Power loss in secondary = IS2RS = (20)2 (0.24) = 96 W
An open circuit test for core losses in a 10 kVA transformer [Example (1)] gives
a reading of 70 W. If the PF of the load is 90%, find efficiency at full load.
VS IS x PF
(VS IS x PF)
= transformer rating = 10 kVA = 10,000 VA
= 0.90; Copper loss = 100 W; Core loss = 70 W
Transformer Operation Under No-Load
If the secondary of a transformer is left open-circuited (Figure 6), primary current is very low
and is called the no-load current. No-load current produces the magnetic flux and supplies the
hysteresis and eddy current losses in the core. The no-load current (IE) consists of two
components: the magnetizing current (Im) and the core loss (IH). Magnetizing current lags
applied voltage by 90°, while core loss is in phase with the applied voltage (Figure 6b). VP and
VS are shown 180° out of phase. IH is very small in comparison with Im, and Im is nearly equal
to IE. No-load current, IE, is also referred to as exciting current.
Figure 6 Open-Circuit Secondary