Transformers TRANSFORMER THEORYPower loss in secondary = I_{S}^{2}R_{S} = (20)^{2} (0.24) = 96 WExample 2: An open circuit test for core losses in a 10 kVA transformer [Example (1)] givesa reading of 70 W. If the PF of the load is 90%, find efficiency at full load.Solution:Eff. =V_{S}I_{S}x PF(V_{S} I_{S} x PF)Copper LossCore Lossx 100V_{S}I_{S}= transformer rating = 10 kVA = 10,000 VAPF = 0.90; Copper loss = 100 W; Core loss = 70 WEff =10,000 (0.90)10,000 (0.90)10070x 10090009170x 10098.2%TransformerOperationUnderNo-LoadIf the secondary of a transformer is left open-circuited (Figure 6), primary current is very lowand is called the no-load current. No-load current produces the magnetic flux and supplies thehysteresis and eddy current losses in the core. The no-load current (I_{E}) consists of twocomponents: the magnetizing current (I_{m}) and the core loss (I_{H}). Magnetizing current lagsapplied voltage by 90°, while core loss is in phase with the applied voltage (Figure 6b). V_{P} andV_{S} are shown 180° out of phase. I_{H} is very small in comparison with I_{m}, and I_{m} is nearly equalto I_{E}. No-load current, I_{E}, is also referred to as exciting current.Figure 6 Open-Circuit SecondaryRev. 0 Page 13 ES-13