VFD line reactor sizing calculator
Pick the input line reactor for a VFD: enter motor power and line voltage, and read the recommended reactor impedance % and inductance in mH.
Line reactor sizing
Reactor impedance
Line frequency
Formula: I (A) = (kW × 1000) / (√3 × V × η·PF) and L (mH) = Z% × V / (√3 × 2πf × I × 100) — the reactor rated current equals the drive input current with no derating.
Note: a 3% impedance reactor typically drops total harmonic distortion (THDi) from ~35% to ~20% for a 6-pulse drive.
6-pulse VFDs draw high current harmonics (30–45% THDi) that heat supply transformers and trip nuisance breakers. A 3–5% line reactor is the cheapest mitigation below a harmonic filter — place it on the line side of the drive, between the supply and the drive input.AO Ctrl supplies drives, reactors and the protection components around them:
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Frequently asked questions
What size line reactor do I need for my VFD?
The calculator sizes the reactor from motor power, line voltage and a combined efficiency × power factor. It first finds full-load current as I (A) = kW × 1000 / (√3 × V × η·PF), then inductance as L (mH) = Z% × V / (√3 × 2πf × I × 100). With the defaults — 11 kW, 400 V, 50 Hz and η·PF = 0.95 — you get a 3% impedance input reactor.
What is the difference between a 3% and a 5% line reactor?
The impedance percentage sets the voltage drop at rated current. 3% is the typical input reactor for a standard 6-pulse drive and the calculator default; 5% is used where harmonic distortion is high, and 1% is for dc-link protection. A 3% reactor typically reduces total harmonic distortion (THDi) from about 35% to about 20% on a 6-pulse drive.
Does a line reactor need to be rated for full-load current?
The reactor rated current equals the drive input current with no derating — the calculator applies a factor of 1.0. A 6-pulse VFD draws 30–45% THDi that heats supply transformers and can nuisance-trip breakers, so the reactor is placed on the line side, between the supply and the drive input, as the cheapest mitigation below a harmonic filter.
AO Ctrl provides this tool for reference only.