Fig: 1 Fig: 2
When two alternators are operating in parallel, each machine has an inherent tendency to remain synchronized. Consider two similar single-phase alternators 1 and 2 operating in parallel at no-load [See Fig. (1)]. Suppose, due to any reason, the speed of machine 2 decreases. This will cause E2 to fall back by a phase angle of a electrical degrees as shown in Fig. (2) (though still E1 = E2). Within the local circuit formed by two alternators, the resultant e.m.f. Er is the phasor difference E1 - E2. This resultant e.m.f. results in the production of synchronizing current Isy which sets up synchronizing torque. The synchronizing torque retards machine 1 and accelerates machine 2 so that synchronism is reestablished. The power associated with synchronizing torque is called synchronizing power.
In Fig. (1), machine 1 is generating and machine 2 is motoring. The power supplied by machine 1 is called synchronizing power. Referring to Fig. (2), we have,
The synchronizing power goes to supply power input to machine 2 and the Cu losses in the local circuit of the two machines.
Note that in this expression, a is in electrical radians.
of bbth machines is negligible