ALTERNATOR REGULATION AND OPERATION 149 



tions, as shown by Fig. 152, where the angle a' between the cur- 

 rent and the emf. EI \< nearly 90. Therefore, the armature 

 current is a maximum when the axes of the armature coils are 

 almost opposite the pole centers as shown in Fig. 139, page 134. 

 A> the armature magnetomotive force has its maximum effect 

 under these conditions, the value of the synchronous impedance 

 so determined is too large for other positions of the coil, as shown 

 in Fig. 137, page 133. 



It will be seen that the value of synchronous impedance deter- 

 mined at short-circuit is too large and will make the calcu- 

 lated value of regulation too high. Therefore, the synchronous 

 impedance method is called the pessimistic method. It is a safe 

 method to use when making a guarantee, because the machine 

 always regulates better than the computed values indicate. 



The following example will illustrate the use of this method: 



'nple. A 50-kv-a., 550-volt, single-phase alternator has an open- 

 circuit electromotive force of 300 volts when the field current is 14 amp. 

 When the machine is short-circuited through an ammeter, the armature 

 current is 160 amp., the field current still being 14 amp. The ohmic resis- 

 tance of the armature between terminals is 0.16 ohm. The ratio of effect ivc 

 to ohmic resistance may be taken as 1.2. (a) Determine the synchronous 

 impedance of the machine. (6) The synchronous reactance, (c) The regu- 

 lat ion at 0.8 power-factor, current lagging. 



The rated current of the machine / = - = 91 amp. 



oou 



JQQ 



(a) The synchronous impedance Z, = .,,., = 1.87 ohms. 



The effective resistance = 1.2 X 0.16 = 0.1D2 ohm. 



(6) X. = V(1.87) - (0.192)' = 1.86 ohms. 



(c) cos = 0.8 sin 6 = 0.6. 

 Applying equation (39), page 139. 



E = V[(r)50^<a8) + (91 X 0.192)]* + [(550 X 0.6) + (91 X 1.86)1* 

 \ i<),000 - 677 volts. 



The definition of rujulntinn for an alternator is the same as for a direct - 

 current generator -Vol. I. pnne '2^2. Par. I'.MI . namely the percenta^ 



ite.l load is taken off the machine. As the I 



QOe was used in the fon^om^ problem, the armature reaction was 

 taken so that the no-load voltage ,,f the machine is pre- 



sumably o77 volts. - . the regulation is 



100 - 100 - 23 , ,. An,. 



It i- tO 1 in the fnre^oiim problem th.il the armature 



/. is practically equal tu the synchronous reactance 



