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Magnetic Disturbances Affecting a Transformer Question

  1. Apr 24, 2014 #1
    1. The problem statement, all variables and given/known data
    Part A:

    A 250kVA, 11kV/50Hz supply single phase transformer has 80-turns on the secondary coil, calculate:
    i. The approximate values of the primary and secondary currents:
    ii. The approximate number of primary turns
    iii. The magnitude of the flux in Wb

    I think I have managed to conquer part A, however it is part B which I am struggling with:

    A transient disturbance signal affects the current on the primary coil and is in the form of a sinusoidal current signal caused by a magnetic coupling. Discuss the effect of this transient upon the system.





    3. The attempt at a solution

    Assuming the secondary voltage is 240:

    The approximate values of the primary and secondary currents:
    250000/11000=22.7A
    250000/240=1041.6A

    ii. The approximate number of primary turns

    ((11000).(80))/240=3667 turns

    iii. The magnitude of the flux in Wb
    240 =(4.44).(80).(50).(ϕ_M)

    φ = 13.5 mWb

    For Part B:

    I think that the transient signal will have very little affect on the system because it is being scaled down to a point of 45 times lower than what is on the primary coil. I therefore believe that the disturbance signal will be so weak on the output that it will have a negligible affect on the system.

    I was wondering if anyone could help clarify if I am on the right lines or I have missed the point all together? Thanks so much in advance.
     
  2. jcsd
  3. Apr 24, 2014 #2

    rude man

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    Homework Helper
    Gold Member

    Don't forget though that the secondary current is many times bigger than the primary current.

    Hint: assume an ideal transformer (primary and secondary inductances = ∞, unity coupling). What does that imply about the ratio between primary and secondary voltages and currents?
     
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