Effects on synchronising action
Effect of speed change on synchronising action:
Fig 1 Fig 2
Suppose, due to any reason, the speed of machine 2 falls. Then e.m.f. E2 will fall back by a phase angle of a electrical degrees as shown in Fig. (1) (though still E1 = E2). There will be resultant e.m.f. Er in the closed circuit formed by the two alternators. This e.m.f. Er will circulate current (known as synchronizing current Isyt) in this closed circuit.
Ra = armature resistance of each alternator
Xs = synchronous reactance of each alternator
Zs = synchronous impedance of each alternator
Since Ra is very small as compared to Xs, θ is nearly 90° so that the current Isy is almost in phase with E1 and in phase opposition to E2. This means that machine 1 is generating and machine 2 is motoring. Consequently, machine 1 tends to slow down and machine 2, by accepting power, tends to accelerate. This restores the status quo i.e., synchronism is re-established. Conversely, if E2 tends to advance in phase [See Fig. (2)], the directions of Er and Isy are changed such that now machine 2 is generating and machine 1 is motoring. Once again the synchronism is restored.
Effect of inequality of e.m.f.s :
The automatic re-establishment of synchronism of two alternators operating in parallel also extends to any changes tending to alter the individual e.m.f.s. When in exact synchronism, then E1 = E2 (magnitude) and they are in exact phase opposition as shown in Fig. (3 (i)). Suppose due to any reason, e.m.f. E1 increases. Then resultant e.m.f. Er exists in the closed circuit formed by the two alternators. Then Er = E1 - E2 and is in phase with E1.
The resultant e.m.f. Er sends synchronizing current Isy in the closed circuit. Here again the current Isy almost lags behind Er. by 90° (since Zs ≃ Xs) as shown in Fig. (3 (ii)). Also Isy lags almost 90° behind E1 and leads E2 almost by 90°. The power produced is practically zero; just enough to overcome copper losses.
The current Isy lags behind E1 and produces a demagnetizing armature reaction effect on machine 1. At the same time, Isy leads E2 and produces magnetizing armature reaction effect on machine 2. Thus E1 tends to fall and E2 tends to rise. The result is that synchronism is re-established. The converse is true for E2 > E1 as shown in Fig. (3 (iii)).