No-Load Transformer And Its Phasor Diagram

Before we start discussing about the no-load condition of a transformer, it is important to first understand that what is meant by no-load.

"A transformer is said to be on no-load when the secondary winding of the transformer is left open-circuited thereby reducing the secondary current to zero".

WHAT HAPPENS WHEN TRANSFORMER IS AT NO-LOAD?

During no-load condition, secondary current in the transformer reduces to zero and when an alternating voltage is applied to the primary, a small current I0 flows in the primary winding.

The current I0 is called no-load current of the transformer and it is 3 to 5 percent of the rated primary current.

No-load current is made up of two components Iu and Im.

Iu is called magnetizing component.It magnetizes the core. We can also say that it sets up the flux in the core and therefore Im is in phase with (flux). It is also called reactive or wattless component of no-load current.

The other component Iw supplies the hysteresis and eddy-current losses in the core. It is in the phase with the applied voltage V1. It is also called active or wattful component of no-load current.

Phasor Diagram At No Load

Phasor diagram for a transformer(real) under no-load condition is shown in the fig. below.

The flux  is taken as the reference phasor.  The magnetizing component of the no-load current Im, magnetizes the core i.e, it sets up the flux in the core. Therefore, Im is in phase with the flux.

Iw is in phase with the applied volage, V1.  The phasor sum of Im and Iw is I0. The angle 0, between Iw and I0 is called no-load power factor angle. The power factor on no load is cos(0).

Iw = I0 . cos(0).

Im = I0 . cos(0)

I0 = v{(Iw) + (Im)}

Since, E1 and E2 are induced by the same flux ?, therefore E1 and E2 will be in same phase , but E1 and E2 lags behind the flux ? by 90 degree. E1 = k. E2 (where k = T1/T2,  T1 & T2 are no. of turns on primary and secondary winding)

E1 and V1 will be opposite to each other due to Lenz Law. If the voltage drop in the primary winding are neglected then V1 will be equal and opposite to the E1.

Power input to the transformer during no-load condition is :-

P0 = V1 . I0. cos (0)