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close this bookElectrical Machines - Basic vocational knowledge (Institut für Berufliche Entwicklung, 144 p.)
close this folder8. Transformer
close this folder8.1. Transformer principle
View the document8.1.1. Operating principle of a transformer
View the document8.1.2. Voltage transformation
View the document8.1.3. Current transformation

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 FK which saturates both coils and a leakage flow FS.

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

According to the transformer equation



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 U1n and U2n are indicated on the rating plate of the transformer.


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


U2 » 230 V