Electrical Machines  Basic vocational knowledge (Institut für Berufliche Entwicklung, 144 p.) 
8. Transformer 
8.2. Operational behaviour of a transformer 

Shortcircuit curves
Secondary current I_{2} increases if load resistance is decreased. Where Z_{a} = 0 the transformer has been shortcircuited.
Primary circuit 
U_{1} is applied 

I_{K} flows 
Secondary circuit 
Z_{a} = 0 

U_{2} = 0 
Shortcircuit voltage
The shortcircuited transformer can be replaced by resistor Z_{1} which corresponds to the transformer internal resistor.
Figure 130  Shortcircuited
transformer
1 Shortcircuit current I_{K}
Figure 131 depicts the commensurate duplicate circuit diagram.
Figure 131  Duplicate circuit
diagram for short circuit run
1 Ohmic winding resistance, 2 Scattered reactance (is made up of the scatter flow of the input and output coils), 3 Inner resistance of the transformer (impedance)
During a shortcircuit attempt (Figure 132) the input voltage given a shortcircuited output winding is increased until primary and secondary nominal currents flow. The voltage applied to the input side is then the shortcircuit voltage U_{K}.
Figure 132  Circuitry to determine
shortcircuit losses
1 Short circuit voltage
The shortcircuit voltage is the overall voltage decrease of a transformer during rated loading.
The relative shortcircuit voltage U_{K} in % is determined by the following equation:
_{}
The relative shortcircuit voltage is, on average, 2 to 10% of input rated voltage (U_{1n}) in mains transformers.
Shortcircuit losses (winding losses)
In the shortcircuit experiment (Figure 132) a power meter indicates shortcircuit losses as the primary and secondary rated currents generate winding losses. The iron core is only slightly magnetised by the applied shortcircuit voltage (U_{K} U_{1}).
The winding losses can be metered during the shortcircuit experiment. They are dependent on the load current (P_{VW} = I_{2} R).