Input and output reactors serve very different purposes. An input (or line) reactor helps protect the VFD from power line disturbances that could cause nuisance tripping or damage, and also reduces harmonics that the VFD can generate back onto the line.

Therefore, input reactors should be used in the following cases:
  • The supply power is subject to disturbances such as surges, spikes, dips, transients, etc.
  • The supply power is very stiff (greater than ten times the kVA rating of the connected VFDs).
  • Harmonics are a concern (VFDs with input reactors almost always meet the IEEE-519 harmonics standard).
An output (or load) reactor, on the other hand, is used to protect the motor if the wiring distance between the VFD and motor is very long. The general rule of thumb is that an output reactor should be used if the motor leads are over 100 feet. As motor lead length increases, voltage spikes generated by the VFD’s output transistors become amplified. In extreme cases, these spikes can result in premature insulation breakdown in the motor. Adding an output reactor protects the motor by reducing the spikes. However, the motor itself has a lot to do with the wiring distance limitation. Most new motors are now made with inverter rated insulation system wire, which allows much longer motor leads (200-300 feet) without the need for an output reactor.

NOTE: If the motor leads are extremely long (500 feet or more), a dV/dt filter should be used instead of a reactor. A dV/dt filter provides better motor protection than a reactor at extreme wiring distances.