CHAPTER XII 

 ALTERNATOR REACTIONS AND REGULATIONS 



1. The maximum value of the effective back ampere-turns with 

 one ampere through an alternator armature is 100. The maximum 

 current is 20; plot on cross-section paper with scales, a curve 

 showing the effectiveness of the armature coils in producing for- 

 ward or back ampere-turns. Show the poles and plot the curves 

 with position of the coil, relative to the poles as abscissae. Plot 

 also the current curve and by multiplication of ordinates plot a 

 curve showing the instantaneous forward and back ampere-turns. 

 Do this for: (a) unity power factor; (b) 30 degrees lag; and (c) 

 30 degrees lead of the current relative to the e.m.f. (30 min.) 



2. A two-phase alternator has poles occupying half the pole 

 pitch. There are 10 turns or 20 conductors in each coil of each 

 phase; the maximum current is 100 amperes. Lay off the pole 

 pieces on a piece of cross-section paper, and using the successive 

 positions of the coils relative to the pole pieces as abscissae, plot 

 a curve showing the combined number of forward or back ampere- 

 turns due to the current in both phases, and for each position of 

 the coil. Do this: (a) for unity power factor; (b) for 30 degrees 

 lagging current; and (c) for 30 degrees leading current. (15 min.) 



3. Same as problem 2, except that the cross ampere-turns 

 are to be plotted. (15 min.) 



4. On short circuit a certain 50-period, 2200-volt, 440-kv-a. 

 generator requires 40-amperes field current to give 200 amperes 

 its full-load current. On open circuit the e.m.f. corresponding 

 to 40 amperes excitation is 1160 volts. What is the synchronous 

 impedance? The resistance being 0.5 ohm, what is the synchro- 

 nous reactance? (3 min.) 



5. Using the "electro-motive force method " of determination, 

 what would be the calculated regulation of the above machine on 

 unity power factor? Also with 30 and 60 degrees lag? (15 min.) 



6. The no-load magnetization curve of an 850-kv-a, 5000-volt, 

 Y-connected alternator is given by the following data: 



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