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 InputPSPPx 100(13-9)EfficiencyPower OutputPower OutputCopper LossCore Lossx 100(13-10)EfficiencyVSISx PF(VS IS 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 RP = 0.3W, find RS and power loss in the secondary.Solution:Copper LossI2P RPI2SRS100 WTo find IP:NPNSISIPIPNSNPIS15204 ampsTo find RS:I2SRS100I2P RPRS100I2P RPI2S1000.3(4)22020.24ES-13 Page 12 Rev. 0
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