A question about single phase input on vfd

joehunt1 · Feb 7, 2017

Hi there.
I'm thinking about installing a vfd in my shop to run a 10hp three phase motor on an old Mazak lathe I just got. I'll just point out right away that I am not a electrician. I understand the need to double the size of the vfd to handle the extra load of converting to three phase. At the risk of sounding like a dumb a.s.s. Could a person run the L1 & L2 to the input of the vfd as normal, but also run another dedicated wire with a dedicated breaker to L3? I realize that this wire would be the same phase as L1 or L2 but was wondering if this would reduce the load on the vfd.
Not to mention the price of a 20hp vfd it would be very close to maxing out the service on my shop.
Is there any other way around this? Can one run a smaller drive with the 10hp motor and just not use it to full load?

Also going to have to run a transformer. Is it ok to run the drive before the transformer or is it better to run it with the higher voltage? It would be nice to keep it at the 230 just gives me the option to run other equipment of different voltages if the need arises.
Also this not along term thing as I plan to be move to a bigger shop with three phase power in the next couple of years. So I would like to keep the cost down and or buy equipment that will be useful down the road.
Looking forward to hearing some input
Joe

Jraef · Feb 7, 2017

No. The issue is not whether power is present on the terminals, it's the nature of that power. Rectifiers draw power in pulses at the peaks of the sine waves. 3 phase has a sine wave peak every 60 degrees, single phase has a sine wave peak every 90 degrees, so it must pull all of the current it needs in fewer pulses. If you simply run another line if single phase into the third terminal is it at the same sine wave point as one of the others, so no net gain,

JST · Feb 7, 2017

joehunt1 said:
Hi there.
I'm thinking about installing a vfd in my shop to run a 10hp three phase motor on an old Mazak lathe I just got. I'll just point out right away that I am not a electrician. I understand the need to double the size of the vfd to handle the extra load of converting to three phase. At the risk of sounding like a dumb a.s.s. Could a person run the L1 & L2 to the input of the vfd as normal, but also run another dedicated wire with a dedicated breaker to L3? I realize that this wire would be the same phase as L1 or L2 but was wondering if this would reduce the load on the vfd.

As mentioned, very little help, the only reason to do it is to fool a missing phase detector

joehunt1 said:
Not to mention the price of a 20hp vfd it would be very close to maxing out the service on my shop.

Not if you do not draw full power out of it.

joehunt1 said:
Is there any other way around this? Can one run a smaller drive with the 10hp motor and just not use it to full load?

Yes, that will work Usually about 1/2 the motor power is the practical limit

joehunt1 said:
Also going to have to run a transformer. Is it ok to run the drive before the transformer or is it better to run it with the higher voltage? It would be nice to keep it at the 230 just gives me the option to run other equipment of different voltages if the need arises.
Also this not along term thing as I plan to be move to a bigger shop with three phase power in the next couple of years. So I would like to keep the cost down and or buy equipment that will be useful down the road.
Looking forward to hearing some input
Joe

A pre-VFD transformer will be single phase, a post VFD transformer will be 3 phase. I suppose cost, and any intent to run the drive as a vector mode drive could push towards having it on the input side. I assume you have 240V and need 480?

Matt Matt · Feb 7, 2017

What you need is a Single phase input 10 hp 7.5KW that requires 50-60A on Single phase side and will produce 30 amp three phase. OR you need a 20 hp then derate it like you assumed.

I'm guessing you have 240 single phase and you want 575 three phase?

joehunt1 · Feb 7, 2017

Thanks for the replies.
I'm trying to make 480volt to the lathe.
The shop I bought the lathe from had 240 three phase. The lathe came with a 240 to
480 volt 15kva transformer. The local used parts store has a 25hp drive in 230 that would work for about 1000$cad but the guy there told me it wouldn't work the best running it through the transformer after. They also have a lot of 480volt drives but not sure if we can run the single phase through the transformer I have?
Also if I can go with the 25hp 230volt drive what would I have to run for braker size and wire size the motor is rated 12.8amps on 480volt.
Another option I have aswell is the motor can be wired 230volt but not sure how the lathe wiring would like this and I'm not like the sound of rewriting the lathe.
Just trying to find the most cost effective way.
Thanks
Joe

SAF · Feb 7, 2017

Phase and Voltage Conversion

Off topic from you question, by may be helpful...

