The impact of high harmonics on the motor mainly has the following aspects.
1, high harmonics make inverter output voltage waveform distortion, the output voltage will be superimposed due to the surge voltage generated when the switch is opened and closed. The peak value of the surge voltage is very high, can have an adverse effect on the motor insulation, or even breakdown insulation.
2, cause additional heating of the motor, resulting in additional temperature rise of the motor.
3, Harmonics can also cause motor torque pulsation, generating vibration and noise.
For these effects, the following proposed some preventive measures.
I. Preventing motor insulation degradation by surge voltage
Ordinary two-level and three-level PWM voltage inverter due to the output voltage jump step is large, the phase voltage reaches half of the DC bus voltage, at the same time, due to the inverter power device switching output faster, will produce a larger rate of change of the voltage, thus generating a surge voltage. Surge voltage will affect the insulation of the motor, especially when the cable distance between the inverter output and the motor is long, due to the prevalence of distributed inductance and distributed capacitance of the line, which will produce traveling wave reflection, so that the voltage change rate is amplified to the motor terminals can be increased by more than double, so that the motor insulation is damaged.
In order to minimize the impact of surge voltage on motor insulation, the following measures can be taken.
1, the distance between the motor and the inverter as short as possible.
2, in the PWM inverter output side access filter to suppress the surge voltage generated by circuit resonance or electromagnetic radiation.
3, the realization of the above measures, if not economical can be changed to PAM control inverter.
4, Improve the insulation strength of the motor.
5, Check the insulation strength of the motor regularly and carry out early diagnosis to prevent the problem before it occurs.
6, Prevent surge voltage with varistor.
Second, to prevent the motor frequency conversion speed control after the increase in temperature rise
Ordinary asynchronous motors are mostly self-ventilated, and when the speed is reduced, the air speed decreases and the air cooling capacity is reduced, which will cause the motor to overheat. In addition, the high harmonic current generated by the frequency converter increases the copper loss and iron loss of the motor. Therefore, the following measures should be taken according to the load status and speed regulation range.
1、It is better to use forced ventilation type motor.
2、Special motor for frequency conversion speed regulation is used.
3、Reduce the speed range and avoid ultra-low speed operation.
Harmonics produce torque pulsation on the motor.
Ordinary current source inverter output current is not sinusoidal, but 120 ° square wave, so the three-phase synthesized magnetic potential is not a constant-speed rotation, but the step magnetic potential, which and the basic constant-speed rotation of the rotor magnetic potential generated by the electromagnetic torque difference is in addition to the average torque, there are pulsating components. Although the average value of the torque pulsation is 0, it causes the rotor speed to be uneven, generating pulsation, and at low motor speeds, stepping phenomena can also occur, and under appropriate conditions, it may cause resonance in the mechanical system consisting of the motor and the load, thus generating vibration and noise.
Pulsating torque is mainly generated by the interaction of fundamental rotating flux and rotor harmonic currents. In three-phase motors, the pulsating torque is mainly generated by the 6n±1th harmonic.6 The output current of the pulse output current inverter contains abundant 5th and 7th harmonics, the rotating magnetic flux generated by the 5th harmonic is inversely phased with the fundamental rotating magnetic flux, the rotating magnetic flux generated by the 7th harmonic is in the same phase as the fundamental rotating magnetic flux, and the electrical rotational speed of the motor rotor is basically close to that of the fundamental magnetic flux, so the 5th harmonic rotating magnetic flux is mainly generated by the interaction between the fundamental rotating magnetic flux and the rotor harmonic current. Therefore, the 5th harmonic magnetic potential and the 7th harmonic magnetic potential will generate a rotor harmonic current 6 times the fundamental frequency in the motor rotor. The combination of the fundamental rotating magnetic potential and the 6 times frequency rotor harmonic current produces a pulsating torque of 6 times frequency. Similarly, the 11th and 13th harmonic currents produce a pulsating torque of 12 times frequency.
The effect of pulsating torque on motor speed is particularly noticeable at low speeds. The speed pulsation is directly proportional to the number of harmonics dug into the inverter output, i.e., the amplitude of the speed pulsation caused by the lower harmonics has a greater effect than the higher harmonics. Therefore, in order to make the motor speed pulsation smaller, the first step is to eliminate or inhibit the low harmonics of the inverter output, and adopt high-frequency PWM method to shift the output harmonics to high frequency, which is an effective way to reduce the speed pulsation.
