Transformer on No-Load

Transformer on No-Load transformer, there is no iron losses and copper losses. The ideal transformer is not practically possible. In case actual transformer these losses are present. When an actual transformer put on load, there is iron loss in the core and copper loss in the windings. When secondary of the transformer is kept open and primary winding is connected to alternating source, the transformer is called transformer on no-load.

When primary is connected to source and transformer is on no load, the transformer draws some current which is not wholly reactive. The primary input current under no load conditions has to supply iron losses in the core and copper losses in the windings (both primary and secondary).


When transformer is connected to source. It draws no load current IO at voltage VI. The current lags the voltage by angle φO which is less than 900. No load current IO has two components. One component is called Active or Working Component and second component is called Reactive or Magnetizing Component.

transformer on no-load

Active or Working component IW is in phase with input voltage V1. Active component supplies the iron loss and small quantity of copper loss. This component is also known as wattfull component or iron loss component.

phasor diagram of transformer on no load

IW = IO Cos φO

Reactive or Magnetizing component is in quadrature with VI. The Function of reactive component is to sustain the alternating flux in the core. This component is also known as wattless component because it does not consume energy.

      Im = IO Sin φO           


  1. The no-load primary current IO is very small in value as compared to the full load primary current. It value may be 2 or 3% of the full load primary current.
  2. The value of input primary current IO is very small. Hence, copper loss is negligibly small. It means input power is equal to the iron loss in the transformer.