Electrical Machines  Basic vocational knowledge (Institut für Berufliche Entwicklung, 144 p.) 
8. Transformer 
8.1. Transformer principle 

Load behaviour of the transformer
If the transformer is outputloaded, current I_{2} flows into coil N_{2}. Current I_{2} generates the magnetic flow F_{2K}. According to Lenz’s Law this magnetic flow is counterpositioned to the cause (F_{1K}).
Figure 126  Loaded transformer
In this manner the magnet flow F_{1K} is weakened and induction voltage U_{10} decreases. Given uniform rated voltage, the difference increases between the two voltages U_{10} and U_{1}.
Consequently, a greater input current I_{1} flows whereby the magnetic flow F_{1K} is increased. The magnetic flow F in the iron core thus remains virtually constant:
F = F_{1K}  F_{2K} = constant
This also applies to the output voltage of the transformer.
The input current I_{1} increases as the load current I_{2} becomes greater.
Transformation ratio
Without heeded the losses of the transformer, the following applies according to the energy conservation law:
s_{1} = s_{2}U_{1} · I_{1} = U_{2} · I_{2}
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.
Example:
A welding transformer takes up 220 (current being 10A). The output voltage is 20V. How great is the welding current?
Solution:
_{}
_{}I_{2} » 110A