VFD as single to 3 phase converter

Kbracing96 · Nov 8, 2016

Ok, So I'm new here... go easy on me, lol. So I just picked up my first piece of equipment for my man cave shop, a 14x40 Birmingham lathe. Came from a school so hours on it are very minimal. SO anyway, in my research I've found I can use a VFD to convert the single phase in my shop to 3 phase to run the motor but it needs to be wire directly to the motor and used as a motor control with all the old controls on the machine wired to low voltage and used to control the VFD.

I understand how that should all go I think but my dad keeps asking me why I can't just use the VFD as a single to 3 phase converter between the wall plug and the lathe and all I can tell him is the internet says I CAN'T! lol I've searched and search and can't find the explanation I need for him. So why is that? How will having on/off switching down stream of the VFD damage it? Thanks!

EJD3 · Nov 8, 2016

disconnecting the vfd output while under power could damage the unit.

JST · Nov 8, 2016

Damage is not likely with modern units.

The BIG issue is that there is NO reason to do it. Disconnecting implies re-connecting. BUT, a matched size VFD is unable to supply starting current to a motor if it is outputting full voltage when the motor is re-connected.

With a matched size VFD, you HAVE to start at low voltage/frequency and then ramp up.

If you have a VFD sufficiently large, you can just set it to mains frequency and switch motors on and off when connected to it. At least a couple folks on PM do just that. Many others have "Phase Perfect" converters. A "Phase Perfect" converter is essentially one section of a big VFD set up to provide the 3rd phase, similar to, but better than an RPC. It has NO problems with motors being switched on and off, if the draw is within its rating.

The limit with most VFDs is going to be the start current draw. That's why it takes an uneconomically large VFD to do the job.

Kbracing96 · Nov 8, 2016

Ok, thanks. I was just wanting to give him an answer other then "the internet told me so..." lol. I need to use this lathe for a job today and don't have time to rewire everything so the VFD controls it all so I'm going to just wire it into the input plug and then use the machine like it was made, but only put it in the run position on the lathe and turn it on and off with the face of the VFD. The VFD I have is a cheap Lapond unit from Amazon for a 3hp motor and the lathe is 3hp 3phase. I do have a good Mitsubishi VFD coming though in case I fry this one, lol.

abarnsley · Nov 8, 2016

VFD is meant to run a motor, not the control transformer/contactors/heaters.

Starting VFD will engage contactors, then motor.. Just the way that VFD's don't like. Manual says no switches/contactors on VFD output.

VFD will probably trip out, before motor starts. Or it might go up in smoke.

You still need to wire VFD output direct to motor input..

Why??? because the VFD manual says so.... It was designed as a smart motor drive unit, it is not a dumb 3 phase power supply..

The flimsy panel membrane switches, are intended only for set up of VFD. Use remote switches to activate VFD, via it's low voltage control terminal strip.

VFD MUST be in an enclosure (in manual). MUST use shielded control wiring (in manual)

Mark Rand · Nov 8, 2016

Once the Mitsubishi one has been delivered, you can try using the cheap one as a replacement 3 phase mains. The biggest problem you're likely to see is contactors/relays chattering when you turn the motor on, due to the VFD limiting it's output current.

I do run all of the machines in my home shop from a single VFD, but it's a big one and the manual specifically states that it's happy with down stream switching.

VFDs have had decent protection circuitry and algorithms for a couple of decades now.

Kbracing96 · Nov 8, 2016

The lapond VFD was $127 and came with only a single sheet of basic programming codes and a basic wiring diagram, no real instructions... Not much info to go off of for setting it up, lol. Not to worried about killing it though. When I get a free day, I'll rework the lathe and correctly wire in the good VFD with proper motor controls via the correct inputs. The Mitsubishi has a MUCH better manual! But at $285, it should! lol

abarnsley · Nov 9, 2016

Good VFD's have decent protection circuits........

Oversized VFD's can handle transients/switching, MUCH better than a ragged edge cost cutting design, run at designed max rating.

Setting VFD amps to protect motor is required, how is each different machine wired to (US) electrical code, if not installed IAW manual???

Why risk damaging even a cheap VFD???, It would be handy for a belt sander or such.

Just install it right, the first time.... The enclosure is required to keep chips/dust/mist out of VFD. One tiny chip, can take out the biggest, nicest VFD.

Mark Rand · Nov 9, 2016

abarnsley said:
Setting VFD amps to protect motor is required, how is each different machine wired to (US) electrical code, if not installed IAW manual???

That's what the heaters in the motor starters are for.

Kbracing96 · Nov 9, 2016

Well, I got it going on the Lapond. Seems to work fine on the slow speed, 2hp side of the motor, but won't start the high speed side. It's letting me get done the little alum bungs I needed to make so it a win for now, lol. But I will properly get it wired up in a couple weeks when I get caught up with my turbo kits.

JST · Nov 9, 2016

Mark Rand said:
That's what the heaters in the motor starters are for.

Yes indeed. If using it as a regular mains source, the machine is left as it was, NO changes, and all those items are still operational.

Chip Chester · Nov 9, 2016

To the OP -- don't feel bad, I think that was my first question here, too.

To Mark Rand -- can you share the make and model of the VFD you're successfully using, and the machines you're using it with?

Chip

Mark Rand · Nov 10, 2016

It's a bloody great 23KVA Danfoss box. It feeds a Hardinge HLV (1.5hp), a Jones&Shipman 1400 surface grinder (2hp+1hp+0.5hp) and a Beaver turret mill (2hp + .25hp). It's fed from the single phase mains via a stepup transformer and has a delta/star(wye) transformer on the output to provide a grounded neutral plus a filter to quieten the machines down.

Bit elaborate for a home shop, but cheaper than getting the electric utility to run 30 foot of cable into a conduit that I've provided them...

kamakazi · May 3, 2017

Did your Lapond come with a manual?

Kbracing96 said:
Ok, thanks. I was just wanting to give him an answer other then "the internet told me so..." lol. I need to use this lathe for a job today and don't have time to rewire everything so the VFD controls it all so I'm going to just wire it into the input plug and then use the machine like it was made, but only put it in the run position on the lathe and turn it on and off with the face of the VFD. The VFD I have is a cheap Lapond unit from Amazon for a 3hp motor and the lathe is 3hp 3phase. I do have a good Mitsubishi VFD coming though in case I fry this one, lol.

I also just purchased a Lapond. However I got mine used on Amazon, and it arrived without a manual so I don't have any of the programming codes to set it up. Did yours come with a paper manual or something in digital form?

I have an email in to Lapond through Amazon, but haven't heard back yet and was hoping to aquire a copy of the manual so I can make sure it works before my Amazon return window ends.

I am sure this Chinese thing is not as good as a unit with an actual brand on it, but $85 to experiment and determine if I want to convert my little 9x20 lathe this way or go with a DC drive actually fit in my budget.

Any help you could provide would be greatly appreciated.

Tim

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