Cover Image
close this bookElectrical Machines - Basic vocational knowledge (Institut für Berufliche Entwicklung, 144 p.)
close this folder8. Transformer
close this folder8.2. Operational behaviour of a transformer
View the document8.2.1. Idling behaviour Idling features
View the document8.2.2. Short-circuit behaviour
View the document8.2.3. Loaded voltage behaviour
View the document8.2.4. Efficiency

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.

Primary circuit

U1 is applied

I1 > I0

Secondary circuit

Za < ¥

I2 > 0

U2 ¹ U20

Voltage curve U2 = f (I2)

As the curve in Figure 133 shows, terminal voltage U2 decreases during loading.

Figure 133 - Voltage behaviour during loaded operation U2 = f (I)2

1 UK small, 2 UK 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 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.

UK = 2...10%

minimal voltage losses

voltage-rigid behaviour

UK = 20...50%

considerable voltage losses

voltage-flexible behaviour


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.

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 no-load voltage.

The output voltage of a transformer depends on the

- degree of load current I2
- the magnitude of relative short-circuit voltage
- the nature of the load (ohmic, inductive or capacitive).