Adding a neutral to a 240 v delta transformer

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voltairzap

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Is there a legal technique to add a neutral to a 240 volt delta three phase transformer? (480 v delta Primary, 240v delta secondary)

The present transformer has no center tap between two power legs to derive 120 volts.

Is it possible to add an "outboard" single phase transformer (or two transformers in series) to create this center tap for 120 volt loads?

To be consistent with present practices, this center tap would have to be grounded as well.

Is this possible? workable? legal?
 
voltairzap said:
Is it possible to add an "outboard" single phase transformer (or two transformers in series) to create this center tap for 120 volt loads?
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If you have room for another transformer you should go with a 240V Delta-208/120V Wye

Roger
 
Beside the knock out answer that Roger gave! :)

Here's a link to 3 trannie that you spoke of.

uomschool.org

found by using Google / images to diagram 240v delta transformer...
 
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roger said:
If you have room for another transformer you should go with a 240V Delta-208/120V Wye
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Interesting. I'd suggest a 480D-to-208Y/120 3ph unit. Why add the new load to the existing unit?

Otherwise, I'd go with a 480-to-120/240v 1ph unit and supply it from the two least-loaded lines.
 
LarryFine said:
Interesting. I'd suggest a 480D-to-208Y/120 3ph unit. Why add the new load to the existing unit?

Otherwise, I'd go with a 480-to-120/240v 1ph unit and supply it from the two least-loaded lines.
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And with the information in the OP how do you know the transformer in question is not POCO owned and the actual service is not a 240 delta?

There may not be any 480 in the facility, the question is only asking about a 240 delta.

Roger
 
roger said:
And with the information in the OP how do you know the transformer in question is not POCO owned and the actual service is not a 240 delta?

There may not be any 480 in the facility, the question is only asking about a 240 delta.

Roger
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Y'all have a lot of POCO's with 480 primary distribution up your way? :grin:
 
mivey said:
Y'all have a lot of POCO's with 480 primary distribution up your way? :grin:
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That wouldn't matter, the problem is reading more into a question or statement than what is asked or stated.

How many tests or exams have you taken that answering more than what was asked worked out with the right answer, please give the exact number. :grin:

Roger
 
roger said:
that wouldn't matter, the problem is reading more into a question or statement than what is asked or stated.

How many tests or exams have you taken that answering more than what was asked worked out with the right answer, please give the exact number. :grin:

Roger
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37 .
 
Project details for this question

Project details for this question

I see that more details are required for my question.
The EXISTING service is 240v delta (4 wire with center tap neutral). We are using a step up transformer to feed the present 480 volt loads. This transformer is delta-delta.

The customer is upgrading his service to 480 volts and wants to refeed the existing 240 volt loads with the same transformer.
Unfortunately, the transformer has no center tap to derive 120 volts.
My question was how to reuse this transformer to refeed his present 240 /120 volt loads.

Any ideas?
 
That makes a bit more sense.

What is the KVA rating of the transformer?

What is the amp rating of the new 480V service?

How many KVA of 120/240V loads do you have?

-Jon
 
The existing transformer is 300KVA.

TOTAL 240 volt three phase loads equals approximately
200 to 250 KVA, including the 120 volt loads.

Thanks for your input

115 KVA ESTIMATED 120 volt load, distributed almost evenly between legs A and C
 
Okay, I have to admit that I was looking at this problem incorrectly, and thought that I had a solution.

For some reason I thought that you only had 120/240V _single phase_ loads to supply, and I was thinking about ways to use your 480:240 delta-delta transformer as a single phase 480:120/240 transformer. (The basic idea: leave one supply phase open and then treat the secondary terminal associated with the open primary terminal as the neutral.) Using this approach, I think that your 300KVA delta-delta transformer could be effectively used as a 200KVA single phase transformer.

Now I realize that you need to supply 240V three phase loads as well.

I think that your original proposal (using some sort of transformer to derive a midpoint tap on one of the delta legs) has merit. This would be the single phase equivalent of using a zig-zag transformer to derive a neutral point on a delta secondary. It seems to me that if you connected a suitable center tapped autotransformer from phase A to phase C you would derive a useable neutral.

However given the size of the loads, I believe that you would be better off with two separate transformers in a more standard configuration rather than trying to add an autotransformer to a delta. I believe that the unbalanced loading that you describe would push the A and C legs very near to full capacity, possibly beyond. Then you have the problem of sizing the autotransformer.

As a first pass, I believe that you would need the full 115 KVA capacity for your neutral deriving transformer. If I had to guess, a half sized weird transformer (as an autotransformer, you only have losses in one coil, so an X KVA isolation transformer can serve as a 2X KVA autotransformer) would not cost less than a full sized standard 3 phase transformer; in fact I'd expect the weird transformer to cost more.

-Jon
 
Further thoughts: if you use the 'center tap deriving transformer', it would only see load equal to the imbalance between the two 120V branches. So one might get away with sizing that transformer for the maximum unbalanced load. However then one has the question of overload protection for the transformer; how to protect the transformer without creating the problem of an open neutral.

Another possibility: use the current transformer as a single phase transformer (described above) and then add two additional small single phase transformers to create a 3 phase bank.

-Jon
 
winnie said:
However given the size of the loads, I believe that you would be better off with two separate transformers in a more standard configuration rather than trying to add an autotransformer to a delta.
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I agree with this. A 240v Delta unit for 3ph loads, and a separate 208Y/120v unit for 1ph loads.

I think you'll be a lot happier when making up your panels, too.
 
LarryFine said:
Why, yes, you have, 'way back in post #2.



Everybody, Roger gets full credit for this suggestion. ;)
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I don't need credit, I'm just wondering why you wanted to copy mine now. :grin:

Roger
 
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