Running CNC Machines and VFDs on a Rotary Phase Converter

Yes — CNC machines with internal VFDs run perfectly on a rotary phase converter, with correct sizing.

How It Works

Modern CNC machining centers (Haas, Fadal, Mazak, Fanuc-based) have internal drives for spindle and servo axes. These drives accept three-phase AC input. A rotary phase converter supplies that three-phase input.

CNC Sizing — Always 2.0x

Spindle HP Converter
10 HP GP20NL
20 HP GP40NL
30 HP GP60NL
40 HP GP75NL
50 HP GP100NL

VFD Rules

  • CNC with internal VFD: size converter to spindle HP x 2.0 — works great.
  • Standalone VFDs: also compatible. Size to motor HP x load multiplier.
  • Never feed VFD output into a converter input — always: converter then VFD then motor.
  • Multiple VFDs: size converter to sum of all connected HP x multiplier.

Questions? Call (800) 417-6568 — we size CNC machines every day.

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CNC and VFD — FAQ

My CNC shows a three-phase fault — is it the converter?

Check voltage balance first. Measure L1-L2, L1-L3, L2-L3 at the machine input under full spindle load. If any leg deviates more than 3% from average, the converter may be undersized or a capacitor may be weak. Call (800) 417-6568 for diagnosis.

Can a phase converter run a Haas, Fanuc, Mazak, or Fadal?

Yes — all of these brands run on Phoenix Phase Converters. Size to 2x spindle HP.

Do I need a line reactor or filter between the converter and CNC?

Generally no. Phoenix Phase Converters produce clean sine wave three-phase output. Line reactors are sometimes used with cheap static converters — not needed with quality rotary units.

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Questions? Talk to an engineer.

Free sizing help. Made in Phoenix, AZ. Lifetime warranty on every unit.

Shop Phase Converters

(800) 417-6568