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.

transformer on no-load

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

phasor diagram of transformer on no-load

No-load current,

Primary power factor at no-load

Core loss, P0 = V1I0 cosΦ0 = V1Iw watts

Magnetising (reactive) volt amperes = V1I0sinΦ0 = V1Im volt amperes