Transformer on NO Load

  • When the supply voltage V1 is given to the transformer primary winding, the no load current I0 flows through the primary winding. This current sets up flux in the core. 
  • Due to varying flux core loss or iron loss occurs in the core material, the no load input power is given by
        W0 = V1I0 Cos F0
        Cos F0 = No load input power factor
               V1 = Primary Supply Voltage
                I0 = No load current


  • Therefore the no load input current has to supply ( I ) Iron losses in the core and ( II ) Small amount of copper loss in the primary winding ( no copper loss in the secondary winding because it is open circuited therefore the no load current lags behind V1 by angle F0. (F0 < 90° )
  • The no load current has two components
( I ) Loss component current ( Iw )
  • The function of the Iw is to supply iron loss in the core. 
  • It is in phase with voltage V1.
        Iw = I0 Cos F0 ……………( 1 )
 ( II ) Magnetizing Current ( Iµ )
  • The function of the magnetizing current Iµ is to create flux in the core. It is in quadrature with voltage V1.
        Iµ = I0 Sin F0……………..( 2 )
        Therefore I0 = √ ( Iµ2 + Iw2 )
         Also tan F0 = Iµ / Iw
What percentage of no load current that of full load primary current? Why?
  • The no load primary current is only 2 to 5% of full load current because the core of the transformer has low reluctance path.

Why the waveform of the exciting current is not sinusoidal?
  • The permeability of the core varies with the exciting current therefore the waveform of the exciting current is not sinusoidal.

Why the angle between supply voltage V1 and no load current is always less than 900 ?
  • The no load current is not wholly reactive. 
  • It has to supply amount of iron losses and relative large amount of magnetizing current therefore the input power factor of the no load current always less than 900.
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