 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}. This 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
( 1 ) No load current I_{0}
( 2 ) Reflected load current component I_{2}'
( 1 ) No load current I_{0}
( 2 ) 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
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.
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}.
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 winding.
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