Polarity testing of transformers is vital before connecting them in parallel. Otherwise, with incorrect polarity, it is not possible to connect them in parallel. The rated voltage is applied to the primary and its two terminals are marked as A1 and A2, respectively, as shown in Figures 1.44(a) and 1.45(b), respectively. The secondary winding terminals are also marked as a1 and a2, shown in Figures 1.44(a) and 1.45(b), respectively. Now a voltmeter is connected across A2 and a2. if it measures the difference of E1 and E2, A2 and a2 are of the same polarity. If it measures the addition of E1 and E2, A2 and a2 are of opposite polarity.
Figure 1.44 Polarity Test of a single-phase Two Winding Transformer