Today we'll discuss the impact of higher harmonics on electric motors, mainly in the following aspects.
1. High-order harmonics distort the inverter's output voltage waveform, and surge voltages generated during switch opening and closing are superimposed on the output voltage. These surge voltages have very high peak values, which can adversely affect the motor insulation and may even cause insulation breakdown.
2. It causes additional heating in the motor, resulting in an extra temperature rise in the motor.
3. Harmonics can also cause torque pulsation in motors, resulting in vibration and noise.
In response to these impacts, some preventative measures are proposed below.
1. Prevent surge voltage from degrading the insulation of the motor.
Ordinary two-level and three-level PWM voltage-type frequency converters have large output voltage jumps, with phase voltages reaching half the DC bus voltage. Simultaneously, the rapid switching output of the inverter's power devices generates a large voltage change rate, resulting in surge voltages. Surge voltages can affect motor insulation, especially when the cable distance between the inverter output and the motor is long. Due to the widespread distributed inductance and capacitance in the line, traveling wave reflection occurs, amplifying the voltage change rate. This can more than double at the motor terminals, damaging the motor insulation. To reduce the impact of surge voltages on motor insulation, the following measures can be taken.
1. The distance between the motor and the frequency converter should be minimized.
2. Connect a filter to the output side of the PWM inverter to suppress surge voltage caused by circuit resonance or electromagnetic radiation.
3. If the above measures are not economical, a PAM-controlled frequency converter can be used instead.
4. Improve the insulation strength of the motor.
5. Regularly check the insulation strength of the motor to make early diagnoses and prevent problems before they occur.
6. Use a varistor to prevent surge voltage.
II. Preventing Increased Temperature Rise in Motors After Variable Frequency Speed Regulation
Most ordinary asynchronous motors use self-ventilation. When the speed decreases, the airflow speed decreases, reducing the cooling capacity and causing the motor to overheat. Furthermore, the high-order harmonic currents generated by the frequency converter increase the motor's copper and iron losses. Therefore, the following measures should be taken based on the load condition and speed range.
1. It is best to use a forced ventilation type motor.
2. Select a dedicated variable frequency speed control motor.
3. Reduce the speed range to avoid ultra-low speed operation.
3. Harmonics cause torque pulsation in the motor.
The output current of a typical current-source inverter is not a sine wave, but a 120° square wave. Therefore, the three-phase combined magnetomotive force (MTF) is not a constant-speed rotating MMF, but a stepping MMF. The difference between this and the electromagnetic torque generated by a basically constant-speed rotating rotor MMF is that, in addition to the average torque, there is a pulsating component. Although the average value of the torque pulsation is 0, it causes uneven rotor speed, producing pulsation. At low motor speeds, stepping phenomena may also occur. Under appropriate conditions, this can cause resonance in the mechanical system composed of the motor and the load, resulting in vibration and noise. The pulsating torque is mainly generated by the interaction of the fundamental rotating magnetic flux and the rotor harmonic current. In a three-phase motor, the 6n±1st harmonic is the primary source of pulsating torque. The output current of a 6-pulse current-type inverter contains abundant 5th and 7th harmonics. The rotating magnetomotive force (MOMF) generated by the 5th harmonic is out of phase with the fundamental OMF, while the rotating MOMF generated by the 7th harmonic is in phase with the fundamental OMF. Since the electrical rotational speed of the motor rotor is close to the rotational speed of the fundamental OMF, both the 5th and 7th harmonic MOMFs induce rotor harmonic currents at six times the fundamental frequency in the motor rotor. The combined effect of the fundamental OMF and the 6th harmonic rotor current produces a 6th harmonic pulsating torque. Similarly, the 11th and 13th harmonic currents also produce a 12th harmonic pulsating torque. The pulsating torque has a particularly significant impact on motor speed at low speeds. The speed pulsation is directly proportional to the harmonic order of the inverter output; that is, the amplitude of the speed pulsation caused by lower harmonics has a greater impact than that caused by higher harmonics.
Therefore, to reduce the speed pulsation of the motor, the low-order harmonics of the inverter output must first be eliminated or suppressed. High-frequency PWM method is adopted to shift the output harmonics to higher frequencies, which is an effective way to reduce speed pulsation.
