# Homework Help: Magnetic Disturbances Affecting a Transformer Question

1. Apr 24, 2014

### lewisbedwell

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. Apr 24, 2014

### rude man

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?