VFD for an Air Compressor vs a Phase Converter on Single Phase

A VFD (variable frequency drive) can run a 3 phase air compressor on single phase power when the compressor is the only machine on the drive and the drive is rated for the motor on single phase input. Lenze says most VFD makers only offer single phase input models up to 3 hp, and a few lines go higher. The compressor's pressure switch also has to be set up to work with the drive.

A rotary or digital phase converter is the other route. If other 3 phase machines will share the power, a rotary converter is the one to pick: a VFD runs one machine, and a rotary converter can run several.

At Built for Production, we put this together from drive makers' documents on single phase input and phase converter makers' guidance on compressors.

If you came here for a variable speed compressor to cut your power bill, that's a different machine and a different purchase. This article is about getting 3 phase power to a compressor you already own or are about to buy.

Why is an air compressor a hard start?

A compressor starts with load on it. The piston has work to do from the first strokes, while a lathe or a table saw spins up with almost nothing to push against. Phase converter makers call this a hard-starting load. They size a rotary converter above the motor's hp for it. Here is how three makers treat compressors:

• Phoenix Phase Converters' sizing guide puts piston compressors on its list of hard-starting loads.
• American Rotary's FAQ says: "For hard-starting loads (compressors, punch presses), add 25-50% capacity."
• North America Phase Converters (NAPCco) puts piston air compressors in the "Very Hard" tier of its motor load chart, the top of its scale.

The amps on your motor's nameplate don't show that starting load. Phoenix's article on sizing by amps says the nameplate "usually shows the running load, not the starting load."

Smaller piston compressors have a part that lightens the start: the unloader valve. Quincy's page on the unloader valve says that on smaller compressors it sits on top of or inside the pressure switch, and that by venting the air it "minimizes the load across the piston." Check whether yours has one.

Our phase converter sizing guide shows each converter maker's rule and the math.

How big a compressor motor can a single phase VFD run?

Lenze Americas' 2022 post "Derating VFDs for Single Phase Power" says: "Most manufacturers of VFDs only offer products up to 3 Horsepower (HP) for single phase input." A few lines go higher:

• Invertek Optidrive E3: 230V single phase input models from 0.5 to 5 hp (0.37 to 4 kW), per Invertek's Optidrive E3 specification page.
• Yaskawa GA500: 240V single phase models from 1/6 to 5 hp, per Yaskawa's GA500 catalog, BL.GA500.01. Yaskawa rates these drives by overload capacity, how far past its rated current a drive can run for 1 minute: 150% under "Heavy Duty" and 110% under "Normal Duty." The 5 hp model has a "Heavy Duty" rating only. The largest "Normal Duty" model is 3 hp. Ask Yaskawa which rating fits a compressor.
• Phase Technologies 1LH Series: VFD panels with 230V single phase input from 5 to 20 hp, per Phase Technologies' 1LH Series page, which lists compressors among their uses.

Drive makers size a drive by the motor's current, not its hp. Danfoss's article How do you size a VFD? says: "VFD sizing should always be based on motor current and voltage, not hp." Give the drive maker your compressor motor's nameplate amps and voltage, and tell them it's a compressor.

Above 5 hp: a larger single phase input drive, or a derated 3 phase drive

Above 5 hp you have two drive routes. One is a VFD with single phase input in a larger size, like the 1LH panels above. The other is a drive built for 3 phase input, fed single phase power and derated, which means run at a lower rating than its 3 phase nameplate. Yaskawa's application note Applying Drives to Single-Phase Input Applications (AN.AFD.15) says the derate keeps single phase input from overstressing parts inside the drive.

ABB's ACS550 single phase derate note (2014) says single phase 230V input gives 3 phase 230V output "with 50% reduction in current." So an ACS550 on single phase needs a current rating about twice your motor's.

The drive also pulls more current from your panel than the motor would on 3 phase power. UL's Question Corner column in IAEI Magazine (March/April 2018) puts the single phase input current at about 1.73 to 2.0 times the 3 phase motor current. Whether your circuit and your service can carry that is a question for a licensed electrician.

What happens if the drive is too small for the compressor?

An undersized drive faults, which means it trips and stops running. ABB's ACS550 note says running continuously above the derated current will cause faults. Yaskawa's AN.AFD.15 says that if the drive shows an input phase loss fault, it generally means the drive is too small for the load it's running.

How does a VFD work with the compressor's pressure switch?

With a VFD, the drive starts and stops the motor. On 3 phase power, the pressure switch does that job, either directly or through a magnetic starter: a switch box that turns the motor on and off when the pressure switch tells it to. On a drive, the pressure switch has to send the drive a run signal instead of switching motor power itself. The drive maker's manual shows how its drive takes that signal.

A magnetic starter between the drive and the motor needs care. Mitsubishi Electric's FAQ on its drives says switching the output side while the drive runs "causes open/close surges that may trigger the protective function," and says to switch it only "while the inverter is not operating." Mitsubishi calls its drives inverters.

Before you buy, ask the drive maker:

• How its drive takes a start and stop signal from a pressure switch.
• If your compressor has a magnetic starter, how its drive should work with it.

A licensed electrician makes the connections, working from the drive maker's manual.

How does a phase converter run an air compressor?

