the noise. Because of the wide working frequency range and wide speed variation range of the motor, it is difficult to avoid the natural vibration frequency of each component of the motor.

2. Adaptability of motor to frequent start and braking

Because the motor can start at very low frequency and voltage without impulse current, it can use various braking modes provided by frequency converter to brake quickly, which creates conditions for the realization of frequent starting and braking, so the mechanical system and electromagnetic system of the motor are under the action of cyclic alternating force, which brings fatigue and accelerated aging problems to the mechanical structure and insulation structure.

3. Cooling problem at low speed

First of all, the impedance of asynchronous motor is not ideal, when the power supply frequency is lower, the loss caused by high order harmonics in the power supply is large. Secondly, when the respeed of the ordinary asynchronous motor is reduced, the cooling air volume and the cubic power of the rotating speed decrease in proportion, which leads to the deterioration of the low speed cooling condition of the motor and the sharp increase of the temperature rise, so it is difficult to realize the constant torque output.