Find out power loss in the secondary:

A 5:1 step-down transformer has a full-load secondary current of 20 amps.    The short circuit test for copper loss at full load provides a wattmeter reading of 100 W. If R_P = 0.3 ?, find out R_S and power loss in the secondary.

Solution:

Copper Loss I²_P   R_P +I²_S R_S=100 W

 To find I_P:

N_P/N_S =  I_S/I_P

I_P = (N_S/ N_P) I_S = (1/5) 20 = 4 amps

To find R_S:

I²_s   R_s -100 - I²_p R_P

 RS =   100 - I²_p R_P / I²_s =(100-0.3(4)²)/20² =0.24

Power loss in secondary = I²_s   R_s =(20)² (0.24) = 96 W