three phase transformer

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sahafi

Member
if I have 10,000 kva, 3 ph, 480/120-208v transformer. can i connect my single phase load 4000kva to the primery side and what is the max current in each primery winding ?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Re: three phase transformer

I started to answer this, than realized you said 10,000 KVA xfmr. Are you sure about the size? According to my calculator, thats 28,000 A @ 208V/3ph.
 

charlie b

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Location
Lockport, IL
Occupation
Retired Electrical Engineer
Re: three phase transformer

While you're at it, would you please explain what you mean by connecting a load to the primary side? Your question is not clear enough for us to help you with an answer.
 

sahafi

Member
Re: three phase transformer

I am sorry if the question not clear, the numbers in va not kva and agian i have three phase transformer but the load is single. my question, can this type of transformer take this type of load? and what is the max. load i can connect it to the transformer for each phase.
 

charlie b

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Location
Lockport, IL
Occupation
Retired Electrical Engineer
Re: three phase transformer

Three phase transformers behave more ?nicely,? if you keep the load balanced among the three phases. But yes, you can put a large, single-phase load on the secondary of a three-phase transformer.

However, the way they build a three-phase, 10,000 VA transformer is to take three individual, single-phase transformers, each rated at 3,333 VA, and connect them in whatever configuration (i.e., Wye or Delta) is needed. So if you want to power a single phase, 4,000 VA load, that would exceed the rating of your transformer. So, no, you can?t do what you propose.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Re: three phase transformer

Good point, engy! Let me revise my statements.

Sahafi?s specific question was how much load can you put on each phase. That answer, as I said above, is 3,333 VA.

But if this is a 208 volt, single-phase load, then it will draw its power from two of the three phases. Therefore, it will draw 2,000 VA from each of the two phases. That would be lower than the phase rating of 3,333 VA. So this is doable.

But if, as I had originally (and perhaps incorrectly) presumed, the load is a 120 volt, single-phase load, then I reaffirm my original statement: This cannot be done.
 

sahafi

Member
Re: three phase transformer

yes, it is 208v, but i did not understand how the load divided between two phases, it is one circuit, i mean the same current will pass throw the phases and the load.
 

rattus

Senior Member
Re: three phase transformer

To be exact, each of the two transformers will provide more than 2000VA.

Iload = 4000VA/208V = 19.2A

120V x 19.2A = 2304VA

A 2KVA transformer would be overloaded. That is, Imax would be 2000VA/120V = 16.6A.

This is a case where apparent power from the transformer is greater than apparent power in the load.

[ May 21, 2005, 11:37 AM: Message edited by: rattus ]
 
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