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 (I_E) consists of two components: the magnetizing current (I_m ) and the core loss (I_H). Magnetizing current lags applied voltage by 90°, while core loss is in phase with the applied voltage (Figure 1b). V_P and V_S are 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 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 ,  (b) I_E   (c) I_H , and (d) I_m

Solution :

(a) Full Load Current = KVA Rating / Vp

(b) I_P at no load current is equal to I_E

(c) I_H = I_E Cos θ = I_E  x PF

I_H = 0.2 x 0.3 = 0.06 amps

(d) I_m = I_E Sin θ

θ = across 0.3 = 72.5°

I_m = 0.19 amps