How to Choose a Phase Converter for a CNC Machine

You can run a 3 phase CNC (computer numerical control) machine on single phase power through a rotary or digital phase converter. Single phase is typical residential power. Most shop machines are designed for 3 phase. First, the CNC maker has to allow a converter on that model. Second, the converter has to keep the voltage on its three legs balanced, meaning close to equal, within what the CNC's electronic control accepts. Haas, for one, says all phases must be balanced.

Skip static converters. A static converter helps a motor start, then leaves it running on single phase. American Rotary says static units should not be used to run a CNC. Size a rotary or digital converter by its own maker's CNC rule, which is usually a multiple of the spindle motor's hp. Those multiples run from 1.5 to 2.5 times, so a 10 hp spindle needs a 15 to 25 hp converter, depending on whose unit you buy. Phase Perfect, a digital converter, sizes by the whole machine's input amps instead.

Chapters: 0:00 Can a CNC run on a phase converter? · 0:44 Does your CNC need one? · 1:35 What the CNC maker says · 2:49 How balanced? · 4:01 Each maker's balance figure · 4:59 Which type · 5:26 The rotary's high leg · 6:26 What size · 8:32 What to have ready

Check in this order: whether your CNC needs a converter at all, what its maker's manual says about converters and balance, then the converter type, then the size. At Built for Production, we size converters by each maker's published rule and show the arithmetic.

Can a CNC machine run on a phase converter?

A CNC machine can run on a phase converter when its maker allows one on that model. Some CNC makers warn against converters. Some ban them on certain machines. Haas gives two reasons for its warnings and bans. Its VF-2 guide says "Phase converters can cause the machine to operate at less than full power." On some newer Haas lathes, a decal says use of phase converters "will cause damage to both converter and the machine." Phase-A-Matic, a phase converter maker, adds a third reason. On a rotary converter, the third leg runs higher than the single phase input voltage at light load, and Phase-A-Matic says some CNC equipment won't work properly at that higher voltage (more under the type table below).

Haas sets out three rules in two documents:

• The Haas VF-2 Pre-Installation Guide: "Do not use a phase converter unless it is necessary."
• The Haas Regen Vector Drive troubleshooting guide, TG0117: phase converters are not compatible with machines that have a Regen Vector Drive.
• The decal shown in TG0117: a decal prohibits phase converters on some Haas lathes. It goes on lathes that have a Regen Vector Drive and were built in 2026 or later. The ST-55 is the exception.

A regen drive can send power back into the supply when the spindle slows. To find out whether your machine has one, look for that decal on a newer Haas lathe, or ask the CNC maker with the model and serial number. If it has a regen drive, ask the CNC maker whether any converter is allowed before you price one.

For every machine, read the pre-install or site requirements page for your exact model first. Our post on running a CNC on a rotary converter covers the Haas wording and regen drives in more detail.

Do you need a phase converter for your CNC at all?

You may not need a phase converter, because some CNC machines are built to run on single phase power. Tormach's 1100M site requirements call for "Single-Phase 230 Vac, 50/60 Hz." Its PCNC 1100 specifications list "230-250 VAC 50/60 Hz single phase (primary)." Haas's VF-2 guide says some Haas machines can run on single phase power. Check your model's electrical specifications table, since Haas notes that some options, such as Through-Spindle Coolant on the Mini Mill, need 3 phase.

On a manual mill or lathe converted to CNC, the spindle may be the only 3 phase motor. A VFD (variable frequency drive) with single phase input can run that one motor. Phoenix's CNC guide puts it this way: "VFDs work well for single-motor applications like spindle-only conversions, but rotary converters are better for complete CNC systems."

How balanced does the power need to be for a CNC?

The CNC maker's manual for your model sets the limit on voltage imbalance. The converter maker's published balance figure has to come in lower than that limit.

Voltage imbalance measures how uneven the three legs are, as a percent. ABB's technical note on three-phase voltage imbalance gives the formula: 100 x (the largest difference of any leg from the average, divided by the average). Here it is on made-up readings from a 240V shop. Legs at 240, 246 and 237V average 241V. The furthest leg, 246V, is 5V off. 5 / 241 x 100 = 2.1%. Lower is better.

Not every CNC manual prints an imbalance percent. The Haas VF-2 guide says: "All phases must be balanced." It also asks that the supply voltage stay within plus or minus 10%. That 10% is a range for the voltage itself, a separate number from imbalance. If your manual gives no imbalance figure, ask the CNC maker what the control accepts.

Which type of phase converter works for a CNC?

Rotary phase converters and digital phase converters feed the whole machine: the control, the spindle, the servos and the coolant pump. A static converter can't run a CNC (more in our guide to static phase converters). A VFD with single phase input runs one motor.

Phase-A-Matic also makes a voltage stabilizer that pairs with a rotary converter and brings its third leg back down toward the input voltage (more on that under the table). Here's the balance figure each maker publishes, as percent imbalance between legs.

Compare your CNC maker's figure with this table. If the manual asks for a lower imbalance percent than the rotary figures, look at digital units. Their makers publish the lowest figures in the table, and the CNC maker's figure still decides.

