Sizing Transformers

[ongmark]

How do you figure out what the amperage is on the secondary that is a 120/240 1/Ph with the primary at 480/277 3/Ph on a 37.5 KVA transformer.

 

[bob]

Re: Sizing Transformers

KVA in = KVA out

 

[bphgravity]

Re: Sizing Transformers

Power-in is equal to power-out so simply use Ohm's Law - W / V = A.

37,500 / 240 = 156.25 Amperes

 

[charlie b]

Re: Sizing Transformers

Is this a trick question? You can't have a 3-phase primary and a single phase secondary. You can use one phase of the three phase system as the primary to a 480 - 120/240 or as the primary to a 277 - 120/240. But in those two cases, the primary is single phase.

So I would have to ask. Is the 35.7 KVA the combined ratings of three single phase transformers, of which you only have an interest in one, so that Bob's formula needs to use 11.9 KVA as the available power?

 

[coulter]

Re: Sizing Transformers

Originally posted by charlie b:
... You can't have a 3-phase primary and a single phase secondary. ...

Well you could, but it takes a twelve-lead secondary - a pair of 120V coils for each phase. I've seen three phase generators wired like this to give a single phase output, but never an xfmr - I don't recall ever seeing an xfmr with a twelve-lead secondary.

Even so, I agree with Charlie - trick question.

But, on the unlikely off chance it is as stated, the output kva is two-thirds of nameplate (I think ---- okay, pretty sure)

carl

 

[5Spot]

Re: Sizing Transformers

Originally posted by ongmark:
How do you figure out what the amperage is on the secondary that is a 120/240 1/Ph with the primary at 480/277 3/Ph on a 37.5 KVA transformer.

Hmm, I'm not aware of an industry standard transformer that's rated at 37.5 KVA and also 3 phase. The only 37.5 KVA industry standard transformer is single phase with a rated full load current on the secondary of 156 amps, an impedance of 1.5%, with the available fault current at 18,018 amps.

 

[rattus]

Re: Sizing Transformers

Carl,

You are right. The xfrmr with the CT delivers 2/3 of the single phase current, the other two deliver the remaining 1/3.

Rattus

 

[coulter]

Re: Sizing Transformers

Originally posted by 5Spot:
Hmm, I'm not aware of an industry standard transformer that's rated at 37.5 KVA and also 3 phase.

Yep, you're right: standard ratings are:
30kva and 45kva in 3ph
10, 15, 25, 37.5 kva in 1ph.

ongmark -
Could you verify the nameplate data and specific primary and secondary connections? We're guessing.

carl

[ December 29, 2005, 02:37 PM: Message edited by: coulter ]