Current on primary side of center tap transformer.

[Do You Like Ham]

Hi--

I'm looking to try to verify what is happening in an app.. I don't think the results are right, and want to be able to help explain to the developers what should be happening.

In the scenario below, the primary has a 277V (L-N) connection.

The secondary is a 240V w/ center tap.

On one of the 120V legs is a 10A load.
On the 240V legs is a 20A load.

How would one properly compute the current on the 277V primary?

 

[synchro]

Can be calculated several ways. Here's two.

1. Using superposition of the two load currents reflected back to the primary using the inverse of the transformer voltage ratio (the inverse of the turns ratio is needed for currents):

(120/277) x 10A + (240/277) x 20A = 21.66A

2. Using fact that input volt-amps is equal to total load volt-amps:

120v x 10A + 240v x 20A = 6000 VA

Input current = 6000 VA / 277V = 21.66A

 

[Do You Like Ham]

Oh, that's great.. I actually didn't expect it to be that intuitive! Thanks much for the quick reply.