American Rotary's CNC blog says CNC machines "require a voltage balance of +/- 5% or better," meaning the difference in voltage among the three phases should be less than 5%. Its FAQ gives its own units a range that goes up to that same 5%. A unit at the top of its range only just meets the 5% American Rotary gives for CNC. If your CNC manual asks for less than 5%, look for a unit or type with a lower published figure.

The high leg on a rotary. A rotary converter uses an idler, a 3 phase motor that spins with no load on it, to make the third leg (the manufactured or wild leg). Phase-A-Matic's voltage stabilizer brochure says that third leg "is normally higher than the input voltage under no-load and light-load conditions." On Phase-A-Matic's 230V models, it says single phase input above 230V can make the output voltage too high. A 240V shop's input is above 230V. The brochure warns: "Some CNC / PLC equipment will not work properly at the higher output voltage."

The stabilizer fix. Phase-A-Matic's voltage stabilizer brings the third leg back down to near the input voltage. CNC PAC is Phase-A-Matic's name for the pair. Its brochure describes the package as "one Rotary Converter and one Voltage Stabilizer of equal HP rating."

Digital. Power electronics build the third leg, with no idler. More in our digital phase converter guide, and side by side in rotary vs digital vs VFD.

What size phase converter for a CNC machine?

Use the CNC rule of the converter maker whose unit you're pricing. Most of those rules are a multiple of the spindle motor's hp, from 1.5 to 2.5 times. Phase Perfect sizes by amps.

Here's each rule on a made-up 240V CNC lathe. It has a 10 hp spindle and servo rapid traverse (servo motors that move the axes at full speed). Its data plate reads 32 A total input.

Phoenix publishes three CNC figures on three pages, so a Phoenix unit can come out at 15, 20 or 25 hp depending on which page you read. If you're pricing Phoenix, ask Phoenix which figure it applies to your machine.

Servo rapids and newer machines push the size up. Phoenix's sizing guide uses 2.5x for CNC machines with servo rapid traverse. Phase-A-Matic says newer CNC models with high spindle speeds or regenerative braking "may require the CNC PAC sized even larger." A regen drive is a question for the CNC maker first, and Phase-A-Matic's note applies where the CNC maker allows a converter. Its brochure also prints: "CONSULT FACTORY FOR SIZING AND OTHER VOLTAGES."

For Phase Perfect, use the whole machine's total input amps from its data plate. The spindle motor's amps alone leave out the control, the servos and the pumps.

What if other machines share the converter?

Add every machine that runs at the same time to the converter size. Phoenix's sizing guide sets the converter at the largest motor times its multiplier, plus the other motors running at once. Its own example: a 10 hp mill running with a 5 hp lathe, 10 x 2 + 5 = 25 hp. Other makers count several machines their own way, so use the method of the maker you're pricing. Our phase converter sizing guide sets each maker's rules side by side.

My CNC is 480V and I have 240V single phase

Phoenix notes that a rotary converter on its own puts out a 3 phase voltage that matches its input: 240V in, 240V out. A 480V machine on 240V single phase needs more than a converter. Phoenix's answer is a rotary converter plus a step-up transformer. The converter maker's engineers size the two as one package. Before you buy, call us at (855) 923-2610 with the voltage and amps off the machine's nameplate. We'll show you each maker's published rule for your numbers and the questions that go to the maker's engineers. Our guide on whether a transformer can make 3 phase from single phase covers what the transformer does and doesn't do.

What to have ready before you buy or call

If you're buying a used CNC, get these before you pay:

• Spindle hp and FLA. FLA (full load amps) is printed on the spindle motor nameplate. The multiplier rules start from the hp.
• Total input amps. From the machine's own data plate, for the whole CNC. Phase Perfect sizes by this number.
• Model, serial number and year built. The pre-install rules depend on the model, and the Haas decal on the year. The CNC maker needs the serial number to answer questions about your machine's drive.
• The CNC maker's input requirement. Voltage range, phase, any balance figure and any line about phase converters.
• Regen drive or not. If you can't tell, ask the CNC maker before anything else.
• Your single phase voltage. Your electrician can measure it at the panel.
• Your panel's amp rating. A licensed electrician decides whether your service can carry the converter. The electrician also makes the connections.
• What else runs on the converter. Every other machine, and which ones run at the same time.

What if your CNC already faults on a converter?

If your CNC throws alarms or resets on the converter you have, call the maker of that converter and a licensed electrician. The maker's number is usually in the converter's manual or on its website. Have the alarm codes and the machine's model ready. You can also call us at (855) 923-2610.

Our post on running a CNC on a rotary converter covers common faults, such as the phase fault alarm.

Get a size for your CNC

Call us at (855) 923-2610 with your spindle hp, the FLA off the spindle motor nameplate, and the machine's total input amps. Add the CNC maker's input requirement if you have it. We'll run each maker's CNC rule on your numbers and show you the math. If you'd rather work out the size yourself, the phase converter sizing calculator applies the published sizing rules of three converter makers to your nameplate numbers: American Rotary, Phoenix and North America Phase Converters (NAPCco). It names the maker's document it used, so you can check the figure at its source. You can also reach us on the contact page.