Transformers Connections

' Can I reconnect this transformer to be used as 50kVA a single ph 480-120/240V?'

No

'The third transformer in the core will be disconnected and shunted.'

How exactly it look like 'disconnected and shunted'

Disconnected and shunted means that the third transformer of the bank in the common core will not be tied to the input and output lines. Shunted means the installation of one jumper on the secondary and primary coil leads to prevent it to work as a CT. I understand that if I leave them opened it can blow up if there is any current flowing thru. Am I correct?

Yes you can. Just make sure you use the outer two winding assys and that they are wire identically in phase with each other.

Transformer can be connected for single phase.

Connection for single phase is the same as for 3 phase, except one phase is left unconnect. Primary and secondary of unconnected phase can be either open circuit or shorted.

First of all, you are not going to get 50kVA out of it - instead you will get smoke if you try to load it with 50kVA load on one phase for too long. Each of the 3 windings of the transformer will be nominally rated for 25kVA (adds up to 75kVA for the 3 together), so connecting to 2 phases of the primary and the associated phase of the secondary phase-to-neutral picks up ONE winding, and will give you 25kVA of 120V single-phase. And you cannot get 240V out of this arrangement no matter how you connect it. Draw the phasor diagram and you will see. Watch out for those disconnected phases! They will have a voltage on them, but not what you want.

Much better to buy the right transformer for your application.

Yup... you got it. Have a GA point on me.

You can get 120/240V 50 kVa from this transformer.

This is what I want to do. But have a doubt of what will happens with the third transformer that is in the same core but disconnected. Naming the leads of the secondary on the two transformers to be in service; A & B for xmer no 1 and C & D for the second transformer; I think A & D are the live leads and B&C connected together is going to be the neutral point. Between A & C, I should obtain 240 volts and 120 to neutral. The primary connection is: both transformers connected in parallel as the drawing you have posted. I just want to be sure this connection is correct and no damage will occur to the transformer.

Carl Pugh, I see you're correct. I should have drawn the winding diagram, not the phasors.

I originally thought that if it could be done, one would need to chop up the winding connections within the transformer. However, I think it can be done at the manufacturer's supplied terminals. For the primary, if you label the terminals A, B, C from top to bottom, you can simply jumper C to A, and connect the 480V across A/C and B. This effectively shorts together the third winding so it is not used. You could probably then get away with just taping off the C-phase secondary terminal (that winding shouldn't develop any voltage) and use A, B, and N as the output connections.

The connection shown will absolutely positively work. However to be on the safe side it is probable advisable to:

Contact the transformer manufacture and obtain his advice.

Use a variable transformer and raise the input voltage slowly while monitoring the input current.

If a variable transformer is not available, use a resistor to limit the input current and quickly disconnect the input voltage is the current is too high.

4 large 120V incandescent light bulbs in series make a low cost resistor for this use.

The unused coil can have the primary shorted or open. Or grounded or floating.
The same for the unused coil secondary.

How did the phone calls with the transformer manufacturer go?