Procedure for mmf Method
Procedure for at Method:
Suppose the alternator is supplying full-load current Ia at operating voltage V and p.f. cos Φ lagging. The procedure for finding voltage regulation for AT method is as under:
(i) From the O.C.C., field current OA required to produce the operating load voltage V (or V IaRa cos f) is determined [See Fig. (1)]. The field current OA is laid off horizontally as shown in Fig. (2).
Fig: 1 Fig: 2
(ii) From S.C.C., the field current OC required for producing full-load current Ia on short-circuit is determined. The phasor AB (= OC) is drawn at an angle of (90° Φ) i.e., Ð OAB = (90° Φ) as shown in Fig. 2.
(iii) The phasor sum of OA and AB gives the total field current OB required. The O.C. voltage E0 corresponding to field current OB on O.C.C. is the no load e.m.f.
This method gives a regulation lower than the actual performance of the machine. For this reason, it is known as Optimistic Method.