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_{m} ) and the core loss (I_{H}). Magnetizing current lags applied voltage by 90Â°, while core loss is in phase with the applied voltage (Figure 1b). V_{P} and V_{S} are shown 180Â° 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 1 : 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} ,Â (b) I_{E}Â Â (c) I_{H} , and (d) I_{m}**

Solution :

(a) Full Load Current = KVA Rating / Vp

(b) I_{P}Â at no load current is equal to I_{E}

(c)Â I_{H}Â =Â I_{E}Â CosÂ Î¸ =Â I_{E}Â x PF

I_{H}Â = 0.2 x 0.3 = 0.06 amps

(d)Â I_{m}Â =Â I_{E}Â Sin Î¸

Î¸ = across 0.3 = 72.5Â°

I_{mÂ }= 0.19 amps