### Transformer on Load

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 I_{2} is flowing through and secondary winding. The magnitude of current I_{2 }depends upon terminal voltage V_{2 }and impedance of the load. The phase angle of the secondary current I_{2} with respect to V_{2 }depends upon the nature of load to be connected on secondary side.

- In case of resistive load current I
_{2} is in phase with V_{2}.
- For inductive load current I
_{2} lags behind the voltage V_{2}.
- For capacitive load I
_{2} leads the voltage V_{2}

**Operation of Transformer On Load**

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 I_{0} produces an mmf *N*_{1}I_{0 }which set up flux in the core.**Step – II**

When the transformer is loaded, then secondary current I_{2} is flowing is secondary winding. I_{2} produces an mmf *N*_{2}I_{2}. This mmf further produces Φ_{2}. This flux opposes the flux which is set up by the current I_{0} according to Lenz’s Law.

**Step – III **

Since Φ_{2 }opposes the flux therefore, the resultant flux tends to decrease and causes the reduction of self induced emf E_{1 }momentarily. Thus V_{1 }predominates over E_{1 }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 V_{1 }= E_{1}. The current I_{1 }is in phase opposition with I_{2 }and is called primary counter balancing current. This additional current I’_{1} produces an mmf N_{1}I’_{1} which sets up flux Φ, in the same direction as that of Φ, and cancels the flux Φ_{2 }set up by *N*_{2}I_{2}.**Step – IV **

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

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