# Size And Type Of 3 Phase Transformer?



## Ropata (Jul 2, 2016)

Hi guys,

I have just purchased a 10hp control board and a 10hp motor to suit, all I need now is a step up transformer but I have no idea what size/type I need. Can anyone help please?


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## Chipper5783 (Jul 2, 2016)

I suppose that depends on what you are planning to do?  If you are putting a transformer between an RPC and a machine, I would suggest a transformer at about 2x the largest motor you intend to run.  For example if you are going to power a 5 HP motor, that is about 4kW.  So go for a transformer rated to at least 8kVa.

What type??  Nothing special here - assuming you are looking at a straight forward 3 phase transformer??  You probably want one of those small, dry type, air cooled boxes (sits in the corner and humms).


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## Ropata (Jul 2, 2016)

Chipper5783 said:


> I suppose that depends on what you are planning to do?  If you are putting a transformer between an RPC and a machine, I would suggest a transformer at about 2x the largest motor you intend to run.  For example if you are going to power a 5 HP motor, that is about 4kW.  So go for a transformer rated to at least 8kVa.
> 
> What type??  Nothing special here - assuming you are looking at a straight forward 3 phase transformer??  You probably want one of those small, dry type, air cooled boxes (sits in the corner and humms).


Cheers for that. This is what I'm trying to run


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## Ropata (Jul 2, 2016)

I've tried looking everywhere for this type of dry transformer to these specs but am confused as hell. Am I after a 7.5kw or 7.5kva?http://www.kayind.com/sites/default/files/pdf_library/Step_up_connection_diagram.pdf
This what I'm trying to achieve, I have all but the transformer. Is there a site that specialises in these transformers?


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## John Hasler (Jul 2, 2016)

Ropata said:


> I've tried looking everywhere for this type of dry transformer to these specs but am confused as hell. Am I after a 7.5kw or 7.5kva?http://www.kayind.com/sites/default/files/pdf_library/Step_up_connection_diagram.pdf
> This what I'm trying to achieve, I have all but the transformer. Is there a site that specialises in these transformers?


You want kva.


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## JimDawson (Jul 2, 2016)

Normally transformers are rated in KVA.  In most cases, when sizing transformers, you can use KVA and KW interchangeably.  So yes, a 7.5 KVA transformer would work for a 10 HP motor, but it looks like you are only going to be running about 2.5 KW.  I assume the 10HP is for the RPC.


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## Chipper5783 (Jul 3, 2016)

My little Maho mill has a 2 speed motor (575V).  On the high speed setting, it is rated at 2.2kW.  I am running it on a 5HP RPC and with a 6KVA transformer.  It does not like starting on the high speed setting (I don't know if it is a transformer issue or something to do with the RPC).  The mill has a 9 speed gear box, and I normally operate on the low motor speed (an adequate selection for most of what I do).   Occasionally I use the high speed, by first pressing the low speed, then push high speed (each has its' own contactor) - works fine.


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## Ropata (Jul 3, 2016)

Yeah I went for a 10hp RPC because I plan on getting a mill soon. So does anyone know where or in what kind of industry I could find a 7.5kva, 3 phase, 220-440 transformer? I take it I'm looking for an auto transformer?


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## JimDawson (Jul 3, 2016)

The transformer in the the RPC drawing is an isolation transformer.  I'm not sure if an auto transformer would work with a RPC, I'd have to think about that for awhile.  You might check with the RPC manufacturer on that one.

Any heavy industrial facility might have something like that around, I would check with some local electricians, especially ones that do industrial.  They might have some leads on a suitable transformer kicking around.  I know where 3 of them are at, but I'm a long ways from you.  You can always go larger on the KVA if something is available for a reasonable price.


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## John Hasler (Jul 3, 2016)

JimDawson said:


> The transformer in the the RPC drawing is an isolation transformer. I'm not sure if an auto transformer would work with a RPC


It work fine as long as you did not forget that your neutral on the output side would be the middle of one of the legs.


