Transformers TRANSFORMER THEORYCurrentRatioThe current in the windings of a transformer is inversely proportional to the voltage in thewindings. This relationship is expressed in Equation (13-5).(13-5)V_{P}V_{S}I_{S}I_{P}whereI_{P}= primary coil currentI_{S}= secondary coil currentSince the voltage ratio is equal to the turns ratio, we can express the current ratio in terms of theturns ratio, as in Equation (13-6).(13-6)N_{P}N_{S}I_{S}I_{P}Example 1: When operated at 120 V in the primary of an iron core transformer, the currentin the primary is 4 amps. Find the current in the secondary if the voltage isstepped up to 500 V.Solution:V_{P}V_{S}I_{S}I_{P}Next, we solve for I_{S}.I_{S}V_{P}V_{S}I_{P}I_{S}1205004 ampsI_{S}0.96 ampsExample 2: A transformer with 480 turns on the primary and 60 turns on the secondary draws0.6 amps from a 120 V line. Find I_{S}.Solution:N_{P}N_{S}I_{S}I_{P}Rev. 0 Page 7 ES-13

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