# Wiring and amp draw for 3 HP rotary phase converter



## Matt in TN (Nov 17, 2014)

I just bought a South Bend 9A that came with a rotary phase converter marked 3 HP and 6KVA.  Does that mean at 120VAC input it'll pull 50A (120V*50A= 6KVA)?  I only have a 1/2 HP motor on the lathe, and am trying to decide what size wire and circuit to run to it.


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## JimDawson (Nov 17, 2014)

Matt in TN said:


> I just bought a South Bend 9A that came with a rotary phase converter marked 3 HP and 6KVA.  Does that mean at 120VAC input it'll pull 50A (120V*50A= 6KVA)?  I only have a 1/2 HP motor on the lathe, and am trying to decide what size wire and circuit to run to it.



I'm a little confused here, Is the motor on the lathe a 3 phase motor?  If so then it probably is 230/460 volt.  I suspect that the input to the motor and phase converter needs to be 230V.

Assuming the input to the rotary converter is approximately 25 amps at 230V then #10 wire and a 30 amp breaker should work fine.  50 amps would require #8 wire and a 50 amp breaker.  

Assuming a 1/2 HP, 3 phase motor (2 amps at 230V), #14 wire would be fine.


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## Matt in TN (Nov 17, 2014)

Previous owner had what I assumed was 120VAC wiring (black, white, and ground) into it.  I didn't think to measure the voltage.  Wiring out was three wire (white, red, black, and ground).  Whites were both connected to "wire 1", blacks were both connected to "wire 2", and red out was connected by itself to "wire 3" of the phase converter.

I guess I'd better look at the motor nameplate more closely.  Is there any place I can read more about how a 3-phase rotary converter works?  I've heard they are very inefficient so I assume power in will be greater than the 1/2 HP the motor requires - but how much more?  And if I add a mill later can I use the same converter as long as I don't buy a mill with more than a 2.5 HP motor?

And right now it's just a grey box with 3 black wires sticking out of it.  I'd love to fully understand what goes on inside the box oher than "it's a motor".


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## JimDawson (Nov 17, 2014)

Many people just grab the nearest chunk of wire to connect their project, so you have to be careful with trying to go by wire color to determine voltages.

Yes, they are a bit inefficient, you have to add the RPC load to the machine motor load to get the power used.  The RPC will have an idle load somewhere around 30% of the Full Load Amps (FLA) of the idler motor.

Here is the best drawing I could find for a RPC

I couldn't find a good explanation of exactly how they work.  

Yes, the 3HP RPC will start a motor at least up to 2.5 HP, maybe a bit more.

If it's in your budget, VFDs are pretty cheap.   For your 1/2 HP lathe motor, you can even get one that has a 120V input and 230V, 3 Phase output.  The advantages here are variable speed, and less wiring in your shop.


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## Matt in TN (Nov 17, 2014)

Good info - thanks.  It turns out afriend of mine has an "extra" VFD, so I'll probably just go that route.  I was wanting to just hook up what I had a get to making chips, but it sounds like the VFD will be a better option in the long run.

Thanks again!


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## Fitch (Nov 20, 2014)

JimDawson said:


> Many people just grab the nearest chunk of wire to connect their project, so you have to be careful with trying to go by wire color to determine voltages.
> 
> Yes, they are a bit inefficient, you have to add the RPC load to the machine motor load to get the power used.  The RPC will have an idle load somewhere around 30% of the Full Load Amps (FLA) of the idler motor.
> 
> ...



You can make a significant reduction in the single phase line current by adding a power factor correction capacitor between terminals C and A on the terminal block in your diagram - it's the same type of capacitor used for the run capacitor but likely a different size. The size of the capacitor depends on the particular RPC motor. Add capacitance, measure line current. Add more, measure again. The single phase line current will decrease and then start back up. Use the value just before the line current was minimum.

It's true that the quadrature current isn't billed at full value, but it is real as far as the wire and circuit breaker size is concerned, and dissipates energy as resistive heating. I run my 5hp RPC and 3hp mill easily off a 20A circuit. Idle current is about 3 amps - from memory. I measured it about 15 years ago when I built the converter.

