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Will a VFD Output Single Phase?
VFD outputting single phase

Dumb question to some maybe, but I never did much with VFD's.

Anyways, VFD in question is a AC Tech 3 hp rated. I know I can input single phase, but never tried output as single phase.

Motor that I would like to run on output end is a Baldor 3 hp single phase on an old Gisholt converted turret lathe. Be nice to have a choice of more than six speeds.

Thank you in advance.

By Bob Peterson on 28 October, 2016 - 6:33 pm

Single phase motors have a starting capacitor. That makes them incompatible with VFDs.

By Steve Myres on 28 October, 2016 - 7:33 pm

Single phase motors will almost never operate well at variable speed due to the way they start. And the drive, if it works at all without this arrangement messing up the current sensing, will not deliver 3hp since you're not using all the IGBT's. Just replace the motor with a 3-phase unit of the same NEMA frame, rpm, and horsepower. Get a used one off Ebay if you don't want to buy a new one.

Hello Mays,

You can use a VFD to drive single phase motor, but the motor can't use a switched starting capacitor, it must be a split capacitor or shaded pole single phase motor. Also single phase output VFD should be used for centrifugal load such as fan and pump.

By Curt Wuollet on 30 October, 2016 - 12:56 am

Even if they did, single phase motors of the usual types wouldn't really work well. Remember the starting switch will drop in if the RPM goes too low and there are other issues with run capacitors and the design of the motors for a single frequency. A synchronous motor might work but they are not run of the mill for industrial equipment.


There are VFDs made for single phase motors, but as previously noted, they will ONLY work on PSC (Permanent Split Capacitor) or Shaded Pole motors. Neither of those are common on machinery, they are mostly used on fans and pumps where low starting torque is not an issue.

If you were to connect a single phase PSC motor to two poles of a 3 phase VFD, you must have a VFD that will NOT detect and trip on the lack of current on the 3rd phase. Most of them will and do not offer a way to disable that protection feature because it is not an approved use.

If you connect a Capacitor Start single phase motor to a VFD, it is a race to see what is destroyed faster; the capacitors in the motor or the transistors in the drive. If you connect a split-phase type single phase motor to a VFD, the start winding becomes damaged when the speed drops below the centrifugal switch speed.

SSAC (now part of ABB) used to have a something that would chop off voltage to small 1~ AC Motors, prbly not up to 3Hp and torque/speed performance probably pretty rough. If issue is voltage source available, get a rotary phase converter to produce 3~ from 1~ (American Rotary is good) and swap out the motor.

If you are going to swap out the motor with a 3 phase version, then the VFD will do the phase conversion for you. Single phase in, three phase out. It's a very common feature at 3HP and below, you can do it at any HP rating with most VFDs by doubling the size of the VFD as well.