Input and output reactors serve very different purposes. Simply put, an input reactor protects the drive and an output reactor protects the motor. However, there are a few more details to determine the correct application.
An input or line reactor helps protect a variable frequency drive (VFD) from input power line disturbances that could cause nuisance tripping or damage to the drive. An input (line) reactor also reduces the harmonics that the VFD generates back onto the line.
Line reactors are sized based on the HP and voltage ratings of the drive in use. Most drive manufacturers use 3% or 5% reactors.
Input (line) reactors should be used in these circumstances:
- The input line power is prone to have disturbances such as surges, spikes, transients, etc.
- The supply line power is very stiff (greater than 10 times the kVA rating of the connected VFD).
- Where harmonic distortion is a concern. (IEEE-519 Harmonic Control in Electrical Power Systems)
Line reactors are connected in series between the input source and the VFD (variable frequency drive) controller. The reactor should be mounted as close to the drive as possible.
An output (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 drive generates a high frequency PWM three-phase output and noise spikes are generated on the leading edge of these signals. These noise spikes get amplified due to the long cable lengths and the additional capacitance of the cable. The resulting voltage can exceed the motor’s peak voltage rating where insulation breakdown occurs.
The general rule of thumb is that an output reactor should be used if the motor wiring is over 1 00 feet, but this value varies depending on the motor. If the motor meets the NEMA MG-1 Part 31 standard, it is possible to have as much as 300 feet of cabling without a reactor. If it does not meet the standard, the maximum cable length should be 100 ft. Also, if the distance is between 300 and 500 feet, a load reactor must be installed.
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