A phase converter feeds 3 phase power to the compressor. The compressor's own pressure switch and starter then turn the motor on and off the same way they would on 3 phase power from the utility. Our rotary vs digital vs VFD guide covers how each type works.

Rotary phase converter. Your single phase service brings in two of the three power lines a 3 phase motor needs. A rotary converter makes the third with an idler, a 3 phase motor that spins with no load on it. One rotary converter can run several machines. For a compressor, makers size it above the motor's hp for the hard start.

A compressor cycles on and off by itself. For it to start on its own, a standard rotary converter has to be running already, and the idler draws power the whole time it runs. American Rotary's FAQ puts that idle draw at about 3 to 10% of the converter's rated hp. Ask the converter maker whether its unit can switch itself on when the compressor calls for air. Our phase converter cost to run article has idle figures by maker.

Digital phase converter. A digital converter makes the third line with electronics instead of an idler motor. Phase Technologies' Gen4 manual says each Phase Perfect unit can start a motor up to the converter's own nameplate hp, so a 10 hp unit starts a 10 hp motor. That holds for models under 50 hp. Whether one digital converter can run several machines depends on the model, so ask the maker about the one you're looking at. Standby draw matters for a compressor that waits between cycles: the Gen4 manual lists 68 W for the 5 hp PT005, 70 W for the 7.5 hp PT007 and 74 W for the 10 hp PT010, at 240V.

Static phase converter. A static converter uses capacitors to start the motor, then lets the motor run on single phase power. Phoenix's comparison of converter types says the motor then puts out about two thirds of its rated hp. NAPCco marks piston and rotary screw air compressors "not recommended" for static converters. Our static phase converter guide covers the horsepower you give up.

VFD vs phase converter for an air compressor, side by side

By American Rotary's FAQ figure of about 3 to 10% of rated hp, a 10 hp rotary converter idles at about 220 to 750 W. Our cost to run calculator shows the same range.

Which one fits your compressor?

Check the motor nameplate voltage first. If it says 480V and your shop has 240V service, the voltage has to change as well as the phase. Take that to the compressor maker and your electrician before you buy anything.

For a 240V compressor:

• One compressor, motor at 3 hp or under, no other 3 phase machines planned. A VFD with single phase input is the first thing to price.
• Motor at 5 hp. A VFD with single phase input still fits: Invertek's E3 and Yaskawa's GA500 reach 5 hp, the GA500 under "Heavy Duty." Compare one with a rotary converter sized above 5 hp, or a digital converter rated for at least 5 hp.
• Motor above 5 hp. Your drive choices are a VFD with single phase input in a larger size, such as Phase Technologies' 1LH panels at 5 to 20 hp, or a derated 3 phase drive, which on ABB's ACS550 needs about twice your motor's current rating. Your converter choices are a rotary sized above the motor's hp, or a digital rated for at least the motor's hp.
• More 3 phase machines coming, like a lathe or a mill. Pick a phase converter. A VFD runs one machine, and a rotary converter can run several. For a digital converter, ask the maker whether the model runs several.
• You'd rather not add a drive or a converter. Fine Woodworking's guide to 3 phase machines in a one-phase shop calls replacing the motor with a single phase one "the first and most obvious option." The guide covers shop machines in general, not compressors, and says a swap may not work when the original motor has special mounting brackets or the shaft has custom threads or splines. Utilities also limit how big a single phase motor your service can start, and they judge it by the motor's starting current and code letter, not hp alone. FPL's service standards, for one, set the line for a 3 hp motor at 230V at code G and 80 A, and assume no more than four starts an hour, which a cycling compressor can pass. Our post Can a 3 phase motor be converted to single phase? walks through the utility check. Ask your utility and a licensed electrician before you go this way.

Will a VFD or phase converter void my compressor's warranty?

No compressor maker we found says whether a VFD or a phase converter affects its warranty, so ask your compressor maker in writing before you buy either one.

Quincy's FAQ ties warranty coverage to electrical requirements in general. It says the electrical requirements, along with piping and foundation, "affect performance, safety and warranty coverage." Check what your compressor's manual says about the power supply.

The drive has its own warranty terms. Yaskawa's AN.AFD.15 says a protection setting must stay on when the drive runs on single phase power: "the drive's input phase loss protection must remain enabled." Turning it off, the note says, "will result in permanent drive damage and void the warranty."

Can a VFD slow the compressor down to save power?

Whether you can run your compressor below full speed on a VFD is the compressor maker's call. Slow running is hard on cooling: Danfoss's FC 51 quick guide says running continuously below half the nominal motor speed "may require extra air cooling." Even compressors built to run at variable speed have a lowest speed: BOGE's white paper on variable speed compressors says their makers set "a minimum turn-down speed for their variable speed machines."

Saving power is what variable speed compressors are sold for, and even those may not pay off in a small shop. In an "Ask the Expert" column in Vehicle Service Pros in 2014, Kaeser's Michael Camber wrote that "a smaller shop with only one or two bays that does not have a constant demand for air, would not get the benefit" of a variable speed compressor.

Next step: size it

Grab your compressor's motor nameplate. Enter its figures in our phase converter sizing calculator, and it shows the converter size each maker's rule gives for your motor. If you're still weighing a VFD against a converter, email a photo of the nameplate to contact@builtforproduction.com or call (855) 923-2610. You can also reach us through our contact page.