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 I _{2}** 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 I_{0} 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 I_{0} lags behind the voltage vector V_{1 }by an angle Φ_{0} which is less than 90^{0}. The angle of lag depends upon the losses in the transformer. The no-load current I_{0} has two components;

**Active or Working Component**

This component of current is represented by I_{w }and in phase with the applied voltage V_{1}. Its function is to overcome the eddy current and hysteresis loss in the core of transformer, secondly a small copper loss I^{2} in the primary winding. It is also called wattfull component of no load current.

*I _{w }= I_{0}cosΦ_{0}*

**Reactive or Magnetising Component**

This component of I_{0} is represented by I_{m }and produces alternating flux in the core. This component does not consume any power. Magnetising component of current I_{m }is in phase with flux, so lags the voltage V_{1 }by π/2. It magnetises the core. It is also called wattless component of no-load current. The no-load current I_{0 }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 I_{0} in primary is not lagging V_{1 }by 90^{0 }

*I _{m }= I_{0}sinΦ_{0}*

From the phasor diagram, when the transformer on no-load

No-load current,

Primary power factor at no-load

Core loss, *P _{0} = V_{1}I_{0 }cosΦ_{0 }= V_{1}I_{w }watts*

Magnetising (reactive) volt amperes = V_{1}I_{0}sinΦ_{0} = V_{1}I_{m }volt amperes