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THREE-PHASE SYNCHRONOUS MACHINES
Thus . I El- I V I x 1 00 % voltage regulat10n = IVI The term rise in voltage used in the above definition assumes that the load is inductive. However, if the load is capacitive i.e., the load is supplying lagging vars to the generator and hence the excitation required is reduced and hence I E I is reduced and the terminal voltage will be higher when the load is connected as compared to when the load is reduced to zero. In such a situation the voltage regulation is negative. If we compare the equivalent circuit of an alternator as shown is Fig. 7.6 with that of the approximate equivalent circuit of a transformer as shown in Fig. 5. 13 we find that both represent identical features and are a series combination of R and X. The physical interpretation ofR and X may be different. We draw the phasor diagram for an alternator both for lag and lead p.f. as shown in Fig. 7.9, using of course the equivalent circuit of Fig. 7.6. E
E
v
IR, v
(a)
(b)
Fig. 7.9. Phasor diagram for an alternator (a) lag p.f. (b) lead p.f.
Here V is the terminal voltage under load condition and E is the induced voltage or the terminal voltage when the load is zero. cos <I> the p.f. of the load and 8 is the angle between E and V and is known as torque angle or power angle or load angle. From the phasor diagram E = V + IRa + j IX8 where E, V and I are phasor quantities. From here usually V, I, cos <j>, Ra and Xs are known, the magnitude of E can be obtained and hence voltage regulation can be obtained. However, if approximate value of voltage regulation is required the following expression can be used as is done in case of a transformer. Approximate voltage regulation IR IX = a cos <j> Âą s sin <j>. V V 7.2.6
Losses and Efficiency
Normally we define efficiency of a machine as the ratio of output to its input. For a generator it is usually convenient to define efficiency as Output Output + losses