transformer question

[al henslin]

I am having a mental block. If you have a single phase transformer, 480 primary, 120/240 secondary with a 120 volt load from each secondary phase to the nuetral are they treated as 2-120 volt loads or as 1 240 volt load? To clarify, 17 amp load phase A to nuetral, 14 amp load phase B to nuetral, what is the amp load on the 480 volt primary phase A? Is it 7.75 or 15.5?

 

[eric stromberg]

7.75

Let's do the numbers.

On the secondary side, you have 3720 VA.
You can either calculate this from 14*240 + 3*120
or from 17*120 + 14*120. It doesn't matter.

Ignoring core losses, we are assuming the transformer is a unity power device. Since 3720 VA are on the secondary, 3720 are on the primary as well.

3720 / 480 = 7.75

Eric

 

[al henslin]

Thanks, that is what I thought but after being challenged by a friend I thought maybe I had forgotten how to do it.