Reversing Transformer connections

[cvirgil467]

Our plant needs 480Y/277v service. We only have 208Y/120V service but do have a 480v delta - 208Y/120v transformer. Can i use this transformer to generate 480Y/277 service by using the 208Y/120v as primary and the 480v as secodary and ground one corner of the 480v delta to get a neutral?

 

[charlie]

Re: Reversing Transformer connections

No, you can get a corner grounded delta that way but it would still be a 480 volt feeder without a neutral (the grounded conductor is not a neutral). You could use a zig zag transformer to derive a neutral but I would suggest just getting the correct transformer to start with.

 

[kiloamp7]

Re: Reversing Transformer connections

I assume that the xfmr that you have is "plain Jane" with the 480V. side being pure 3-wire Delta connected. If that is the case, no, you cannot get 480/277V., with a usable "neutral" from it.

 

[templdl]

Re: Reversing Transformer connections

You omitted the KVA that you require but standard DTDT drive isolation transformers are available 208-460y/266 up to about 300kva.
DTDTs can be special ordered as well a the larger KVA liquid cooled transformers.

 

[steve66]

Re: Reversing Transformer connections

If the neutral wasn't required, could the 480v delta to 208/120V transformer be reversed to get straight 480V 3 phase?

Steve

 

[tom baker]

Re: Reversing Transformer connections

I don't think you can reverse a 480 x 120/208 transfomer. I had the same situation to provide 480 from 208 and there a listed step up transformer ( I got mine from Sq D), its delta wye, and reversing would get you wye delta. My point is the transformer has to wired per the instructions, reversing the wiring would be violation of 110.3(b).
Of course the listed step up transformer is a lot more expensive...

 

[jim dungar]

Re: Reversing Transformer connections

Tom,

Square D's transformer instructions do not list primary and secondary, instead they show high voltage and low voltage connections.


There are application considerations to reverse connections but probably not "listing" violations. Always confirm this type of application with the specific manufacturer of the transformer rather than using generalities.

 

[templdl]

Re: Reversing Transformer connections

I agree with Jim. As a former applications engineer for a large electrical manufacturer that impart supported DTDTs we never advised a customer that it would violate a warranty if a 480-208y/120 transformer was applied as a step-up transformer and that its only to be applied for step down use. It was a matter of installing it correctly, that is unbonding the neutral(X0) from the enclosure which it is normally supplied with and feeding X1, X2, and X3 only, and feeding and addressing the 480v delta as to if it should be corner grounded or not. Since the taps are on the HV side, adjusting the taps is to though of in reverse which can be a real brain teaser.

 

[jaxx]

Re: Reversing Transformer connections

Just what are you trying to accomplish?