Transformer as a Constant Flux Device ( Transformer on LOAD )

Transformer ON Load
  • When the supply voltage V1 is given to the transformer primary winding, primary current I1 flows through the winding which produces maximum flux Fm in the core. 
  • When the secondary winding of the transformer is loaded, the secondary current I2 flows through it. 
  • However the phase of current I2 with respect to secondary voltage V2 depends upon type of load. i.e. inductive, capacitive or resistive load. 


  • The secondary current I2 sets up its own mmf N2I2 and its own flux F2 which opposes the main or primary flux Fm
  • Therefore the resultant flux and primary induced emf is also decreased.

        Resultant flux F = FmF2

  • The supply system draws more current from the primary winding due to difference between supply voltage V1 and induced emf E1 increases until the original value of flux or main flux Fm is obtained.
  • Let the additional primary current be I2'. 
  • This current is equal in magnitude with Ibut anti phase with I2. This current I2' is known as reflected load current. 
  • The current I2' sets up flux F2' in magnitude. 
  • Therefore the mmf N1I2' and mmf N2I2 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 
       F2' = F2
       N1I2' = N2I2
       I2' = ( N2 / N1 ) I2 = K I2 …………… ( 1 )
Therefore the primary current consists of vector addition of two current
( 1 ) No load current I0
( 2 ) Reflected load current component I2'
Primary current I1 = I0 + I2’…………….( 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
       V1 = E1 and V2 = E2

Vector Diagram For Lagging power factor
The vector diagram shows that the secondary current I2 lags behind the load voltage by angle F2.


Vector Diagram For Unity power factor
  • The vector diagram shows that the secondary current I2 is in phase with the load voltage. 
  • It means that the phase angel between V2 and I2 is equal to zero.


Vector Diagram For Leading power factor
  • The vector diagram shows that the secondary current I2 leads the load voltage V2 by angle F2.  

What is Demagnetizing MMF? Describe its function.
  • The mmf sets up by the load current in the secondary winding is known as demagnetizing mmf ( N2I2 ). 
  • Its function is to reduce main flux Fm.
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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