8.2.3. Loaded voltage behaviour
In contrast to operational idling, during loading the secondary circuit is closed through an external resistance Za (Figure 126). Secondary current I2 flows. According to the energy conservation law the transformer must also take up commensurate primary power, thus a primary current I1 also flows.
Voltage curve U2 = f (I2)
As the curve in Figure 133 shows, terminal voltage U2 decreases during loading.
1 UK small, 2 UK big
Figure 134 depicts the duplicate circuit diagram for the loaded transformer.
The duplicate circuit diagram corresponds to a transformer with a transformation ratio
As rated current flows the short-circuit voltage UK decreases at the internal transformer resistance Zi as a result of which the terminal voltage U2 declines by the power decrease of the short-circuit voltage UK.
Transformers with considerable short-circuit voltage UK have powerful internal resistors, that is to say pronounced voltage changes as load alters.
A 220/42 V transformer has a short-circuit voltage of 10%.
How great is the voltage change between idling and rated current loading?
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.
1 Idling, 2 Rated load
Given capacitive load, the output voltage may even be greater than no-load voltage.
The output voltage of a transformer depends on the
- degree of load current I2
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