When the primary side of a transformer is connected to the source of alternating current supply and secondary side is kept open, it is said to be transformer on no-load i.e. there is no load on secondary side. The secondary current I2 is thus zero. In this case, neither the secondary winding has any effect on the magnetic flux in the core nor it has any effect on the primary current.
In actual transformer, the losses cannot be neglected. Therefore, if transformer is on no load, a small current I0 called exciting current drawn by the primary. This current has to supply the iron losses (eddy current and hysteresis losses) in the core and a very small amount of copper loss in the primary. As discussed, no current flows in secondary side, so that secondary copper losses are neglected.
Therefore, current I0 lags behind the voltage vector V1 by an angle Φ0 which is less than 900. The angle of lag depends upon the losses in the transformer. The no-load current I0 has two components;
Active or Working Component
This component of current is represented by Iw and in phase with the applied voltage V1. Its function is to overcome the eddy current and hysteresis loss in the core of transformer, secondly a small copper loss I2 in the primary winding. It is also called wattfull component of no load current.
Iw = I0cosΦ0
Reactive or Magnetising Component
This component of I0 is represented by Im and produces alternating flux in the core. This component does not consume any power. Magnetising component of current Im is in phase with flux, so lags the voltage V1 by π/2. It magnetises the core. It is also called wattless component of no-load current. The no-load current I0 is small of the order of 3 to 5 percent of the rated current of the primary. Due to the eddy current and hysteresis loss, the current I0 in primary is not lagging V1 by 900
Im = I0sinΦ0
From the phasor diagram, when the transformer on no-load
Primary power factor at no-load
Core loss, P0 = V1I0 cosΦ0 = V1Iw watts
Magnetising (reactive) volt amperes = V1I0sinΦ0 = V1Im volt amperes