Practical Transformer on No Load

Please read Ideal Transformer and Practical Transformer before continuing.

Consider a practical transformer on no load i.e., secondary on open-circuit as shown in figure below.

Practical%2Btransformer%2Bon%2Bno%2Bload

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 = V₁I₀cosΦ₀

As seen from the phasor diagram, the no-load primary current I₀can be resolved into two rectangular components viz.

Phasor%2Bdiagram%2B Practical%2Btransformer%2Bon%2Bno%2Bload

(i) The component I_w in phase with the applied voltage V_1. This is known as active or working or iron loss component and supplies the iron loss and a very small primary copper loss.

I_w = I₀ cosΦ₀

(ii) The component I_m lagging behind V₁ by 90° and is known as magnetizing component. It is this component which produces the mutual flux Φ in the core.

I_m = I₀ sinΦ₀

Clearly, I₀ is phasor sum of I_m and I_w.

I₀ = √ (I_m² +I_w²)

No load power factor, cosΦ₀ = I_w∕I₀

It is emphasized here that no load primary copper loss (i.e. I₀²R₁ ) is very small and may be neglected. Therefore, the no load primary input power is practically equal to the iron loss in the transformer i.e.,

No load input power, W₀ = Iron loss

Note.

At no load, there is no current in the secondary so that V₂ = E₂.

On the primary side, the drops in R₁ and X₁, due to I₀ are also very small because of the smallness of I₀.

Hence, we can say that at no load, V₁ = E_1.