For the determination of the series equivalent impedance of transformers, the short-circuit test is used and the same procedure will be applied to the autotransformers, with some small differences.
Determining the serial impedance of a three-terminal autotransformer, primary, secondary, and tertiary, is more common than that of only two windings. When an autotransformer does not have a tertiary winding, the test is done in the same way as in the conventional transformer.
As the equipment has three windings, three “binary” tests, ie a short test between the primary and the secondary with the tertiary open, a test between the primary and the tertiary with the secondary open and a test Between the secondary and the tertiary, with the primary open.
For the conventional transformer it is not possible to determine the value of the impedances of each of the separate windings by the short-circuit test, and so we find that Z1 = 2Z ‘= 0.5 Zeq, however, in the autotransformer test Possible to generate the impedance of each of the windings separately.