Issues with a single-phase motor on a VFD
Issues with a single-phase motor on a VFD
(OP)
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!
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 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
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
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
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
RE: Issues with a single-phase motor on a VFD
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
RE: Issues with a single-phase motor on a VFD
Special Boost Phase
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