Will a Machine With Its Own VFD Run on a Rotary Phase Converter?

A machine with its own VFD (variable frequency drive), such as a CNC mill or lathe, can often run on a rotary phase converter if the machine's maker allows a converter. The converter also has to be big enough for the machine. And its three output voltages have to stay close enough to each other to meet the drive's limit, which the machine maker sets.

Converter makers such as Phoenix Phase Converters and American Rotary say their rotary units run CNC machines with built-in drives. That's each maker's claim about its own units. The machine maker gets the final say. Haas, for one, says phase converters are not compatible with its machines that have a Regen Vector Drive, a newer Haas drive design.

Once the machine maker allows a converter, size comes next. Each converter maker publishes its own sizing rule. Phoenix's CNC rules, for example, usually size the converter at twice the spindle hp. When you bring Built for Production a sizing question, we apply each maker's rule to your machine's numbers and show you the math.

Does your machine's maker allow a phase converter?

Start with the machine maker, because a converter maker can only speak for its own converter. The machine's manual, pre-install guide or service line tells you whether a converter is allowed and what input the drive accepts.

Haas puts its rules in writing. These apply to Haas machines only:

• The Haas VF-2 Pre-Installation Guide says "Do not use a phase converter unless it is necessary." It also says "All phases must be balanced."
• The Haas Regen Vector Drive troubleshooting guide, TG0117, says phase converters are "not compatible with a machine equipped with a Regen Vector Drive."
• The same guide shows a decal that goes on Haas lathes built in 2026 or later, except the ST-55. It reads: "Use of Phase converters is prohibited as it will cause damage to both converter and the machine."

If you're shopping for a used Haas, call Haas service with the model and serial number. Ask whether that machine has a Regen Vector Drive before you look at anything else.

For any other maker, read the electrical or input power section of the manual. If it doesn't mention phase converters, call the maker's service line with the model and serial number. Ask whether a rotary phase converter is allowed on that machine. While you have them, ask for the drive's voltage imbalance limit: how far apart its three supply voltages can be.

Why does a VFD care what a rotary converter puts out?

A VFD turns incoming AC power into DC, then builds new 3 phase power for the motor from that DC. The converter feeds the drive through three power wires, called legs. Two of them carry your incoming single phase power. The converter makes the third one itself. ABB's technical note on 3 phase voltage imbalance says a small difference in voltage between the legs causes a relatively large difference in the current each leg carries. In ABB's words, that "could cause excessive heating and it may even shorten the life of the capacitors" in the drive.

How balanced does the power need to be?

A typical drive allows a voltage imbalance of about 3% at its input. Imbalance is a percent. It compares the leg voltage furthest from the average of all three legs with that average. ABB's note says that at a drive's input, "a relatively small voltage imbalance of only up to 3-percent is typically allowed."

ABB lays out the way to figure imbalance, from the NEMA MG1 motor standard. Your electrician reads the voltage between each pair of legs: L1 to L2, L1 to L3 and L2 to L3. Average those three readings. Take the reading furthest from that average, divide the difference by the average, and multiply by 100. Here's the math on three example readings:

• Readings: 236V, 240V and 247V
• Average: (236 + 240 + 247) / 3 = 241V
• Biggest difference from the average: 247 - 241 = 6V
• Imbalance: 6 / 241 x 100 = 2.5%

That example is under ABB's typical 3%.

ABB wrote its 3% for drives in general. Its note doesn't mention phase converters, so the figure wasn't written with converter power in mind. Your machine maker's manual sets the limit for your drive.

Converter makers publish their own balance figures. A rotary converter makes the third leg with a spinning idler motor. A digital converter makes it with solid state electronics. A digital rotary is a rotary unit with a digital control. Each figure below is the maker's own, with the condition the maker states.

The converter makers also say what balance a CNC needs, and Phoenix's figure is tighter than American Rotary's. Phoenix's "The Complete Guide to Phase Converters for CNC Machines (2026)" says "Phase balance should be within 2-3% for optimal CNC performance." American Rotary's CNC article says "three-phase CNC machines require a voltage balance of +/- 5% or better."

American Rotary's 5% is looser than ABB's typical 3% drive limit. A converter that meets American Rotary's CNC figure could still fall outside what your drive accepts. Compare two numbers: the converter's published balance from the table, and the imbalance limit in your machine maker's manual. The machine maker's limit is the one that decides.

Why a reading at idle can fool you

A rotary converter's balance changes with the load on it. Chuck Yung of the Electrical Apparatus Service Association (EASA) wrote in Control Engineering that a rotary converter's output can be "drastically unbalanced" when few machines are running, or when all of them are heavily loaded.

That's why the reading that counts is the one taken while the machine works. Phoenix's article "Operating CNC, or VFD's with a Rotary Phase Converter" says: "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." If any leg is more than 3% off, whatever the cause, take it to the converter maker. Those readings come from inside a live panel, so have a licensed electrician take them.

What size rotary phase converter for a CNC?

Phoenix's CNC pages most often call for a converter of twice the spindle hp. Across its CNC pages, the multiples run from 1.5x to 2.5x. North America Phase Converters (NAPCco), another converter maker, lists a converter model for each spindle hp. American Rotary's general rule is also twice the hp of the load. For a machine with a drive, though, its FAQ says to size by the drive's input rating, so that's the American Rotary rule for your machine. The input rating is the power the drive is rated to draw, printed on the drive's nameplate or in the machine's electrical documents. Here are all of them on the same 10 hp spindle.