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## markba633csi (Jul 3, 2016)

Could you elaborate on that John? I don't see a neutral on the drawing he posted.  Also couldn't he use a delta or wye type transformer, depending on what's available?  Wye types do have neutrals.
MS


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## John Hasler (Jul 3, 2016)

markba633csi said:


> Could you elaborate on that John? I don't see a neutral on the drawing he posted.  Also couldn't he use a delta or wye type transformer, depending on what's available?  Wye types do have neutrals.
> MS


I should have written "grounded conductor", not "neutral".  One of the wires coming out of the RPC is the grounded white wire of the single-phase supply.  This doen't matter with an isolation transformer: the secondary is a seperately derived system and the grounded conductor will be whatever you make it.  With an autotransformer, however, that grounded conductor connects to a tap so that all three of the autotransformer output terminals will be hot with respect to ground (but not symmetrically as they would be in a three-phase system with a grounded neutral).  It'll work fine for motors, though.


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## markba633csi (Jul 3, 2016)

Got it- I don't know how common 3-phase step-up auto formers are, I suspect the OP will end up with an isolation tranny so the neutral/hot tap stuff won't matter.  Ropata: post a pic and/or specs of the transformer you are considering and we will advise you.  Happy shopping...
Mark S.


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## John Hasler (Jul 3, 2016)

markba633csi said:


> Got it- I don't know how common 3-phase step-up auto formers are, I suspect the OP will end up with an isolation tranny so the neutral/hot tap stuff won't matter.  Ropata: post a pic and/or specs of the transformer you are considering and we will advise you.  Happy shopping...
> Mark S.


You *can* use three single phase transformers.


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## Ropata (Jul 4, 2016)

I would love to post a picture of one I'm looking to buy but damned If I can find anything even remotely close to what I need.


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## Ropata (Jul 4, 2016)

Would I need a 60 or 50hz transformer? This site looks promising. http://www.temcotransformer.com/3-phase-transformers.html

Second thoughts, they all look like they weigh too much to ship.


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## markba633csi (Jul 4, 2016)

It will be heavy that is for sure.  50 or 60 Hz would work OK.  Also like John said you could use three single phase and wire 'em up in a triplet.  Look at Temco TT 1772 it is only 55 pounds but expensive at 2,000 $. You might check ebay for something like that...
MS


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## Ropata (Jul 11, 2016)

How does this one look guys?

http://www.ebay.com.au/itm/11184960...99&_trkparms=gh1g=I111849606697.N36.S1.R2.TR4


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## JimDawson (Jul 11, 2016)

Might be a little small for your purpose.  0.4 KVA  vs the 7.5 KVA that you need for 10 HP


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## Ropata (Jul 11, 2016)

Yeah sorry. Thought it said 400v


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## talvare (Jul 11, 2016)

I'm guessing that the motor in your lathe isn't dual voltage (220/440) so that you could just re-configure the wiring at the motor ? If the motor is a single voltage, 440V motor, have you considered the possibility of changing out the motor to a 220V, 3 phase, 60hZ motor. It may not be any more expensive than the transformer you're trying to find. Of course, you would also have to make sure that the control voltage requirement for your machine isn't also 440V if there are components such as relays, magnetic starters, transformers, etc.  Just some food for thought.

Ted


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## John Hasler (Jul 11, 2016)

Have you looked into having the motor rewound for 240?


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## Ropata (Jul 11, 2016)

No I'm afraid the motor is 440 only 2 speed which rules out rewire and VFD. I wanted to go the rpc route to future proof for other machines I may purchase and to keep the lathe all original. If only I knew the transformer would be so hard/expensive to acquire, maybe I would have just changed the motor.


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## markba633csi (Jul 11, 2016)

I think you might have rushed into this- Myself I probably would have done the step-up first in single phase then used a vfd to get the 440 three phase... If I was determined to use the original motor.  You still could do that and maybe use the rpc for other stuff.  The direction you are going now won't be the most efficient and the 3 phase transformer will be pretty buzzy.  Not to mention expensive and BIG.  
Mark S


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## John Hasler (Jul 11, 2016)

Ropata said:


> No I'm afraid the motor is 440 only 2 speed which rules out rewire and VFD. I


I'm suggesting having it rewound by a motor shop, not just changing the connections.


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## Ropata (Jul 12, 2016)

http://www.trademe.co.nz/building-renovation/tools/power-tools/other/auction-1116648638.htm

The guy selling this has told me I could run my 3hp motor off a 2.2kw version of this unit shown in the link. Is he right or would the 4kw be a better option?


