- When the supply
voltage V
_{1 }is given to the transformer primary winding, primary current I_{1}flows through the winding which produces maximum flux F_{m}in the core. - When the secondary winding of the transformer is loaded, the
secondary current I
_{2}flows through it. - However the phase of current
I
_{2}with respect to secondary voltage V_{2}depends upon type of load. i.e. inductive, capacitive or resistive load.

- The secondary current I
_{2}sets up its own mmf N_{2}I_{2}and its own flux F_{2}which opposes the main or primary flux F_{m}. - Therefore the resultant flux and primary induced emf is also decreased.

Resultant flux F
= F

_{m}– F_{2}- The supply
system draws more current from the primary winding due to difference between
supply voltage V
_{1}and induced emf E_{1}increases until the original value of flux or main flux F_{m}is obtained. - Let the
additional primary current be I
_{2}'. This current is equal in magnitude with I_{2 }but anti phase with I_{2}. - The current I
_{2}' is known as reflected load current. - The current I
_{2}' sets up flux F_{2}' in magnitude. Therefore the mmf N_{1}I_{2}' and mmf N_{2}I_{2}cancel each other. - We can say that the magnitude and phase of load component of primary current depends upon the type of load.
- The net flux passing through the core remains constant whatever the load conditions if we neglect primary and secondary leakage fluxes.
- From the above discussion

F

_{2}' = F_{2}
N

_{1}I_{2}' = N_{2}I_{2}
I

_{2}' = ( N_{2 }/ N_{1}) I_{2}= K I_{2}…………… ( 1 )- Therefore the primary current consists of vector addition of two current

( II ) Reflected
load current component I

_{2}'- Primary current
I
_{1}= I_{0}+ I_{2}’…………( 2 ) - Figure B shows the vector diagram of transformer under different load conditions. In this diagram total voltage drops of both winding are neglected therefore

V

_{1}= E_{1}and V_{2}= E_{2}###
_{Vector Diagram For Lagging Power Factor}

- The vector diagram shows that the secondary current I
_{2}lags behind the load voltage by angle F_{2}.

_{}

###
_{Vector Diagram For Unity Power Factor}

- The vector
diagram shows that the secondary current I
_{2}is in phase with the load voltage. - It means that the phase angel between V
_{2}and I_{2}is equal to zero.

###
_{Vector Diagram For Leading Power Factor}

- The vector
diagram shows that the secondary current I
_{2}leads the load voltage V_{2}by angle F_{2}.

**What is demagnetizing mmf ? Describe its function.**

- The mmf sets up
by the load current in the secondary winding is known as demagnetizing mmf ( N
_{2}I_{2}). - Its function is to reduce main flux F
_{m}.

**The transformer is a constant flux device, isn’t it?**

- Yes, the core flux in the transformer remains constant whatever the load condition if we neglect leakage flux in the primary as well as secondary windings.

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