Consider a practical transformer on no load i.e., secondary on open-circuit as shown in figure below.
The primary will draw a small current I0 to supply
(i) the iron losses and
(ii) a very small amount of copper loss in the primary.
Hence the primary no load current I0 is not 90° behind the applied voltage V1 but lags it by an angle Φ0 < 90° as shown in the phasor diagram.
No load input power, W0 = V1 I0 cosΦ0
As seen from the phasor diagram, the no-load primary current I0 can be resolved into two rectangular components viz.