|Electrical Machines - Basic vocational knowledge (Institut für Berufliche Entwicklung, 144 p.)|
|8.2. Operational behaviour of a transformer|
Secondary current I2 increases if load resistance is decreased. Where Za = 0 the transformer has been short-circuited.
U1 is applied
Za = 0
U2 = 0
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).