# excitation branch in the equivalent circuit for transformers

Discussion in 'Homework Help' started by Erica, May 18, 2011.

1. ### Erica Thread Starter New Member

May 18, 2011
15
1
I am reading a textbook about transformers. According to the book, the excitation branch in the equivalent circuit of a transformer are two elements (Rc and Xm) in parallel. It says the total excitation admittance is Y = 1/Rc - (1/Xm)j. Any idea why the indutance is minus instead of postive?

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
Because this is talking about admittance (Y) - rather than impedance (Z).

3. ### jegues Well-Known Member

Sep 13, 2010
735
43
Ahhh! That makes much more sense!
So, $Z = R_{c} + jX_{m}$

$Y = \frac{1}{Z} = \frac{1}{R_{c}} + \frac{1}{jX_{m}} = \frac{1}{R_{c}} - \frac{j}{X_{m}}$

Last edited: May 18, 2011
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4. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
No quite right ....

Zm=Rc||jXm

$Z_m=\frac{R_c*jX_m}{R_c+jX_m}$

and so on ...

5. ### Erica Thread Starter New Member

May 18, 2011
15
1
You are right. I just prove that (1/Rc)-(1/Xm)j=1/Z. Thanks so much!