No load Transformer means a transformer which has no load connection at secondary winding only normal voltage is applied at the primary winding. Let V1 is applied at the primary winding. After applying A.C voltage V1, it is seen that small amount of current I0 flows through the primary winding. In case of Ideal Transformer, no load primary current (I0) will be equal to magnetizing current (Iµ) of the transformer. We assumed there is no core losses and copper loss, So I0 =Iµ . But, in case of actual transformer, there is two losses, i.e i) Iron Losses in the core i.e hysteresis loss and eddy current loss , ii) and a very small amount copper loss in the primary winding.
So, the primary current I0 has two components:
Hench, the primary current I0 is vector summation of Iµ & Iw , So, we can write that I0 = (Iµ2 + Iw2) and is not a 90⁰ behind V1, but lags it by an angle φ < 90⁰ Which is shown in figure. And no load input power, W 0 = V1 I0Cose φ0 . The magnitude of no load primary current is very small as compared to the full-load primary current. It is 1 percent of the full-load current. As I0 is very small, the no load primary Cu loss is negligible which means that no load primary input is practically equal to the iron loss in the transformer.