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 (I_E) consists of two components: the magnetizing current (I.) and the core loss (I_H). Magnetizing current lags applied voltage by 90, while core loss is in phase with the applied voltage (Figure 6b). V_p and V_sare shown 180 out of phase. I_H is very small in comparison with I_m, and I_m is nearly equal to I_E.No-load current, I_E,is also referred to as exciting current.

Figure 6 Open-Circuit Secondary

Example: When the secondary of a 120/440 V transformer is open, primary current is 0.2 amps at a PF of .3. The transformer is a 5 kVA transformer. Find: (a) I_P, (b) I_E, (c) I_H, and (d) I_m