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 480v would have 4 times as many or 980 turns.
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