TRANSFORMER THEORY Transformers Core losses are caused by two factors:  hysteresis and eddy current losses.  Hysteresis loss is that energy lost by reversing the magnetic field in the core as the magnetizing AC rises and falls and reverses 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) Efficiency Power  Output Power  Input P_S P_P x  100 (13-9) Efficiency Power  Output Power  Output Copper  Loss Core  Loss x  100 (13-10) Efficiency V_S  I_S  x  PF (V_S  I_S  x  PF) Copper  Loss Core  Loss x  100 where PF  =    power factor of the load Example 1: A  5:1  step-down  transformer  has  a  full-load  secondary  current  of  20  amps.   A short 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  Loss I²_P  R_P I²_S  R_S 100  W To find I_P: N_P N_S I_S I_P I_P N_S N_P I_S 1 5 20 4  amps To find R_S: I²_S  R_S 100 I²_P  R_P R_S 100 I²_P  R_P I²_S 100 0.3(4)² 20² 0.24 ES-13 Page 12 Rev. 0