Standard Three phase drives comprise a three phase bridge rectifier on the input. In older drives, this rectifier is a standard three phase rectifier made up of Six rectifiers. In more modern drives, this is commonly a controlled rectifier comprising three SCRs and three rectifiers.
The output of the rectifier is applied to a large capacitor bank to smooth the high voltage DC applied to the output stage.
In some drives, the rectifier is directly connected to the capacitor bank, and in others, the rectifier is connected via a DC Bus choke. The third option is a three pahse reactor installed on the AC side of the rectifier.
The presence of the AC reactor or the DC bus choke, enhances the smoothing of the voltage on the DC bus and reduces the peak currents flowing through the rectifier inputs.
The input AC reactor, or the DC Bus choke should be a 3% reactor for the best results at full load. As the VFD loading is reduced, the advantages of the reactor are reduced also.
The reactor effectively reduces the peak current and lengthens the time time that the current is flowing. This change in conduction angle of the current flowing through the rectifiers alters the harmonic content of the input current.
The THDi (Total Harmonic Distortion of the current) at the input of the drive that does not include reactors is in the order of 90% - 115%. The addition of 3% reactors or DC bus chokes will reduce the THDi down to around 40%.
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