The motor exchanges energy with the power grid through the frequency converter in the way of PWM pulsation. This current demand makes the frequency converter produce obvious current distortion and voltage depression on the power side, resulting in a large amount of harmonic current and harmonic voltage.
The input part of the commonly used AC-DC-AC voltage inverter is the rectifier circuit that converts the AC power supply to the DC power supply. Since there is a large capacitor in parallel between the rectifier and the inverter, the current supplied by the power supply to the inverter is actually the charge and discharge current of the capacitor, only when the power supply voltage is greater than the capacitor voltage is charged, and the other time is discharged, so the input current is seriously distorted, from the original sine wave into a distorted waveform containing a large number of harmonics, due to the waveform distortion, the input harmonics are generated. Coupled with the harmonics generated by the rectifier, the devices that return to the upper power supply and distribution system and are connected to the same power supply cause adverse effects such as overheating, noise, and misoperation.
Input harmonics
Cause: The input harmonics of the inverter are mainly due to the fact that the rectifier part of the inverter usually uses three-phase full wave uncontrollable rectification, and the DC circuit uses large capacitors as filters. In this way, although the input voltage waveform of the network side of the inverter is basically a sine wave, the input current is a pulsed charging current, which is rich in harmonics.
Impact: Input harmonics distort the voltage waveform of the power grid, increase the reactive power of the power grid, reduce the power factor of the power grid, and affect the normal operation of other devices.
Suppression measures: In order to reduce the input harmonics, an AC reactor can be installed at the input end of the inverter to increase the line impedance, thereby improving the input current waveform. In addition, advanced harmonic control solutions such as active filters can be used to perfectly solve harmonics.
Output harmonics
Cause: The output harmonics of the inverter are mainly because the variable frequency inverter part of the inverter usually adopts PWM (pulse width modulation) technology, and the output voltage is non-sinusoidal voltage, which will produce higher-order harmonic components, which will affect the motor and other equipment connected behind the inverter.
Impact: Output harmonics may cause motor vibration to intensify, temperature rise to increase, and may even cause motor damage. At the same time, harmonics will also affect the surrounding electronic equipment through electromagnetic coupling and other ways, causing interference.
Suppression measures: In order to reduce harmonics on the output side, an output reactor or EMI filter can be installed on the output end of the variable frequency drive, which can absorb part of the harmonic energy and reduce the harmonic level. In addition, the optimized PWM modulation strategy is also an effective harmonic suppression method. In summary, both the input side and the output side of the inverter produce harmonics, which can adversely affect the power grid and connected equipment. Therefore, when designing and using the inverter system, appropriate measures must be taken to suppress harmonics to ensure the stability and reliability of the system.
Post time: Dec-19-2024