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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

8.1.3. Current transformation

Load behaviour of the transformer

If the transformer is output-loaded, current I2 flows into coil N2. Current I2 generates the magnetic flow F2K. According to Lenz’s Law this magnetic flow is counter-positioned to the cause (F1K).

Figure 126 - Loaded transformer

In this manner the magnet flow F1K is weakened and induction voltage U10 decreases. Given uniform rated voltage, the difference increases between the two voltages U10 and U1.

Consequently, a greater input current I1 flows whereby the magnetic flow F1K is increased. The magnetic flow F in the iron core thus remains virtually constant:

F = F1K - F2K = constant

This also applies to the output voltage of the transformer.

The input current I1 increases as the load current I2 becomes greater.

Transformation ratio

Without heeded the losses of the transformer, the following applies according to the energy conservation law:

s1 = s2

U1 · I1 = U2 · I2

If we arrange the equation so that the voltage and current values appears on respective sides, then


The following relationships may be cited for current ratio:

Conversely the currents are proportional to the voltages or numbers of turns. A transformer converts high currents into low ones or low currents into higher ones.


A welding transformer takes up 220 (current being 10A). The output voltage is 20V. How great is the welding current?


I2 » 110A