# Rotary phase converter amperage draw



## Jonathans (Nov 15, 2014)

Ok. Newbie question here.
I may have a lead for a very inexpensive 40hp rotary phase converter.
I would like to use it to power a 5hp hydraulic forging press.
is the amperage draw based upon the 40hp converter, or the 5hp motor on the press?
It's an American Rotary converter


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## JimDawson (Nov 15, 2014)

An interesting question, I had to look this one up.

My best information is that the 40hp motor will draw about 25 to 35% of it's full load nameplate current at idle, or somewhere around 30 amps or so.  This assumes it is connected to a 3 phase supply, I would expect that it would draw a bit more when supplied with single phase.  The starting load is probably in the range of 250 amps or so, this could be mitigated by spinning the motor up with a small ''pony'' motor.

Add the full load amps of the 5hp motor, about 14 amps.  So total system draw would be in the 50 to 60 amp range.

Somebody else jump in here and point out the flaws in what I said.


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## Karl_T (Nov 15, 2014)

I run a 15 + 20 system in the big shop. No measurements ever taken but you can't see it change the speed of how fast the power company meter spins.  Anyway my guess Jim's numbers are way too high.

Now starting load on a straight 40 would be a REAL issue. I've got a thread on the 5 + 20 system I built my son last summer. The 5 was used to establish a 3 phase circuit so the 20 could come in smoothly without dimming the lights. It would have worked even better if the motors had been coupled with a pulley to get it spinning. In the case of a 40, I think you'll want to get it spinning either with this route or a single phase before bringing it in.

My two cents, unless you are adding more equipment, this is way too much converter for you.

Karl


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## chuckorlando (Nov 15, 2014)

Lotta juice there. I dont know what a good deal is number wise but building a small RPC is as cheap as the motor more or less. I use a cheap static converter to start my motor. Flip the switch on the static and she's ready to run.

So if you can find a decent motor for a decent price you could run a much smaller rpc. Of coarse mine is much much smaller then your thinking. I have 1hp on the bridgeport and run a 3hp rpc.

I'm electrically challenged so anyone can pull it off


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## John Hasler (Nov 15, 2014)

JimDawson said:


> An interesting question, I had to look this one up.
> 
> My best information is that the 40hp motor will draw about 25 to 35% of it's full load nameplate current at idle, or somewhere around 30 amps or so.  This assumes it is connected to a 3 phase supply, I would expect that it would draw a bit more when supplied with single phase.  The starting load is probably in the range of 250 amps or so, this could be mitigated by spinning the motor up with a small ''pony'' motor.
> 
> ...



Most of that 30 amps will be inductive current so the motor won't be as expensive to run as it may seem.  It'll draw about the same on single phase as on three phase.  You'll definitely want a pony motor to start it.  Otherwise it will dim the lights in the house and make the pole pig squeal.


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## Ulma Doctor (Nov 15, 2014)

Jonathans said:


> Ok. Newbie question here.
> I may have a lead for a very inexpensive 40hp rotary phase converter.
> I would like to use it to power a 5hp hydraulic forging press.
> is the amperage draw based upon the 40hp converter, or the 5hp motor on the press?
> It's an American Rotary converter




IMHO, you would be way better off finding a used 3 phase 7.5 hp motor to make the RPC.
you will save enough money on electrical savings to fund the entire venture, then you will have the added savings every month.
if you don't plan on using the RPC for other machinery it is really double overkill to use a 40hp, no matter the initial cost. and i like to over kill but that is way too much.
 here's a link to my design for a 3 phase motor starter, you would use it to start and run the RPC.
if you have any questions i'd be happy to assist.
click the link below for the plans

http://www.hobby-machinist.com/showthread.php/12712-Rpc-simple-design-unbalanced


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## Jonathans (Nov 15, 2014)

Thanks guys. I didn't realize how much the 40hp motor would draw as a converter, even with nothing running off of it.  I'm limiting to adding a 50amp circuit in the shop.  Or, perhaps I should consider swapping out the 3 phase motor with a single phase 220 5hp motor.  I do have a Chinese 5hp VFD that I was going to use, but friends said it wouldn't handle a 5hp hydraulic pump.
I could try it and the worst that could happen is I burn up the motor or the vfd or both, and then get a new 5hp single phase motor!


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## Ulma Doctor (Nov 15, 2014)

IMHO, i'd put the VFD to the test. 
If you run the VFD at 60hz, i seriously doubt you will inflict damage on the electric motor for the hyd pump.
Plus, you already have the VFD and a 50 amp circuit would be more than adequate.


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