The RPC runs as an induction generator, it is self synching to line frequency.

Fitch


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## Karl_T (Nov 20, 2014)

I just want to say "Hi there Fitch"

You helped balance all three of my RPCs in about 2002. Haven't touched them sense. 

I did build "the Kid" one last summer.

Karl


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## countryguy (Nov 20, 2014)

to quote Barney Stinson: "Challenge Accepted"  (tell me ya'll have seen some of these please?). 
http://www.americanrotary.com/phase-converter-101
http://www.americanrotary.com/rotary-phase-converter-details




JimDawson said:


> Many people just grab the nearest chunk of wire to connect their project, so you have to be careful with trying to go by wire color to determine voltages.
> I couldn't find a good explanation of exactly how they work.


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## Ulma Doctor (Nov 20, 2014)

countryguy said:


> to quote Barney Stinson: "Challenge Accepted"  (tell me ya'll have seen some of these please?).
> http://www.americanrotary.com/phase-converter-101
> http://www.americanrotary.com/rotary-phase-converter-details
> 
> ...


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## Phils69 (Nov 20, 2014)

I guess one factor in the RPC vs VFD debate is if you have multiple 3 phase motors to power it just seems cheaper to run them off one one RPC than have multiple VFD's. The downside to the RPC's is that you have to listen to it run when you have the 3 phase in use. I run a 30 hp RPC and it does make make a hummmmmm. Like Jim said, #10 wire and a 30 amp breaker and it will be fine. Just remember that it needs 220v not 110, from a double pole breaker.


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## jbollman (Nov 20, 2014)

Matt in TN said:


> Previous owner had what I assumed was 120VAC wiring (black, white, and ground) into it.  I didn't think to measure the voltage.  Wiring out was three wire (white, red, black, and ground).  Whites were both connected to "wire 1", blacks were both connected to "wire 2", and red out was connected by itself to "wire 3" of the phase converter.
> 
> I guess I'd better look at the motor nameplate more closely.  Is there any place I can read more about how a 3-phase rotary converter works?  I've heard they are very inefficient so I assume power in will be greater than the 1/2 HP the motor requires - but how much more?  And if I add a mill later can I use the same converter as long as I don't buy a mill with more than a 2.5 HP motor?
> 
> And right now it's just a grey box with 3 black wires sticking out of it.  I'd love to fully understand what goes on inside the box oher than "it's a motor".



11-20-14
Dear phase converter owner:  I have used a phase converter in my shop for the last 18 years.  I also built a phase converter ( 5HP) for an aquantence of mine.  Running  single phase power to a 3 phase motor ( correct HP) & change  it to a converter then 3 phase power comes 
out of the 3 phase motor.  This set up works very well.  The start circuit consists of starting capacitors across 2 legs. This arangemest
is controlled by a contactor of which it is controlled by a 0-2 second timer.. 


The 5 HP converter I built I was able to get all 3 legs within 2 volts of each other.  Balancing all three legs results in the lowest amp draw and motor will not heat up very much.   Some time later I will send a schematic of how to do this project.  Once one understands the function it is quite an easy project.  The balance procedure is to get all 3 legs in the closest volt balance.  This is done by adding AC Capacitors across the 
legs in a particular fashion I realize that this may be hard to understand at this time but in the next week or so I will put a schematic in the "forum" area. 

Regards - jbollman)


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## Matt in TN (Nov 24, 2014)

I'm still waiting on the VFD, so to ensure that it comes tomorrow I went ahead and wired up the RPC.  I ran 240VAC (single phase) to it and everything works great - thanks everyone for the help!


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## countryguy (Nov 24, 2014)

*CNC rated RPC Vendor. : Wiring and amp draw for 3 HP rotary phase converter*

Just to add to the thread for future searches.   This ebay store was sent to me by a guy who uses these.    Of course the motor's are the pricey parts, but I have found some nice AC motors if you dig... Be ya'll know that anyway. 

http://www.ebay.com/itm/10-HP-CNC-G...147?pt=LH_DefaultDomain_0&hash=item45f8c18f63

Surplus Motors:
http://www.surpluscenter.com/Pages/Contact-Us/

http://www.surplusindustrialsupply.com/


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