Lecture
The external characteristic of a transformer is the dependence of the secondary winding voltage U2 on the current I2. If we neglect the no-load current, the equivalent circuit of the transformer under rated conditions takes the form shown in Figure 1.
Figure 1 Equivalent circuit of a transformer for constructing its external characteristic
Given that Zsc = Rsc + jXsc, the voltage on the secondary winding of the transformer can be expressed as follows:
(1)
Let us construct a vector diagram, relative to the current vector 
, as shown in Figure 2.
Figure 2 Vector diagram of the currents and voltages of the transformer's secondary winding
If the load resistance 
is resistive (active), then the voltage 
is in phase with the current . The sum 
. Since the applied voltage U1= const, it is obvious that as the current changes, the voltage drop ΔU across the internal resistance of the transformer will also change. That is, the external characteristic has a drooping shape, as shown in Fig. 2.25.
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