# Effective turns ratio of 3 phase bank

Discussion in 'Homework Help' started by Sparky, Sep 20, 2010.

1. ### Sparky Thread Starter AAC Fanatic!

Aug 1, 2005
75
0
Greetings,

I've been brushing up on my "power" knowledge.

I found this problem and solution but need a little clarification.

Problem:
" The following transformer is composed of 3 single phase transformers or pri:sec voltage ratio 480:120 volts. Each transformer is rated 4.5kVA. The voltage source is a balanced 3phase positive sequence source with VA - 277.1 (0 degrees) v.

Find the effective 3-phase turns ratio."

a = VLL_primary / (VLL_Sec) = (VLN_Pri / VLN_Sec) = 2.309

I guess first question -

In the solution
The 480 is labeled the LL_Pri while 120 is labeled LN_Sec

The LN_Primary is 277.1.

How do we know the 480 is the LL_Primary and how do we know the 120 is the LN_secondary?

Thanks
-Sparky

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
You are not given this explicitly - I guess the implicit assumption is that the transformer is a distribution type with a typical primary line-to-line value of 480V and a typical secondary line-to-neutral value of 120V.

It would have made the issue abundantly clear if the question had simply stated that this was the case.

3. ### Sparky Thread Starter AAC Fanatic!

Aug 1, 2005
75
0
Thanks

I was hoping someone would agree with my take on the way the problem was worded.

-Sparky