ByAPSEEE

When primary side of transformer is connected to a.c. source and secondary side of transformer is put on load then it is said that transformer is on load. When the secondary side of a transformer is loaded, secondary current I2 is flowing through and secondary winding. The magnitude of current I2 depends upon terminal voltage V2 and impedance of the load. The phase angle of the secondary current I2 with respect to V2 depends upon the nature of load to be connected on secondary side.

• In case of resistive load current I2 is in phase with V2.
• For inductive load current I2 lags behind the voltage V2.

The operation of the transformer on load is explained with help of four steps;

Step-I

When the primary of transformer is connected to a.c. supply and secondary is open then it will draw no load current from the mains. The no load current I0 produces an mmf  N1Iwhich set up flux in the core.Step – II

When the transformer is loaded, then secondary current I2 is flowing is secondary winding. I2 produces an mmf N2I2. This mmf further produces Φ2. This flux opposes the flux which is set up by the current I0 according to Lenz’s Law.

Step – III

Since Φ2 opposes the flux therefore, the resultant flux tends to decrease and causes the reduction of self induced emf E1 momentarily. Thus V1 predominates over E1 causing additional primary current I’2 drawn from the supply mains. The amount of this additional current is such that the original conditions i.e. flux in the core must be restored to original value Φ, so that V1 = E1. The current I1 is in phase opposition with I2 and is called primary counter balancing current. This additional current I’1 produces an mmf N1I’1 which sets up flux Φ, in the same direction as that of Φ, and cancels the flux Φ2 set up by N2I2.Step – IV

Thus we have seen that flux is restored to its original value Φ.Phasor Diagram of a Loaded Transformer