VFD vs. Rotary Phase Converter: What Is the Difference?
Both VFDs (Variable Frequency Drives) and rotary phase converters convert single-phase power to three-phase — but they work differently and serve different applications. Here is a clear breakdown:
How They Work
- Rotary Phase Converter: Uses an idler motor to generate a true third phase. Produces real, balanced three-phase AC power — the same type your utility delivers. Powers any three-phase equipment including CNC, multi-motor shops, and equipment with its own VFD.
- VFD (Variable Frequency Drive): Converts single-phase AC to DC, then inverts it to three-phase AC at variable frequency. Controls motor speed. Each VFD drives one motor only.
Key Differences
| Feature | Rotary Phase Converter | VFD |
|---|---|---|
| Motors served | Multiple simultaneously | One per VFD |
| Speed control | No (fixed speed) | Yes (variable Hz) |
| CNC / servo compatible | Yes | Not directly — CNC has its own drives |
| Power quality | True sine wave | PWM (modified wave) |
| Whole-shop solution | Yes | No — one motor only |
| Typical cost | $600–$15,000 | $200–$3,000 per motor |
Which Should You Choose?
- Choose a rotary phase converter if you have a CNC machine, multiple motors, or any equipment that already has its own VFD or servo drives built in.
- Choose a VFD only if you have a single simple motor and need variable speed control (e.g., a lathe or fan where speed adjustment matters).
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VFD vs Rotary — FAQ
My equipment dealer suggested a VFD — is that right?
If you have a single simple motor and want variable speed control, a VFD per motor may work. If you have CNC equipment, multiple motors, or the equipment already has internal VFDs, you need a rotary phase converter. A VFD cannot power a Haas or similar CNC machine.
Can I use both a VFD and a phase converter?
Yes — the phase converter provides three-phase power to the whole shop, and individual VFDs (built into CNC machines or added to specific motors) operate normally from that three-phase supply.
Related Articles
- How to Size a Rotary Phase Converter
- What Is a Rotary Phase Converter?
- Rotary vs Digital Phase Converters
- Phase Converter FAQ
Questions? Talk to an engineer.
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