1. Electromagnetic design

For ordinary asynchronous motor, the main performance parameters in redesign are overload capacity, start-up performance, efficiency and power factor. Because the critical slip rate is inversely proportional to the power supply frequency, frequency motor can start directly when the critical slip rate is close to 1. Therefore, the overload capacity and start-up performance do not need to be considered too much, and the key problem to be solved is how to improve the adaptability of the motor to non-sinusoidal power supply. The general approach is as follows.

1) Reduce stator and rotor resistance as much as possible. By reducing the stator resistance, the fundamental copper consumption can be reduced to make up for the increase of copper consumption caused by high order harmonics.

2) In order to suppress the high order harmonics in the current, the inductance of the motor should be increased appropriately. However, the skin effect of rotor slot leakage reactance is also large, and the copper consumption of high order harmonics is also increased. Therefore, the leakage reactance of the motor should take into account the rationality of impedance matching in the whole speed range.

3) The main magnetic circuit of variable frequency motor is generally designed to be unsaturated. One is to consider that high order harmonics will deepen the saturation of magnetic circuit. The other is to consider that in order to improve the output torque. The output voltage of frequency converter should be properly increased at low frequency.

2. Structural design

In the restructure design, the influence of the characteristics of non-sinusoidal power supply on the insulation structure, vibration, noise cooling mode and so on of the variable frequency motor are also taken into account. The following problems are generally paid attention to.

1) Insulation grade is generally F or higher, which strengthens the insulation strength of ground insulation and turn insulation, especially the ability of insulation to withstand impulse voltage.

2) For the vibration and noise of the motor, we should fully consider the rigidity of the electric mechanism and the whole, and try our best to improve its natural frequency in order to avoid the resonance with each force wave.

3) Cooling mode: forced ventilation is generally used to cool.

4) For constant power variable frequency motor, when the speed exceeds 3000/min, special grease with high temperature resistance should be used to compensate for the increase of bearing temperature.