Harmonic Reactor
Calculator
Size a line reactor (AC choke) for a VFD or non-linear load. Calculates rated current, reactor inductance in mH, and estimated THD reduction for 3% or 5% impedance reactors.
Sizing a Line Reactor for a VFD
Enter the drive's input kVA rating (from the drive spec sheet) or the motor horsepower. The rated current is the basis for reactor sizing — the reactor must carry the drive's rated input current continuously without overheating or saturating.
3% is standard for most VFD installations and is the minimum recommended by IEEE 519. It provides a good balance between harmonic reduction and voltage drop. Use 5% where utility harmonic limits are strict or the system has high background THD.
The calculated inductance in mH is the value to specify on the reactor nameplate. At 60 Hz, a 3% reactor on a 100 kVA, 480 V load is approximately 0.44 mH. Reactor manufacturers rate their products in mH at a specific frequency and rated current.
The reactor causes a voltage drop at rated load. For a 3% reactor, the drop is 3% of line voltage (≈ 14.4 V on a 480 V system). Ensure the voltage reaching the drive input is within its specified input voltage tolerance, typically ±10%.
A harmonic reactor (also called a line reactor or AC line choke) is an inductor connected in series with a variable frequency drive (VFD), rectifier, or other non-linear load. It limits the rate of current change during rectifier switching, reducing peak current, lowering harmonic distortion (THD), and protecting against voltage spikes and transients from the supply.
3% reactance is the most common specification for VFD applications — it reduces THD from a typical 80–100% (no reactor) to approximately 35–45%, while causing only a small voltage drop. 5% reactors provide better harmonic reduction but a larger voltage drop. Values above 5% are used for very sensitive applications or where utility harmonic limits are strict.
Both perform similar functions. An AC line reactor is installed on the AC input side of the drive — it is simpler to specify and protects against supply-side transients. A DC bus choke is internal to the drive and acts between the rectifier and capacitor bank — it is typically more effective per unit of inductance. Many modern VFDs include a DC choke; add an AC reactor in addition if the supply has significant harmonic distortion or voltage unbalance.
No — a passive line reactor reduces harmonics but does not eliminate them. A 3% reactor on a 6-pulse drive reduces THD from ~80% to ~35–45%. To achieve IEEE 519 limits (< 5% THD at the PCC), active harmonic filters or 12-pulse / 18-pulse rectifier topologies are required. Reactors are cost-effective first-step mitigation and should always be installed when using VFDs.
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