- Once synchronized properly, two alternators continue to run in synchronism. Any tendency on the part of one to drop out of synchronism is immediately counteracted by the production of a synchronizing torque, which brings it back to synchronism.
- When in exact synchronism, the two alternators have equal terminal p.d.’s and are in exact phase opposition, so far as the local circuit (consisting of their armatures) is concerned. Hence, there is no current circulating round the local circuit. As shown in Fig. 1 (b) e.m.f. E1 of machine No. 1 is in exact phase opposition to the e.m.f. of machine No. 2 i.e. E2.
- It should be clearly understood that the two e.m.f.s. are in opposition, so far as their local circuit is concerned but are in the same direction with respect to the external circuit.
- Hence, there is no resultant voltage (assuming E1 = E2 in magnitude) round the local circuit. But now suppose that due to change in the speed of the governor of second machine, E2 falls back by a phase angle of α electrical degrees, as shown in Fig. 1 (c) (though still E1 = E2).
- Now, they have a resultant voltage Er, which when acting on the local circuit, circulates a current known as synchronizing current. The value of this current is given by ISY = Er /ZS where ZS is the synchronous impedance of the phase windings of both the machines (or of one machine only if it is connected to infinite bus-bars).
- The current ISY lags behind Er by an angle θ given by tan θ = XS / Ra where XS is the combined synchronous reactance of the two machines and Ra their armature resistance. Since Ra is negligibly small, θ is almost 90 degrees.
- So ISY lags Er by 90° and is almost in phase with E1. It is seen that ISY is generating current with respect to machine No.1 and motoring current with respect to machine No. 2 (remember when the current flows in the same direction as e.m.f., then the alternator acts as a generator, and when it flows in the opposite direction, the machine acts as a motor).
- This current ISY sets up a synchronising torque, which tends to retard the generating machine (i.e. No. 1) and accelerate the motoring machine (i.e. No. 2).
- Similarly, if E2 tends to advance in phase [Fig. 1 (d)], then ISY, being generating current for machine No. 2, tends to retard it and being motoring current for machine No. 1 tends to accelerate it. Hence, any departure from synchronism results in the production of a synchronizing current ISY which sets up synchronizing torque.
- This re-establishes synchronism between the two machines by retarding the leading machine and by accelerating the lagging one. This current ISY, it should be noted, is superimposed on the load currents in case the machines are loaded.