Test of A Series Transformer

The series transformer, as its name indicates, has its primary winding in series with the line, and its primary current is determined by the main circuit, the primary E.M.F. being the drop across the transformer due to its impedance. If the secondary is on open circuit the primary circuit will set up a magnetic field in the iron of the transformer and generate in the secondary E.M.F. equal to the voltage drop in the primary multiplied by the ratio of secondary to primary turns. As there is no opposing flux (on open circuit) there results an abnormally large secondary voltage. Hence the secondary of a series transformer should always be short circuited.