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## John Hasler (Jul 12, 2016)

Ropata said:


> http://www.trademe.co.nz/building-renovation/tools/power-tools/other/auction-1116648638.htm
> 
> The guy selling this has told me I could run my 3hp motor off a 2.2kw version of this unit shown in the link. Is he right or would the 4kw be a better option?


He's right.  That isn't a motor-generator, though.  If it was there would be no need for an autotransformer.


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## Ropata (Jul 12, 2016)

Please explain?


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## John Hasler (Jul 12, 2016)

Ropata said:


> Please explain?


A motor generator would consist of a single-phase synchronous motor connected by a shaft to a three-phase synchronous generator (they might share a common frame).  There would be two sets of terminals: one set for the motor and one for the generator.  The transformer would be unneeded as the generator could directly output 400V.


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## Ropata (Jul 12, 2016)

Ok, so he has just worded it wrong?


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## 4GSR (Jul 12, 2016)

Be a lot cheaper to have the motor re-wired to run on 220V's as John said to do.

I've done this same thing on two-speed motors over the years. One case, even re-wired it back to 440V!


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## John Hasler (Jul 12, 2016)

4gsr said:


> Be a lot cheaper to have the motor re-wired to run on 220V's as John said to do.
> 
> I've done this same thing on two-speed motors over the years. One case, even re-wired it back to 440V!


I think there may be some confusion here.  Rewinding, which is what I suggested, involves taking the motor completely apart, unwinding the armature, and rewinding it for the new voltage.  Not cheap, but probably cheaper than a new motor.  It's what motor rewinding shops do (among other things).  Rewiring would usually involve changing the existing windings from series to parallel.  That option is evidently not available here.


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## Chipper5783 (Jul 13, 2016)

Hi Ropata,  I hope I have not steered you in the wrong direction?



markba633csi said:


> I think you might have rushed into this- Myself I probably would have done the step-up first in single phase then used a vfd to get the 440 three phase... If I was determined to use the original motor.  You still could do that and maybe use the rpc for other stuff.  The direction you are going now won't be the most efficient and the 3 phase transformer will be pretty buzzy.  Not to mention expensive and BIG.
> Mark S



My experience with the RPC + transformer performs well.  I can't comment on how efficient it is - nothing gets very hot, the power bill hasn't changed much, the machines work well.  Of course, the more devices in the system, the lower the efficiency & reliability are going to be.  Mark is right about the buzzing noise, though it is still quieter than my RPC - I do have them located in a seperated part of the garage (along with the air compressor) - with the dividing door closed all is good.

Big?  Perhaps that depends on how much room a person has.  My 6kva transformer has a bit smaller foot print than my 5HP RPC.  The spec sheet should identify what the dimensions are and you could decide if that would be workable in your situation.

For multiple machines and easy controls, it is hard to beat the RPC option.

Reading about your efforts to find a transformer, I consider myself very fortunate to have found a, tested working, 240-575V, 3 ph, 6kva in close driving distance for $100.  I certainly thought it was a good deal at the time and acted on it immediately (I didn't haggle on price and got there before he changed his mind).

Good luck, David


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## JimDawson (Jul 13, 2016)

Chipper5783 said:


> I consider myself very fortunate to have found a, tested working, 240-575V, 3 ph, 6kva in close driving distance for $100.



That was a deal!


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## John Hasler (Jul 13, 2016)

Chipper5783 said:


> Hi Ropata,  I hope I have not steered you in the wrong direction?
> 
> 
> 
> ...


Three phase transformers are smaller and lighter for the same KVA than single phase ones.


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## Ropata (Jul 21, 2016)

Gunrunner you need to order a set of these for the Z axis hand crank. Bill did you just spit coffee on your monitor? :rofl:

View attachment 253356


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## Ropata (Jul 21, 2016)




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## John Hasler (Jul 21, 2016)

I still think you should take the motor to a motor shop and get a price on rewinding it for 240.


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## Ropata (Jul 21, 2016)

I was told It wasn't possible to re-wind. 
	

		
			
		

		
	



	

		
			
		

		
	
 Can you see any reason why not? Is re-winding quite cheap?


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