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VFDs and Harmonics

Six Pulse Drives

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%.

DC Bus Chokes.

  • DC current so has copper loss and no iron loss.
  • Physically smaller than AC Line Reactor
  • Usually supplied built in to the drive, harder to add externally.
  • Connected between the rectifier and the Capacitor Bank.
  • Can be combined with an AC reactor if rectifier failures are an issue


AC Line Reactors.

  • Reduces Transient voltages applied to the rectifier.
  • Can reduce rectifier failures.
  • AC current so has copper loss and iron loss.
  • Physically larger than DC Choke
  • Reduces AC voltage applied to the rectifier, reduces DC bus voltage and therefore limits maximum output voltage.
  • Can result in higher current on Motor and higher motor losses.

Twelve Pulse Drives

Eighteen Pulse Drives

Twenty Four Pulse Drives