8.1.2. Voltage transformation

A few field lines already close before reaching the output coil (Figure 125) so that flow F₁ can be divided into a maximum flow F_K which saturates both coils and a leakage flow F_S.

The leakage flow may be ignored in regard to the unloaded transformer (idling). Therefore the following applies:

According to the transformer equation

and

.

If we relate both equation then

Shortening gives us

During idling no current flows into the output winding, thus there is no voltage decrease. Consequently the induced voltage U₂₀ equal to the terminal voltage U₂ (Cp Figure 125):

Figure 125 - Transformer principle

1 Input winding/upper voltage winding/primary winding, 2 Output winding/under voltage winding/secondary winding

U₂₀ = U₂

In the event of minimal idling current I voltage decrease in the input winding is negligibly minimal. We therefore have

U₁₀ = U₁

which results in

The voltages behave like the numbers of turns.

The interrelationship of the numbers of turns is known as the transformation ratio We have:

The rated voltages U_1n and U_2n are indicated on the rating plate of the transformer.

Example:

What secondary terminal voltage arises in a transformer where 380 V is applied to the primary winding of 980 turns and the secondary winding has 594 turns?

Given: U₁ = 380 V; N₁ = 980; N₂ = 594

Sought: U₂

Solution:

U₂ » 230 V