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Issues with a single-phase motor on a VFD

Issues with a single-phase motor on a VFD

Issues with a single-phase motor on a VFD

A well pump equipped with a 230V, single-phase, 1.5 HP, PSC motor with a nominal full-load current of 9A is being controlled through a VFD. Upon startup of the pump, it immediately draws approximately 18A (Allegedly measured by the electrical contractor at both the line-side and the load-side of the VFD) and causes the VFD to trip on an overload condition. The VFD is supposedly rated for single-phase in, single-phase out applications.

When the VFD is bypassed through an across-the-line starter, the pump draws 9A and operates with no issues.

The VFD and the pump were specified by another consultant, so the choice to go with a single-phase configuration was out of our hands. Years ago, I recall reading that controlling a single-phase motor via a VFD was basically asking for trouble. Is that still true today?

Any thoughts on what could be going on?

Thanks, everyone!

RE: Issues with a single-phase motor on a VFD

I believe the 18A number.

I had a dual voltage single phase 1HP fan that would draw ~48A from a 115V circuit for a few ms at startup.
It was used on a show display, and more current or more voltage would require an extortionate charge from the show facility.
I added a timer and a relay, so that it would start up wired for 230V and stay that way for ~6 seconds, drawing a peak of ~32A on 115V, after which the wiring would switch to the 115V configuration and the fan would continue accelerating to its normal speed, and could run from a normal 115V/15A circuit without reliably blowing a breaker.

If I had it to do over, I'd have ordered a 3 phase fan and used a single phase in VFD to ramp it with an arbitrary current limit.

As far as controlling the speed of a single phase motor with a VFD, AFAIK, it's still a bad idea that sometimes works well enough to allow you to say that it works.

Mike Halloran
Pembroke Pines, FL, USA

RE: Issues with a single-phase motor on a VFD

Very lame. The consultant should be dipped in sugar and rolled in ants.

No reason what-so-ever to not use a cheaper more reliable 3ph motor run by a VFD, fed single phase power.

As for the current draw a motor typically draws around 7 times its full load current while starting. 18A is less than I'd expect for this abused motor.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Issues with a single-phase motor on a VFD

Okay, amended; I believe the 18A number, given that the contractor probably used a cheapish slow current meter that couldn't catch the real peak current.

In my case, long ago, I spent ~$500 of company money on a nice meter that had both a peak capture and a graphical display that was fast enough to capture a few seconds of current history.
... and a whole lot of other things that were useful for debugging power electrics.
Nobody else knew how to use the meter anyway; I wish I'd stolen it when I left.
... and I wish I remembered where I got it, or even its manufacturer and model number.

Mike Halloran
Pembroke Pines, FL, USA

RE: Issues with a single-phase motor on a VFD

That cap is expecting 60Hz and will perform differently at different frequencies.

RE: Issues with a single-phase motor on a VFD


One would expect up to 7x FLA when starting a motor across-the-line, but in the case of a VFD, shouldn't the current draw be similar to that of a soft starter?

RE: Issues with a single-phase motor on a VFD


You raise an excellent point. It also makes me wonder if this motor is even inverter-duty rated.

RE: Issues with a single-phase motor on a VFD

You could try with simple voltage fan regulator instead of VFD.

RE: Issues with a single-phase motor on a VFD

Yes jmb. I was pointing out that while 63A DOL is reasonable your mangy setup is still considerably less than that and so not particularly onerous.

Since this single phase motor is being run with a VFD that can't depend on the physical amenities of the natural rotational exigent of a three phase motor, I'd never expect to see the same minimal current capabilities as being available to your setup.

As for a soft starter; even with a three phase motor they need at least 3x FLA to get motors started. Your situation is less than that and seems to be starting. I think you're okay with what you're seeing.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Issues with a single-phase motor on a VFD

Care to share the VFD make and model?

RE: Issues with a single-phase motor on a VFD

There are only about 3 manufacturers of true single phase MOTOR drives (albeit several more brand-name versions of the same 3). The main one I am familiar with is made by Invertek out of the UK, also sold under the names Anacon and Bardac here in the US, but they are all the exact same product. I have used one on a PSC motor in the past, their algorithm PURPOSELY provides a fixed 60Hz frequency at initiation in order to charge the capacitor and make the motor turn. That's something that you can't really get away from when using a single phase motor; it's inherent in the motor design. The purpose of the VFD is to control the speed after it starts, but because it's a single phase motor, that higher inrush current is going to be there no matter what you do. They do ramp the voltage, which helps to reduce that inrush current somewhat, but it is still going to be higher at first. Here is how they describe it:
Special Boost Phase


To ensure reliable starting of single phase motors, the drive initially ramps the motor voltage up to rated voltage whilst maintaining a fixed starting frequency, before reducing the frequency and voltage to the desired operating point.

I don't know anything about the other two I have seen advertised, but I can't imagine they do anything different with regard to that issue.

It's also possible that the VFD in question was not actually designed to operate a single phase motor. I have seen several cheap no-name Chinese drives being sold over the internet that have web page or eBay ads that CLAIM the drives are suitable for single phase motors, but that is incorrect, only evidenced by the manual, which they don't provide unless you buy the drive. it's likely a translation issue, but most of the sellers are fly-by-night outfits that sell off a chunk of inventory, then disappear (by changing their account on Fleabay or Amazon), leaving frustrated buyers in the dust.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

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