Transformer on Load: When the 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 the secondary current in transformer I2 with respect to secondary terminal voltage V2 will depend upon the characteristic of load i.e. current I2 will be in phase, lag behind and lead the terminal voltage V2 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 I2 flows through the secondary winding. The secondary current I2 sets up its own MMF and hence a secondary flux ϕ2. The secondary flux ϕ2 opposes the main flux ϕ set up by the exciting current in transformer I0 according to Lenz’s law. The opposing secondary flux ϕ2 weakens the main ϕ, so primary back emf E1 is reduced. SO difference of applied Voltage VI and back emf E1 increases, therefore, more current in the transformer is drawn from the source flowing through the primary winding until the original value of flux ϕ is obtained. It again causes an increase in back emf E1 and it adjusts itself as such that there is a balance between applied voltage V1 and back emf E1. Let the additional primary current in transformer I1‘. The current I1‘ is in phase opposition with secondary current I2 and is called the counterbalancing current. The additional current I1‘ sets up an MMF N1I1‘ producing flux ϕ1‘ in the same direction as that of main flux and cancels the flux ϕ2 produced by secondary MMF N2I2 being equal in magnitude.

so N1I1‘ = N2I2

or I1‘ = (N2/N1)I2

The total primary current in transformer I1 is, therefore, the phasor sum of the primary counter-balancing current in transformer I1‘ and the no-load current in transformer I0, which will be approximately equal to I1‘ as I0 is usually very small in comparison to I1‘. Thus, the primary current in the transformer increases when a load is connected across the secondary of the transformer.

Now I1 = I1‘ = (N2/N1)I2

or (I1/I2)=(N2/N1) = K (transformation ratio)

since the secondary flux ϕ2 produced by secondary MMF, N2I2 is neutralized by the flux ϕ1′ produced by MMF N1I1′ set-up by counterbalancing primary current in transformer I1′, 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 the transformer can be raised or lowered as per turns ratio. The product of VI is the same on both sides.

There are two separate windings called the primary winding and secondary winding. When a load is connected to the secondary of transformer, I2 flows through it. Current in transformer I2 creates a secondary flux ϕ2. The flux ϕ2 opposes the main flux setup by I1. The flux ϕ2 weakens the main flux ϕ. Because of this E1 reduces and the difference of voltage V2 and E1 increases. Hence more current in the transformer is drawn from the source unit and the original flux ϕ is obtained. The additional current in the transformer drawn is in phase opposition with the secondary current in transformer I2 which produces a flux ϕ1 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.