How to size a frequency inverter for a compressor
By DigiEntropy Engineering · 2026-07-19 · 8 min read
Which current sizes a VFD for a compressor — full-load, locked-rotor, or something else? The answer is the max operating current at the top frequency, checked against two criteria the compressor and drive makers spell out. Here is the sizing rule, sourced, with the tool that runs it.
About this post — Authored by an AI assistant using DigiEntropy's polynomial corpus, the universal compressor predictor, and the project's chart-generation tooling. Charts are produced by Python scripts that read the same database the live site queries; tables and formulas are pulled from the same engineering modules. Findings reflect the project's current dataset and methodology — send corrections or deeper questions to admin@digientropy.com. Ask ten refrigeration engineers which current sizes a variable-speed drive for a compressor and you get three answers: the nameplate full-load amps, the locked-rotor amps, or "whatever the kW rating says." Two of those are wrong, and the third is only half the story. This is the sizing rule we settled on for DigiEntropy's inverter matcher — grounded in the compressor and drive makers' own selection documents, not a rule of thumb. It is not the locked-rotor current The instinct to size on locked-rotor current — the big inrush a motor pulls at standstill — is understandable. It is the number that sizes contactors, breakers, and star-delta starters, and it runs five to eight times the running current, so it feels like the worst case the drive must survive. But a frequency inverter never puts the motor through that inrush. It ramps the output frequency up from near zero, so the rotor is only ever slightly behind the rotating field. Slip stays small, current stays controlled — a soft start is the entire reason to fit a drive. Size the drive to the locked-rotor current and you would buy something five to eight times too big. Locked-rotor current still matters — for the contactor, the breaker, and a drive's optional bypass. Just not for the drive's own continuous rating. Size on current, not power Danfoss puts it bluntly in their drive-sizing guide: "VFD sizing should always be based on motor current and voltage, not hp." A drive does not produce kilowatts; it supplies current at a controlled voltage and frequency. Two motors of the same kW can draw different amps depending on voltage and power factor, and the drive's output stage is current-limited. So the whole calculation lives in amps. The current that matters: max operating current, at the top speed The right basis is the compressor's maximum operating current — the highest steady current it draws anywhere in its allowed envelope — taken at the highest frequency it will run. Current climbs with speed, so a 50 Hz nameplate figure is not enough when the compressor will over-speed to 70 Hz. Two more qualifiers hide inside "max current." It has to be the maximum…