These auxiliary devices are necessary for frequency inverters with high power output. Different manufacturers provide detailed explanations in their user guides.
Let's take a preliminary look at the function of the "AC input reactor" installed at the input end of the frequency converter. Its main purpose is to improve the power factor of the AC power supply on the input side; effectively eliminate high-order harmonics on the input side and prevent damage to other electrical equipment caused by voltage distortion; and eliminate input current imbalance caused by phase imbalance of the three-phase AC power supply.
Generally, when adding an AC input reactor, an "EMC input filter" should also be installed on the input side of the frequency converter. Its main purpose is to minimize the conduction and radiation of the frequency converter itself to the outside world. The figure below is a schematic diagram of the frequency converter system configuration.
For domestically produced frequency converters that are compatible with motors with a capacity of 37kW (50HP), a power input capacity of 114kVA, an input current of 157A, and an output current of 150A or more, all use external DC reactors as standard. The model of the DC reactor needs to be adapted according to different power requirements.
During installation, the user needs to remove the short-circuit copper busbar between the main circuit terminals P and (+) of the frequency converter, and then connect the DC reactor between P and (+). There should be no polarity between the reactor terminals and the frequency converter terminal O (+). After installing the DC reactor, the short-circuit copper busbar between P and (+) is no longer needed.
The DC reactor is connected in series between the rectifier module and the filter capacitor inside the frequency converter. Its functions are to improve the power factor on the input side; improve the overall efficiency and thermal performance of the frequency converter; effectively eliminate the influence of high-order harmonics on the input side on the frequency converter; and reduce the conducted and radiated electromagnetic interference of the frequency converter itself.
"AC reactors" are installed close to the inverter, between its output side and the driven motor. Their main function is filtering, suppressing inrush current in the input circuit and mitigating the overcurrent effects caused by power supply voltage imbalances. It is generally recommended to install an output AC reactor if the distance between the inverter and the motor exceeds 100 meters. This is because the inverter output typically contains a significant amount of high-order harmonics. When the distance between the motor and the inverter is large, the distributed capacitance in the cables can cause certain harmonics to resonate in the circuit, severely damaging the motor's insulation and eventually destroying it. Furthermore, the long wiring can generate significant leakage current, causing frequent inverter shutdowns due to protection mechanisms.
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