8.2.2. Short-circuit behaviour

Short-circuit curves

Secondary current I₂ increases if load resistance is decreased. Where Z_a = 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 I_K

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 U_K.

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 U_K in % is determined by the following equation:

The relative short-circuit voltage is, on average, 2 to 10% of input rated voltage (U_1n) 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 (U_K U₁).

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