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.
10% - continuous reserve - on top of the max operating current
Bitzer's selection rule is explicit: the inverter must continuously supply the compressor's max operating current under all expected conditions, and "at least 10% additional reserve should be planned for."
1.48 - 50 to 70 Hz current factor - not the 1.40 a pure speed ratio gives
Across the 16 Bitzer models that publish both a 50 Hz and a 70 Hz current, an 18.0 A figure at 50 Hz becomes about 26.6 A at 70 Hz — a factor of about 1.48, not the 1.40 a pure speed ratio would give.
1.6 - starting factor, four-cylinder recip - 1.2 for a screw
Bitzer publishes 1.6 for a four-cylinder reciprocating compressor and 1.2 for a screw, and notes the factor climbs as cylinder count falls.
150% - Heavy-Duty overload - for 60 seconds
A Heavy-Duty drive — the correct rating class for a constant-torque load like a positive-displacement compressor — provides about 150% overload for 60 seconds.
90 A - drive for a Bitzer 6FE-40Y - sizing floor 78.7 A
The sizing floor is 78.7 A (running plus 10%); the smallest VARIPACK tier that clears it is 90 A. That drive's 150% overload reaches 135 A, comfortably above the 100.1 A starting current.
357 - drives in the catalog - across eight brands
The recommendation is drawn from a real catalog: 357 drives across eight brands — ABB, Danfoss, Schneider, Inovance, Bitzer VARIPACK, Copeland, Yaskawa, and Invertek.
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.
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