Differences between variable frequency motor and ordinary asynchronous motor

Ordinary asynchronous motors are designed according to the principle of constant frequency and constant voltage, which cannot fully meet the requirements of variable frequency speed regulation. The influence of frequency converter on motor is as follows:

1. Motor efficiency and temperature rise
No matter what form of frequency converter, it will generate harmonic voltage and current of varying degrees during operation, making the motor operate under non sinusoidal voltage and current. In spite of data introduction, take the widely used sine wave PWM frequency converter as an example, its low harmonic is basically zero, and the remaining high harmonic components that are about twice the carrier frequency are 2u+1 (u is the modulation ratio). Higher harmonics will cause the increase of stator copper loss, rotor copper (aluminum) loss, iron loss and additional loss of the motor, especially rotor copper (aluminum) loss. Because the asynchronous motor rotates at a synchronous speed close to that corresponding to the fundamental frequency, high order harmonic voltage will cause large rotor loss after cutting the rotor bar with a large slip. In addition, additional copper consumption due to skin effect shall also be considered. These losses will cause the motor to generate extra heat, reduce efficiency and output power. If the ordinary three-phase asynchronous motor is operated under the non sinusoidal power supply output by the frequency converter, its temperature rise will generally increase by 10% - 20%.

2. Motor insulation strength
At present, many medium and small frequency converters adopt PWM control mode. Its carrier frequency is about thousands to tens of kilohertz, which makes the motor stator winding bear a high voltage rise rate, which is equivalent to applying a steep impulse voltage to the motor, so that the motor's turn to turn insulation can withstand a more severe test. In addition, the rectangular chopping impulse voltage generated by PWM frequency converter superimposed on the motor operating voltage will pose a threat to the motor insulation to ground, and the insulation to ground will accelerate aging under the repeated impact of high voltage.

3. Harmonic electromagnetic noise and vibration
When the common asynchronous motor is powered by a frequency converter, the vibration and noise caused by electromagnetic, mechanical, ventilation and other factors will become more complex. The time harmonics contained in the variable frequency power supply interfere with the inherent space harmonics of the electromagnetic part of the motor to form various electromagnetic excitation forces. When the frequency of the electromagnetic force wave is consistent with or close to the natural vibration frequency of the motor body, resonance will occur, thus increasing the noise. Due to the wide working frequency range and large speed variation range of the motor, the frequency of various electromagnetic force waves is difficult to avoid the natural vibration frequency of various components of the motor.