Transformer on No-load

TRANSFORMER ON NO-LOAD

We assumed an ideal transformer, that is there were no core losses and copper loses. But practical conditions require that certain modifications be made in the forgoing theory. When an actual transformer is put on load, these is iron loss in the core and copper loss in the windings and these losses are not entirely negligible.

Even when the transformer is on no-load, the primary input current is not wholly reactive. The primary input current under no-load conditions has to supply (i) iron losses in the core (ii) very small amount of copper loss in primary.

Hence, the no-load primary input current I₀ is not at 90° behind V, but lags it by an angle ϕ₀ <90°, no load input power.

W₀ = V₁ I₀ cos ϕ₀

where cos ϕ₀ is primary power factor under no-load condition.

From the Figure 2.36, primary current I has two components

(i) One in phase with V₁. This is known as active (or) working (or) iron loss component I_w.It mainly supplies the iron loss plus small quantity of primary Cu loss.

I_w = I₀ cos ϕ₀

(ii) The other components is in quadrature with V₁ and is known as magnetising

component I_μ.

I_μ = I₀ sin ϕ₀

I₀ is the vector sum of I_w and I_μ hence

I₀ = (I_μ² + I_w²)

Example: 18

A 2200/200 V transformer draws a no-load primary current of 0.6 A and absorbs 400 watts. Find the magnetising and iron loss currents.

Solution:

Iron-loss current

 = No-load input in watts / Primary voltages

= 400/2200 = 0182 A

Iron-loss current = 0.182 A

I₀² = I_w²+ I_µ²

Magnetising component I_μ = √(0.6² -0.182²)

I_μ = 0.572 A

Example: 19

A 2200/250 v transformer takes 0.5A at a pf of 0.3 on open circuit. Find magnetising and working components of no-load primary current.

Solution:

I₀ = 0.5 A, cos ϕ₀ = 0.3

I_w = I₀ cosϕ₀ =⇒ 0.5 × 0.3 ⇒ 0.15 A

I_μ = √0.5² - 0.15² = 0.476A

I_μ = 0.476 A

Voltage regulation of transformer:

The regulation is defined as change in the magnitude of the secondary terminal voltage, when full load. That is rated load of specified power factor supplied at rated voltage is reduced to no load, with primary voltage maintained constant expressed as the percentage of the rated terminal voltage.

Let

E₂= secondary terminal voltage on no load.

V₂ = secondary terminal voltage on given load.