Transformer on No load Condition

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 (I_µ) of the transformer. We assumed there is no core losses and copper loss, So  I₀ =I_µ . 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:

1. I_w = Iron loss component which is same ph of applied voltage V₁.
2. I_µ = magnetizing component which is 90⁰ behind V₁.

 

Hench, the primary current I₀ is vector summation of I_µ & I_w , So, we can write that I₀ = (I_µ² + I_w²)  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 φ_0  .   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.