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

In contrast to operational idling, during loading the secondary circuit is closed through an external resistance Z_{a} (Figure 126). Secondary current I_{2} flows. According to the energy conservation law the transformer must also take up commensurate primary power, thus a primary current I_{1} also flows.
Primary circuit 
U_{1} is applied 

I_{1} > I_{0} 
Secondary circuit 
Z_{a} < ¥ 

I_{2} > 0 

U_{2} ¹ U_{20} 
Voltage curve U_{2} = f (I_{2})
As the curve in Figure 133 shows, terminal voltage U_{2} decreases during loading.
Figure 133  Voltage behaviour
during loaded operation U_{2} = f (I)_{2}
1 U_{K} small, 2 U_{K} big
Figure 134 depicts the duplicate circuit diagram for the loaded transformer.
Figure 134  Duplicate circuit for
the loaded transformer with a transformation ratio = 1:1
The duplicate circuit diagram corresponds to a transformer with a transformation ratio _{}
_{}
_{}
As rated current flows the shortcircuit voltage U_{K} decreases at the internal transformer resistance Z_{i} as a result of which the terminal voltage U_{2} declines by the power decrease of the shortcircuit voltage U_{K}.
Transformers with considerable shortcircuit voltage U_{K} have powerful internal resistors, that is to say pronounced voltage changes as load alters.
U_{K} = 2...10% 
minimal voltage losses 

voltagerigid behaviour 
U_{K} = 20...50% 
considerable voltage losses 

voltageflexible behaviour 
Example:
A 220/42 V transformer has a shortcircuit voltage of 10%.
How great is the voltage change between idling and rated current loading?
Solution:
_{}
Output voltages at differing loads
Given differing loads with ohmic, inductive or capacitive external resistance gives rise to the dependence of output voltage on load current as shown in Figure 135.
Figure 135  Secondary terminal
voltage depending on the degree and nature of loading
1 Idling, 2 Rated load
Given capacitive load, the output voltage may even be greater than noload voltage.
The output voltage of a transformer depends on the
 degree of load current I_{2}
 the magnitude of relative shortcircuit voltage
 the nature of the load (ohmic, inductive or capacitive).