Transformer Operation Under No-Load:

If the secondary of a transformer is left open-circuited that was show in the figure, primary current is extremely low and is known as the no-load current.  No-load current generates the magnetic flux and supplies the hysteresis and eddy current losses in the core. The no-load current (I_E) consists of two parts: the magnetizing current (I_m) and the core loss (I_H).  A Magnetizing current lags applied voltage through 90°, although core loss is in phase along with the applied voltage that was show in the figure.  V_P and V_S are display 180° out of phase.  I_H is extremely small in comparison with I_m, and I_m is nearly equivalent to I_E.  No-load current, I_E, is also referred to as exciting current.

Figure: Open-Circuit Secondary

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

(a)        Full load current = kVA Rating/VP

(b)      IP at no load is equal to I_E

I_E = 0.2 amp

(c)        I_H - I_E   cosθ - I_E   x PF

0.2 (0.3)

I_H =     0.06 amps

(d)       I_M - I_E   sinθ

θ = arccos 0.3 =         72.5°

= (0.2) sin 72.5° =      (0.2) (0.95)

I_M =     0.19 amps