transformer question

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

  1. yoamocuy

    Thread Starter Active Member

    Oct 7, 2009
    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'

    3. The attempt at a solution

    Full Load

    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
    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
    Ok thanks I got it.