# transformer question

Discussion in 'Homework Help' started by yoamocuy, Oct 7, 2009.

1. ### yoamocuy Thread Starter Active Member

Oct 7, 2009
84
0
1. The problem statement, all variables and given/known data
A single phase 500V/250V transformer that supplies a rated load current of 10 A with a power factor of 0.8 lagging. At no load, the supplied current is 0.25 A with a power factor of 0.10 lagging. I am supposed to find the full-load primary current and express it in polar form. Winding impedance voltage drop can be neglected.

2. Relevant equations
I=Io + I2'
I2'=(N2/N1)*I2
V2/V1=N2/N1

3. The attempt at a solution

Full Load

500V/250V=N1/N2
I think that rated load current is = to I2
therefore, I2'=(250/500)*10 A
I2'=5 A

No Load

Io=0.25 A

using the equation I=Io + I2'
I= 5 A + 0.25 A
I=5.25 A

The correct answer is supposed to be 5.17 A at an angle of -38.9o

I have no idea how to find the angle, and I'm not sure where I went wrong with finding the magnitude of the current. If someone could help me with even just half of the question it would be greatly appreciated.

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
Refer the secondary full load current to the primary and add this referred value to the primary no load current (using complex numbers) - then you have the answer. You must use complex addition as the currents are not in phase.

3. ### yoamocuy Thread Starter Active Member

Oct 7, 2009
84
0
Ok thanks I got it.