**Transformer On Load**

**Transformer on Load :** When load is connected across secondary terminals, the **transformer is loaded** and **current in transformer** flows through the secondary winding and the load. The magnitude and phase of secondary **current in transformer **I_{2} with respect to secondary terminal voltage V_{2} will depend upon the characteristic of load i.e. current I_{2} will be in phase, lag behind and lead the terminal voltage V_{2} respectively when the load is resistive, inductive and capacitive.

When a **Load** is connected across the secondary terminals, as shown in fig. **current in** **transformer **I_{2} flows through the secondary winding. The secondary current I_{2} sets up its own mmf and hence a secondary flux ϕ_{2} . The secondary flux ϕ_{2} opposes the main flux ϕ set up by the exciting current in transformer I_{0} according to **Lenz’s law**. The opposing secondary flux ϕ_{2} weakens the main ϕ, so primary back emf E_{1} is reduced. SO difference of applied Voltage VI and back emf E_{1} increases, therefore, more current in transformer is drawn from the source flowing through the primary winding until the original value of flux ϕ is obtained. It again causes increase in back emf E_{1} and it adjusts itself as such that there is a balance between applied voltage V_{1} and back emf E_{1}. Let the additional primary current in transformer I_{1}‘ . The current I_{1}‘ is in phase opposition with secondary current I_{2} and is called the counter – balancing current. The additional current I_{1}‘ sets up a mmf N_{1}I_{1}‘ producing flux ϕ_{1}‘ in the same direction as that of main flux and cancels the flux ϕ_{2} produced by secondary mmf N_{2}I_{2} being equal in magnitude.

so N_{1}I_{1}‘ = N_{2}I_{2}

or I_{1}‘ = (N_{2}/N_{1})I_{2}

The total primary current in transformer I_{1} is, therefore, phasor sum of primary counter-balancing current in transformer I_{1}‘ and **no-load **current in transformer I_{0} ,which will be approximately equal to I_{1}‘ as I_{0} is usually very small in comparison to I_{1}‘ . Thus, the primary current in transformer increases when a **load** is connected across secondary of **transformer**.

Now I_{1} = I_{1}‘ = (N_{2}/N_{1})I_{2}

or (I_{1}/I_{2})=(N_{2}/N_{1}) = K (transformation ratio)

since the secondary flux ϕ_{2} produced by secondary mmf N_{2}I_{2} is neutralized by the flux ϕ_{1}‘ produced by mmf N_{1}I_{1}‘ set-up by counter balancing primary current in transformer I_{1}‘ , so the flux in the **transformer core **remains almost contact from **no-load to full load**.

Hence, the ratio of primary current to secondary current in transformer is exactly equal to the ratio of secondary turns to primary turns.

**Transformer Is Constant Flux Device :**

**Transformer Is Constant Flux Device: **The **transformer is a static device** and works only on AC. There is no rotating part. It transmits power from one circuit to another without changing frequency but voltage and current in transformer can be raised or lowered as per turns ratio. The product of VI is same on both sides.

There are two separate windings called primary winding and secondary winding. When a **load** is connected to **secondary of transformer**, I_{2} flows through it. **Current in transformer I _{2} **creates a

**secondary flux ϕ**. The flux ϕ

_{2}_{2}opposes main flux setup by I

_{1}. The flux ϕ

_{2}weakens the main flux ϕ. Because of this E

_{1}reduces and difference of voltage V

_{2}and E

_{1}increases. Hence more current in transformer is drawn from the source unit and the original flux ϕ is obtained. The additional current in transformer drawn is in phase opposition with

**secondary current**

**in transformer**I

_{2}which produces a flux ϕ

_{1}in the same direction as the main flux ϕ and cancels the flux ϕ. Thus flux in the

**transformer core**remains

**constant at no**load to

**full load**.

**Hence Transformer is a constant flux device.**