VFD to run 3-Phase on Single phase supply

[CMax]

Hi all,

In a previous thread, I raised the idea of getting some used machinery, and one thing I've noticed is that on eBay and suchlike, there seems to be a surplus of 3-phase machines at good prices. Now, I've never used these kinds of things and know very little about electrics. That said, some initial googling seems to suggest that it's fairly trivial to use a VFD to run a 3-phase machine (apparently it has to be an induction motor, is that correct?)

Has anyone done this, and is it as simple as it seems, or am I woefully informed and clutching the wrong end of the stick?

 

[Dr.Al]

I've done it for my metalworking lathe and also on my woodturning lathe (which is home-made, but uses a three-phase motor I bought on ebay) and in both cases it was very straightforward.

For a three-phase induction motor, they typically come star-wound, like the diagram on the right here:


That'll mean they're expecting 415 V three phase. A lot of motors have the six connections (either end of the coils) coming out to screw terminals in a terminal box on the top of the motor. This one is star wound (I can tell as there are three wire ends that are connected together):



If that's the case, you can disconnect the windings and re-connect them as delta-wound (left diagram at the top). You now have a three-phase 230 V motor.

Sometimes motors even come with wiring diagrams embossed into the body showing how to do delta (left) and star (right):



Once you've got it wound in delta form, you buy a cheap inverter (aka variable speed drive or VSD, aka variable frequency drive or VFD), wire it up and configure it as per the manual and you're away.

If the motor on your machine isn't rewireable to the delta arrangement, you'll need a more expensive inverter that also scales the voltage up to 415 V. These are readily available, they just cost about twice as much.

 

[fuse]

As per Dr Al -check motor configuration first
It can be magically simple - or not
My experience of them is they need rating well above the intended device and the cheap ones are a gamble in the quality of the build (and also the documentation if you need more than simple on/off/speed)

 

[Dr.Al]

Yep, I agree with all that Martin has said. If the motor is a 1.5 kW one and you're getting a cheap inverter, go for a 3+ kW inverter.

The one I bought for my metalworking lathe (from a decent brand, a Jaguar Cub or something like that if memory serves me correctly) came with a small book for a manual.

The cheap (< £50) one I bought from amazon for the woodturning lathe had a single sheet of paper for a manual and it included several things that turned out to be incorrect (including simple stuff like the output voltage for the control signals, which was listed as 10 V but was actually 5 V).

 

[Doug]

I’ve had a couple of inverters over the last 20 years the first was on an old Wadkin planer, I couldn’t re-wire the motor to Delta & found the cheaper option was to put another 3 phase motor on it that could be wire in delta & wired that through a VFD.

The second was on my Meddings pillar drill, this could be wired delta. I bought a station with the VFD & this made setting it up a breeze as I didn’t have to setup any parameters, just wired the motor & station to the VFD & switch on.
The station gave me stop/start, reverse & speed control in one unit

 

[AndyT]

If you want a bit more guidance than the excellent answers above, I'd like to mention that this forum used to have a resident motor expert, Bob "9fingers" Minchin. He wrote a guide to motors which covers three phase to single and inverters.

It's in a Dropbox, linked to from the forum.

Use the three line menu to go to Resources, then Machine Manuals, then Induction Motors issue 3.

You don't need to install Dropbox so dismiss any prompts/ads if you don't want it.