TRANSFORMER THEORY TransformersCore losses are caused by two factors: hysteresis and eddy current losses. Hysteresis loss is thatenergy lost by reversing the magnetic field in the core as the magnetizing AC rises and falls andreverses direction. Eddy current loss is a result of induced currents circulating in the core.The efficiency of a transformer can be calculated using Equations (13-8), (13-9), and (13-10).(13-8)EfficiencyPower OutputPower InputP_{S}P_{P}x 100(13-9)EfficiencyPower OutputPower OutputCopper LossCore Lossx 100(13-10)EfficiencyV_{S}I_{S}x PF(V_{S} I_{S} x PF)Copper LossCore Lossx 100wherePF = power factor of the loadExample 1: A 5:1 step-down transformer has a full-load secondary current of 20 amps. Ashort circuit test for copper loss at full load gives a wattmeter reading of 100 W.If R_{P} = 0.3W, find R_{S} and power loss in the secondary.Solution:Copper LossI^{2}_{P} R_{P}I^{2}_{S}R_{S}100 WTo find I_{P}:N_{P}N_{S}I_{S}I_{P}I_{P}N_{S}N_{P}I_{S}15204 ampsTo find R_{S}:I^{2}_{S}R_{S}100I^{2}_{P} R_{P}R_{S}100I^{2}_{P} R_{P}I^{2}_{S}1000.3(4)^{2}20^{2}0.24ES-13 Page 12 Rev. 0

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