How Are Transformer Turns Calculated Using Voltage Measurements?

[encoder007]

Question: The nameplate on a 50 kVA transformer shows a primary voltage of 480 V and a secondary voltage of 120 V.We wish to determine the approximate number of turns on the primary and secondary windings. Toward this end, three turns of wire are wound around the external winding, and a voltmeter is connected across the 3-turn coil. A voltage of 76 V is then applied to the 120 V winding, and the voltage across the 3-turn winding is found to be 0.93 V. How many turns are there on the 480 V and 120 V windings (approximately)?

Answer: With 76 volts applied, the constant is 0.93v/3T = 0.31 V/T. Correcting that for 120 volts instead of 76, that becomes (0.31 V/T)(120/76) = 0.489 V/T.So the 120 volt winding has 120 V / 0.489 V/T = 245 turns the 480 would have 4 times as many or 980 turns.

Thats the answer what my teacher gave me but i don't understand it,if that's right can anyone explain me in detail. Or Should it not be like this when i tryed to solve it:

The worked out ratio is 0.31 V/T
Primary turn = 1548 turns for 480 V
Secondary turn = 387 turns for 120 V

Please any suggetions and explanations will be helpful.Thank you.

 

[NascentOxygen]

Hi encoder007! http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif

The answers you get would be correct had the test been performed with 120V applied to the 120V winding. But that wasn't so. Only 76V was applied, so you have to scale up the measurement of volts/turn to what would apply when using 120V.

For future questions, note that the forum for homework help is the homework subforum.

 

[rude man]

This problem seems a bit bizzarre. The only way the computations make sense is if the 3 turns of wire saw the same transformer flux as the transformer's own primary and secondary windings. But these are obvioulsy not accessible from outside the transformer.

What does "3 turns of wire are wound around the external winding" mean? What external winding?

 

[I_am_learning]

I guess the three turns were wound on top of the windings; outside the insulations. It would work that way.

 

[CWatters]

I did it like this...

The 120V winding has an unknown number of turns = X.

During the test the ratio of the voltages = ratio of turns eg...

76/0.93 = X/3

Rearrange to give..

X = 3 * 76/0.93 = 245

So the 120V winding has 245 turns.

The 480V winding has an unknown numbe of turns Y:

480/120 = Y/245

Y = 245 * 480/120 = 980

So the 480V winding has 980 turns.