- When two or more alternators have been connected in parallel, they will remain in stable operation under all normal conditions i.e., voltage, frequency, speed and phase equality will continue.
- In other words, once synchronized properly, the alternators will continue to run in synchronism under all normal conditions. If one alternator tries to fall out of synchronism, it is immediately counteracted by the production of a synchronizing torque which brings it back to synchronism. This automatic action is called the synchronizing action of the alternators.
- Consider two similar single-phase alternators 1 and 2 operating in parallel as shown in Fig. (1 (i)). For simplicity, let us assume that the alternators are at no-load. When in exact synchronism, the magnitudes of the e.m.f.s E1 (machine 1) and E2 (machine 2) are equal.
- These e.m.f.s are acting in the same direction with respect to the external circuit [See Fig. (1 (ii))]. But in relation to each other, these e.m.f.s are in phase opposition i.e., if we trace the closed circuit formed by the two alternators we find that the e.m.f.s oppose each other [See Fig. (1 (iii))].
- When the alternators are in exact synchronism, E1 and E2 are in exact phase opposition. Since E1 = E2 in magnitude, no current flows in the closed circuit formed by the two alternators.