Today, I will talk to you about the influence of high harmonics on the motor mainly in the following aspects.

1. High-order harmonics distort the output voltage waveform of the inverter, and the output voltage will be superimposed with the surge voltage generated when the switch is turned on and off. The peak value of this surge voltage is so high that it can adversely affect the motor insulation and even break down the insulation.

2. Cause additional heating of the motor, resulting in additional temperature rise of the motor.

3. Harmonics can also cause torque pulsation of the motor, resulting in vibration and noise.

In response to these effects, some preventive measures are proposed below.

1. Prevent surge voltage from deteriorating motor insulation

Ordinary two-level and three-level PWM voltage inverters have large output voltage jump steps, and the phase voltage reaches half of the DC bus voltage. At the same time, due to the fast switching output of the inverter power device, a large voltage will be generated rate of change, resulting in a surge voltage. The 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 ubiquity of distributed inductance and distributed capacitance of the line, it will produce traveling wave reflection, which will amplify the voltage change rate and reach the motor terminal. The place can be more than doubled, so that the motor insulation is damaged.

In order to reduce the impact of surge voltage on the motor insulation, the following measures can be taken.

1. The distance between the motor and the inverter should be as short as possible.

2. Connect a filter to the output side of the PWM inverter to suppress the surge voltage caused by circuit resonance or electromagnetic radiation.

3. To achieve the above measures, if it is not economical, PAM control frequency converter can be used instead.

4. Improve the insulation strength of the motor.

5. Regularly check the insulation strength of the motor, and carry out early diagnosis to prevent problems before they happen.

6. Use a varistor to prevent surge voltage.

2. Prevent the temperature rise of the motor after frequency conversion and speed regulation

Ordinary asynchronous motors mostly adopt self-ventilation mode. When the speed decreases, the wind speed decreases and the air cooling capacity decreases, which will cause the motor to overheat. In addition, the copper loss and iron loss of the motor increase due to the high-order harmonic current generated by the frequency converter. Therefore, according to the load status and speed range, take the following measures.

1. It is best to use a forced ventilation motor.

2. Use a special motor for frequency conversion speed regulation.

3. Reduce the range of speed regulation to avoid ultra-low speed operation.

3. Harmonics produce torque ripple to the motor

The output current of the ordinary current source inverter is not a sine wave, but a 120° square wave. Therefore, the three-phase composite magnetomotive force is not a constant-speed rotation, but a step-by-step magnetomotive force. The difference between the electromagnetic torque generated by the magnetomotive force is that in addition to the average torque, there is also a ripple component. Although the average value of torque ripple is 0, it will cause the rotor speed to be uneven, resulting in pulsation. When the motor is at a low speed, a stepping phenomenon will also occur. Under appropriate conditions, it may cause a mechanical system composed of the motor and the load. resonance, resulting in vibration and noise.

The pulsating torque is mainly generated by the interaction between the fundamental rotating flux and the rotor harmonic current. In a three-phase motor, the pulsating torque is mainly generated by 6n±1 harmonics. The output current of the 6-pulse output current-type inverter contains rich 5th and 7th harmonics. The rotating magnetomotive force generated by the 5th The momentum is in phase with the fundamental rotating magnetomotive force, and the electrical rotation speed of the motor rotor is basically close to the rotating speed of the fundamental wave magnetomotive force, so the 5th harmonic magnetomotive force and the 7th harmonic magnetomotive force will be in the motor rotor Induction generates rotor harmonic currents that are 6 times the fundamental frequency. The fundamental rotating magnetomotive force and the 6-fold frequency rotor harmonic current work together to generate a 6-fold frequency pulsating torque. Similarly, the 11th and 13th harmonic currents will also produce a pulsating torque with a 12-fold frequency.

The impact of ripple torque on the motor speed is particularly obvious at low speeds. The speed pulsation is directly proportional to the harmonic order output by the frequency converter, that is, the amplitude of the speed pulsation caused by low-order harmonics is more influential than high-order harmonics. Therefore, in order to make the speed pulsation of the motor smaller, the low-order harmonics output by the frequency converter must first be eliminated or suppressed, and the high-frequency PWM method is adopted to shift the output harmonics to high frequencies. This is an effective way to reduce the speed pulsation.