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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

8.2.2. Short-circuit behaviour

Short-circuit curves

Secondary current I2 increases if load resistance is decreased. Where Za = 0 the transformer has been short-circuited.

Primary circuit

U1 is applied

IK flows

Secondary circuit

Za = 0

U2 = 0

Short-circuit voltage

The short-circuited transformer can be replaced by resistor Z1 which corresponds to the transformer internal resistor.

Figure 130 - Short-circuited transformer

1 Short-circuit current IK

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 short-circuit attempt (Figure 132) the input voltage given a short-circuited output winding is increased until primary and secondary nominal currents flow. The voltage applied to the input side is then the short-circuit voltage UK.

Figure 132 - Circuitry to determine short-circuit losses

1 Short circuit voltage

The short-circuit voltage is the overall voltage decrease of a transformer during rated loading.

The relative short-circuit voltage UK in % is determined by the following equation:

The relative short-circuit voltage is, on average, 2 to 10% of input rated voltage (U1n) in mains transformers.

Short-circuit losses (winding losses)

In the short-circuit experiment (Figure 132) a power meter indicates short-circuit losses as the primary and secondary rated currents generate winding losses. The iron core is only slightly magnetised by the applied short-circuit voltage (UK U1).

The winding losses can be metered during the short-circuit experiment. They are dependent on the load current (PVW = I2 R).