What Each One Actually Does
Let's clear this up once and for all. These two tools are used for two different jobs.
Phase Converter
Produces 3-phase power from single-phase
Primary Function
Takes your 240V single-phase power and generates true three-phase power by producing the missing third leg.
Best For
- Powering your entire shop
- CNC machines with multiple motors
- Equipment with built-in controls
- Running several machines
- Anything requiring 3-phase
Limitations
- Doesn't control motor speed
- Fixed frequency output (60Hz)
VFD (Variable Frequency Drive)
Controls motor speed by varying frequency
Primary Function
Converts your incoming AC to DC, then to AC again at any frequency you like, so you can vary speeds.
Best For
- Variable speed applications
- Soft starting motors
- Energy savings on pumps/fans
- Single motor applications
- Speed control is the priority
Limitations
- Powers only ONE motor
- 50% derating on 1-phase input
- Can't power controls/electronics
Head-to-Head Comparison
| Feature | Phase Converter | VFD |
|---|---|---|
| Creates 3-phase power | ✓ Yes — true 3-phase output | Sort of — but only to one motor |
| Controls motor speed | ✗ No | ✓ Yes — that's its whole job |
| Powers multiple machines | ✓ Yes — size it right and run your whole shop | ✗ No — one VFD per motor |
| Works with CNC machines | ✓ Yes — powers everything including controls | ✗ No — CNCs need more than one motor |
| Single-phase input capacity | Full rated output | Derated 50% (10HP VFD = 5HP usable) |
| Works with VFD-controlled equipment | ✓ Rotary converters work great | N/A |
| Powers welders, plasma cutters | ✓ Yes | ✗ No |
| Typical cost (10HP equivalent) | $1,500 - $2,500 | $500 - $1,000 (but 50% derated) |
Why People Get This Wrong
Here's where the confusion comes from: some VFDs can accept single-phase input and output three-phase to a motor. So people think "hey, it converts single-phase to three-phase — that's a phase converter!"
Not quite. Here's what's actually happening inside a VFD:
Single-phase AC → DC → Three-phase AC (at variable frequency)
The VFD rectifies your incoming power to DC, then uses an inverter to create a new AC waveform at whatever frequency you want. The "three-phase output" is really just a side effect of how the inverter works — and it comes with serious limitations:
⚠️ The 50% Derating Problem
When you feed a VFD single-phase power, you have to derate it by 50%. That 10HP VFD you bought? It's now a 5HP VFD. The DC bus capacitors can't handle the ripple current from single-phase input at full power. Run it at full rated HP and you'll cook those capacitors.
A rotary phase converter, on the other hand, actually generates a third phase using an idler motor. The output is true three-phase power at full rated capacity — no derating, no limitations on what you can connect to it.
Which One Do You Need?
Here are the most common scenarios we see:
CNC Mill or Lathe
CNCs have spindle motors, servo motors, coolant pumps, and control electronics. A phase converter powers the whole machine. A single VFD can't do this.
Multiple Shop Machines
One properly-sized phase converter can power your mill, lathe, surface grinder, and air compressor from a single installation.
Welder or Plasma Cutter
These have transformers and rectifiers that need real three-phase power. VFDs don't work here — you need a phase converter.
Single Motor, Variable Speed Needed
Drill press, lathe spindle, or conveyor where you want to dial in the exact RPM. A VFD gives you that control (but remember the 50% derating).
Pump or Fan Application
HVAC blowers, dust collectors, irrigation pumps — anywhere you want to adjust flow rate. VFDs excel here and save energy at partial speeds.
Bridgeport Mill with Power Feed
Phase converter powers the whole machine at proper voltage. VFD on the spindle lets you vary speed without changing belts. Best of both worlds.
Using a Phase Converter and VFD Together
This is actually a common and smart setup. Here's how it works:
The phase converter feeds clean three-phase power to your shop's subpanel. Everything gets proper voltage and the converter handles the heavy lifting of phase generation.
VFDs on individual machines then take that three-phase power and give you variable speed control where you need it — the spindle on a manual mill, for instance, or the motor on a belt sander.
✓ Why This Works Better
The VFD gets three-phase input (no derating!), works at its full rated capacity, and you still get variable speed control. The phase converter handles phase conversion; the VFD handles speed control. Each does what it's designed for.
Important: Rotary Converters Only
If you're running VFDs downstream of a phase converter, use a rotary phase converter. Digital/electronic phase converters don't play nice with VFDs — the switching harmonics cause problems. Stick with rotary and you're good.
Common Questions
Can I just use a VFD instead of a phase converter for my CNC?
No. Your CNC has a spindle motor (which might have its own built-in VFD), plus servo motors, a coolant pump, control electronics, and probably a chip conveyor. A single VFD can only power one motor. You need a phase converter to power the entire machine — the CNC's internal VFD will take care of spindle speed control.
Why is the VFD cheaper? Should I just try that first?
VFDs are cheaper because they're simpler devices solving a narrower problem. But "cheaper" is relative — after the 50% derating, you need a bigger VFD than you think. And if your application doesn't actually fit a VFD's limitations (single motor, nothing else connected), you'll end up buying a phase converter anyway. Better to match the tool to the job.
My Bridgeport has a VFD. Do I still need a phase converter?
If the VFD was added to give variable spindle speed and the original motor was replaced with a 3-phase motor, then yes — the VFD needs proper 3-phase input to work right (or you're dealing with the 50% derating issue). A phase converter feeding the VFD is the clean solution.
I've heard VFDs are bad for motors. Is that true?
VFDs can stress motor insulation due to voltage spikes in the PWM waveform, especially with long cable runs. This matters more for older motors or cheap imports. Modern "inverter-duty" motors handle it fine. A phase converter outputs clean sine-wave power — no stress on motor insulation. For vintage machines with original motors, this is worth considering.
What about those "digital phase converters"? Are they like VFDs?
They're in the same family — both use power electronics to synthesize AC waveforms. Digital phase converters are designed for phase conversion rather than speed control, but they share the VFD's problems with VFD-controlled loads downstream. That's why we recommend rotary converters for shops with modern equipment that might have VFDs built in.
Not Sure What You Need?
Tell us what equipment you're trying to run. We'll help you figure out if you need a phase converter, a VFD, or both — and we won't try to sell you something you don't need.