If the lathe has electro-mechanical controls as existing, and using a VFD for phase conversion, will require machine wiring modifications. You normally can't power the controls from the VFD and retain the variable speed function. The motor needs to be wired directly to the VFD, and the machine control switches need to be ported over to the VFD from the existing operator controls.

You didn't want to get into the machine wiring, and expect to get utility 3 phase in the future. And are looking to power other equipment and be cost effective.

You may want to consider a rotary converter or a Phase Perfect, electronic phase converter. With either, the lathe could remain untouched, and you can use the transformer you have, downstream of the phase converter for the voltage conversion. This would also allow you to run other equipment at either voltage.

With of VFD you can power one motor, at a single voltage and requires rewiring of controls.

The lathe "may be" readily converted to low volt operation, it depends on the actual machine controls. It's incorporated in some control panels and not in others. The wiring, fuses and motor starters need to be twice the size for low voltage operation.If the controls and wiring are sized for low voltage operation it could be relatively simple to convert the machine. If not, it's not practical to change everything out, and larger components may not fit the enclosure.

Do you have a industrial electrician in your circle that could look at your controls? If you get a building with 3 phase, do you know what your likely voltage there will be? Will that voltage be different from the voltage your considering now?

SAF Ω

JST · Feb 7, 2017

I suppose that another temporary solution could be to run the lathe on a "static converter", which means essentially running it "single phased", with a circuit in the box that gets it started. Cheaper than a drive, probably cheaper than an RPC, likely not a problem, especially if you do not run more than a 4 or 5 HP cut with it.

The motor will run, but will probably get too hot with a bigger load on it than about half rated.

No change to the wiring, you DO need to make sure the lathe controls are powered by the two wires that stay powered up after starting. The instructions will explain that

http://www.phase-a-matic.com/StaticDescription.htm

http://www.phase-a-matic.com/PDF/SB.pdf

joehunt1 · Feb 8, 2017

SAF said:
Off topic from you question, by may be helpful...

If the lathe has electro-mechanical controls as existing, and using a VFD for phase conversion, will require machine wiring modifications. You normally can't power the controls from the VFD and retain the variable speed function. The motor needs to be wired directly to the VFD, and the machine control switches need to be ported over to the VFD from the existing operator controls.

You didn't want to get into the machine wiring, and expect to get utility 3 phase in the future. And are looking to power other equipment and be cost effective.

You may want to consider a rotary converter or a Phase Perfect, electronic phase converter. With either, the lathe could remain untouched, and you can use the transformer you have, downstream of the phase converter for the voltage conversion. This would also allow you to run other equipment at either voltage.

With of VFD you can power one motor, at a single voltage and requires rewiring of controls.

The lathe "may be" readily converted to low volt operation, it depends on the actual machine controls. It's incorporated in some control panels and not in others. The wiring, fuses and motor starters need to be twice the size for low voltage operation.If the controls and wiring are sized for low voltage operation it could be relatively simple to convert the machine. If not, it's not practical to change everything out, and larger components may not fit the enclosure.

Do you have a industrial electrician in your circle that could look at your controls? If you get a building with 3 phase, do you know what your likely voltage there will be? Will that voltage be different from the voltage your considering now?

SAF Ω

I don't think that I would care much if the speed control portion didn't work.

Realistically it could be a couple of years before I move to a shop with three phase power.

Most of the newer shops around here are 575 there are some around with 230. I've never seen one with 480.

I have a friend coming to have a look in a day or two that is an industrial electrician.

Thanks for the help.

joehunt1 · Feb 8, 2017

JST said:
I suppose that another temporary solution could be to run the lathe on a "static converter", which means essentially running it "single phased", with a circuit in the box that gets it started. Cheaper than a drive, probably cheaper than an RPC, likely not a problem, especially if you do not run more than a 4 or 5 HP cut with it.

The motor will run, but will probably get too hot with a bigger load on it than about half rated.

No change to the wiring, you DO need to make sure the lathe controls are powered by the two wires that stay powered up after starting. The instructions will explain that

http://www.phase-a-matic.com/StaticDescription.htm

http://www.phase-a-matic.com/PDF/SB.pdf

It looks like a good sultion. I think that I would like to try and get close to full power but if all else fails.
Thanks
Joe

joehunt1 · Feb 8, 2017

Well thanks for the info.
I'm thinking it looks like I'm going to have to rewire a the lathe to make it work with the vfd. Might just start looking for a rotophase or maybe I can make a primitive one if I can find a 15hp motor to use.
Joe

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