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Transformer on No load Condition

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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:

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

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.


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