Phoenix's sizing guide uses "servo rapid traverse" in the heading for its 2.5x rule and doesn't define it. If you're not sure it applies to your machine, ask the machine maker.

Use the rules of the maker whose rotary phase converter you're pricing. Phoenix lists three CNC multiples, so ask Phoenix which one fits your machine. If other machines will run on the same converter at the same time, the converter has to cover them too. Our phase converter sizing guide shows the math for running several machines on one converter under American Rotary's and Phoenix's rules.

Where does the manufactured leg go?

The manufactured leg (the generated or wild leg) is the third line, T3, that the converter makes itself. The other two output lines, T1 and T2, carry your incoming single phase power. Both converter makers below say to keep control circuits and single phase loads off T3, and their manuals put them on T1 and T2 instead. Here is the voltage each manual says T3 reads, and where each says control circuits go.

Haas machines have a transformer inside whose setting depends on the incoming voltage. Haas's conversion kit instructions, AD0627, say "The incoming voltage determines the location of the jumper on the T5 transformer." Ask the machine maker which incoming lines feed the machine's transformers and its control. The converter makers want control circuits kept off T3, so you and your electrician need to know whether anything in the control takes power from that line.

We don't give wiring advice. A licensed electrician makes the connections, with both makers' documents in hand.

Common problems running a CNC on a phase converter

These are problems that makers' documents say converter power can cause, with who to ask about each.

If a CNC faults on an install that's already running, take your readings and the alarm codes to the machine maker's and the converter maker's engineers. They have the numbers for their own equipment.

Can a rotary phase converter handle regen braking?

Published documents don't settle it, so ask the machine maker and the converter maker before you buy.

Regen is what happens when a drive slows the spindle: energy from the spinning motor flows back into the drive. Fuji Electric's note "Regenerative Loads: When Do You Need a Dynamic Braking Resistor?" says that energy raises the voltage on the drive's DC bus, where the drive holds its DC power. If the drive can't get rid of the energy, it trips on DC bus overvoltage. A braking resistor is a part that turns that extra energy into heat. Fuji says one may be the answer when slowing the motor more gently isn't practical.

None of the machine or drive maker documents we read says how a regenerative drive behaves on a rotary phase converter. Haas's troubleshooting guide says phase converters are not compatible with its Regen Vector Drive. Haas lists that line under Alarm 646, Electrical Supply Error, and points to the incoming power as the cause. The decal quoted above says converter use damages both the converter and the machine. Neither one names regenerative braking as the reason, so don't assume regen is why Haas rules converters out. Phoenix's article "Digital (VFD) vs Rotary Phase Converters: Why Rotary Is the Smarter Choice" says a rotary converter "Can be used easily, even with regenerative load." That's Phoenix's claim about its own converters.

Ask the machine maker whether the drive turns braking energy into heat in a braking resistor, or sends it back out toward the supply line, which on your install means toward the converter. Then take that answer to the converter maker's engineers. It tells them what their unit would have to take.

Would a digital converter or single phase power to the drive work better?

If your machine maker's imbalance limit is tighter than a rotary maker's published figure, a digital phase converter may fit better. The two rows marked Digital in the balance table above run from 2% or less down to better than 1%. Our guide to rotary vs digital vs VFD compares the three types in more depth.

Some owners skip the converter and feed the machine's own drive with single phase power. A 3 phase drive fed single phase has to be derated, which means it can carry less than its 3 phase rating. Yaskawa's application note "Applying Drives to Single-Phase Input Applications" says "single-phase use requires the three-phase VFD power rating be reduced (derated) to avoid over stressing the rectifier and DC link components." Chuck Yung adds, in Control Engineering: "It is strongly recommended for the user to work with the drive manufacturer in selecting and sizing the VFD for this use."

Before you try it, ask:

• The drive maker or the machine maker whether the drive can run on single phase, and how far to derate it.
• The machine maker whether anything else in the machine needs 3 phase, such as a coolant pump.

Our guide to running 3 phase machines on single phase power covers four ways to do it: static, rotary and digital converters, and a VFD with single phase input.

What to ask before you buy a used CNC for single phase power

Get these answers from the seller, the manual or the machine maker before you pay:

• The nameplate voltage. If the machine runs only on 480V and your shop has 240V, a rotary converter alone won't do it. Phoenix's 480V page says a rotary converter puts out the same voltage it takes in. Its answer for a 480V machine is a rotary converter plus a step-up transformer. Phoenix also offers a call with its engineers. More on voltages in our post on 3 phase voltage.
• Whether the machine needs wye or delta power.
• The spindle drive's hp and input rating.
• The maker's input voltage range and imbalance limit, from the manual or the service line.
• Whether the drive sends braking energy back to the supply line or uses a braking resistor. On a Haas, whether it has a Regen Vector Drive.
• The model and serial number, so the maker's service line can look it up.
• The electrical or pre-install section of the manual. If the seller doesn't have it, the maker may.
• What else will run on the converter at the same time, such as an air compressor.

Get the converter sized for your machine

Once you have those numbers, run them through our phase converter sizing calculator. Its CNC option shows Phoenix's 2x and 2.5x rules, NAPCco's spindle hp chart and American Rotary's general 2:1 rule, with the math. Or call us at (855) 923-2610, or send your numbers through our contact page. Have the spindle hp, the drive's input rating, the machine's voltage and a list of what else runs on the converter. We'll go through each maker's rule for your machine with you.