SYNCHRONOUS GENERATORS 45 



(a) What is the greatest combined load that the alternators 

 can deliver without overloading cither by more than 25 per 

 cent.? 



(b) What is the load on each alternator when the first is run- 

 ning at 91.6 rev. per min.? 



(c) What is the frequency when they are delivering a com- 

 bined load of 2000 kw.? 



79. Two 3-phase, 60-cycle alternating-current generators are 

 operating in parallel. The first has a capacity of 1000 and the 

 second a capacity of 1500 kv.-a. The first is driven by a prime 

 mover so adjusted that the frequency falls from 61 cycles at no 

 load to 59.6 cycles at full load. The second has a different speed- 

 load characteristic, the frequency falling from 61.4 cycles at no 

 load to 59.2 cycles at full load. 



When these alternators are jointly delivering 2000 kw. what is 

 the load on each? What is the frequency? If the speed-load 

 characteristic of the second is shifted parallel to itself until the 

 alternators divide this load properly, what is the new value of the 

 no-load frequency of this alternator? At what frequency will 

 they now operate when delivering 2000 kw.? 



80. Two 3-phase, 11,000-volt, 60-cycle alternating-current 

 generators, operating in parallel, are driven by prime movers 

 which have the same speed-load characteristic. The armature 

 windings of the alternators are Y-connected and have an effective 

 resistance of 0.94 ohm and a synchronous reactance of 56 ohms per 

 phase. The total load supplied is 1700 kw. at 0.83 power factor. 

 The excitations are adjusted so that the terminal volt a 



1 1 ,000 volts and one of them is operating at unit power factor. 

 What are the excitation voltages and the phase an^le between 

 them? 



81. Two identical 3-phase, Y-connected alternators operatini: 

 in parallel are driven by prime movers that have such dissimilar 

 speed-load characteristics that when the fir-t alternator i- 

 delivering 400 kw. at 0.8 power factor the second is runn 



no load, (a) If the excitations of the alternator- arc adju-ted 

 that the terminal voltage is 5000 volts and tin ai rp.it UK current 

 of the second alternator i- /< ro, what are tin- excitation voltages 

 and their phase di-plac ment '.' ibi If the ex citations are adjusted 

 so that the terminal volta^ i .MM )0 volts and the total armature 

 copper 1" lured to it- least value what current will each 



alternator deliver'.' The effective armature resistance and the 



