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#205927 - 04/27/12 01:49 PM Transformer/engineering question
sparkyinak Offline
Member

Registered: 07/08/07
Posts: 1286
Loc: Alaska
I have a situation where I need to power a intermittent single phase large load with a three phase transformer so I will not be using one leg. Its short term hook up for two to three years at best so I do not want to buy a transformer ($$$$$) when I have a 3 phase dry transformer collecting dust of sufficent size. I have not come up on this type of config before so I have my conserns. Such as

*Would it be kosher to wire and tap only two of the three coils?

*if not, what issues would I have if I were to wire the high side 3 phase and only use two legs where the amperage in theory be upto 100 amps, 90 more realistically and only intermittenly?
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#205928 - 04/27/12 04:45 PM Re: Transformer/engineering question [Re: sparkyinak]
HotLine1 Offline

Member

Registered: 04/03/02
Posts: 6785
Loc: Brick, NJ USA
I've seen this done over the years, as having 3 phase wired as normal, secondaries going to a disconnect/panel, and single phase load being connected.

Can't say I heard of any problems.

IMHO, as long as the xfr is of sufficient KVA, you should be OK. Efficiency? May be an issue.

Scott....jump in here please!
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#205931 - 04/28/12 11:35 AM Re: Transformer/engineering question [Re: sparkyinak]
LarryC Offline
Member

Registered: 07/05/04
Posts: 775
Loc: Winchester, NH, US
Does the transformer have each end of the secondaries available for separate connection? If so, you could hook them up in a "dog leg" connection to get single phase out of a three phase source.

It is the same method that is used on three phase generators when they need to supply single phase loads.

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#205932 - 04/28/12 12:00 PM Re: Transformer/engineering question [Re: LarryC]
LarryC Offline
Member

Registered: 07/05/04
Posts: 775
Loc: Winchester, NH, US
I timed out while editting. Use the following "Scott" drawing.

12 lead generator drawings

Basically it is a open delta where Phases A & B are connected in opposing series and Phase C is connected in aiding series to the combined A & B. The output "L1"is connected to the free end of A, the "neutral" is the connection between B & C, and "L2" is the free end of C. The connection between A & B is not used.

This limits the unit's maximum power to 2/3 of full rating but is does load the 3 phase input equally.

Larry C

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#205939 - 04/29/12 04:32 PM Re: Transformer/engineering question [Re: LarryC]
sparkyinak Offline
Member

Registered: 07/08/07
Posts: 1286
Loc: Alaska
I am not sure if it's a 12 wire transformer or not. I currently do not have access to it.

Larry, I think the the 12 lead generator link is hinky. It takes me to some web site. I do have schems of generators and I didn't thInk of looking at it from your perspective.

Thanx for everyone's input
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#205940 - 04/29/12 07:12 PM Re: Transformer/engineering question [Re: sparkyinak]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9012
Loc: Estero,Fl,usa
Maybe I am missing something but why not terminate in a 3 pole disconnect and feed a single phase panel from there or just use a 3p panel with one 2 pole breaker plugged into it.

The transformer won't care.
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#205944 - 04/29/12 09:52 PM Re: Transformer/engineering question [Re: gfretwell]
sparkyinak Offline
Member

Registered: 07/08/07
Posts: 1286
Loc: Alaska
That is what I'm planning to do. By maxing two of three legs while having nothing on the third leg, I dont know such an unbalance would cause any issues for the transformer and or the circuits or not. That would be a rather expensive smoke test if overlooked something
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#205947 - 04/30/12 09:20 AM Re: Transformer/engineering question [Re: sparkyinak]
LarryC Offline
Member

Registered: 07/05/04
Posts: 775
Loc: Winchester, NH, US
The link is hinky because address got changed from electrical-contractor .... to electrical.contractor ....

The link is supposed to go to the technical drawings on this site.

Larry

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#205949 - 04/30/12 02:41 PM Re: Transformer/engineering question [Re: LarryC]
sparkyinak Offline
Member

Registered: 07/08/07
Posts: 1286
Loc: Alaska
Ah ha! That did it. Thanx Larry.

Btw, I'll get the rest of my barge pics posted some day. Got super busy. Finally getting on top of things.
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#205950 - 04/30/12 02:45 PM Re: Transformer/engineering question [Re: sparkyinak]
sparkyinak Offline
Member

Registered: 07/08/07
Posts: 1286
Loc: Alaska
And nice drawings Scott
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