S YNCHRONO US GENERA TORS 47 



The effective armature resistances are respectively 0.40 and 0.52 

 ohm per phase. 



What should be the armature current of each alternator in 

 order that their combined armature copper loss will be reduced 

 to its least value? 



86. An alternator which has a capacity of 1650 kv.-a. is 

 operated in parallel with one which has a capacity of 1000 kv.-a. 

 Their armature resistances are 1.56 ohms and 2.08 ohms re- 

 spectively. For a combined load of 2200 kw. at 0.83 power 

 factor what load should each deliver and at what power factor 

 should it operate if the current and power outputs are propor- 

 tional to their ratings? With this division of the load the excita- 

 tions are adjusted so that the combined armature copper loss 

 is reduced to a minimum. At what power factor should each 

 alternator operate for this latter condition? 



87. Two identical 3-phase, Y-connected alternators are ope- 

 rating in parallel with a common potential difference of 2200 

 volts. The effective armature resistance and the synchronous 

 reactance of these alternators are respectively 0.158 and 2.12 ohms 

 per phase. The characteristics of the prime movers are so dis- 

 similar that of the total load of 1500 kw. at 0.85 power factor 

 the first alternator supplies 840 kw. The excitations of the 

 alternators are adjusted so that both are operating at the same 

 power factor. What are their excitation voltages and the phase 

 displacement between them? 



88. Two identical 3-phase, Y-connected alternators, operat- 

 ing in parallel, are driven by prime movers which have the same 

 speed-load characteristic. The effective armature resistance 

 and the synchronous reactance of the alternators are respec- 

 tively 2.18 ohms and 92 ohms per phase. 



The alternators supply 1830 kw. at 13,800 volts to an induction 

 motor load that is operating at 0.83 power factor. The excita- 

 tions of the alternators are adjusted so that the first supplies a 

 current of 40 amperes. 



(a) What current does the second alternator supply? 



(b) What are the excitation voltages of the alternators and 

 their phase displacement? 



89. Two identical 3-phase, Y-connected alternators, operating 

 in parallel, are driven by prime movers that have the same speed- 

 load characteristic. The alternators are rated to deliver 1000 

 kv.-a. at 2400 volts. The effective armature resistance and the 



