50 PROBLEMS IN ALTERNATING CURRENT MACHINERY 



it is found that their corresponding field poles are displaced by 

 an angle of 1 degree and 6 minutes. 



(a) When they deliver 1500 kw. at 0.87 power factor and 

 their rated voltage, what is the output of each alternator if the 

 field excitations are equal? 



(b) If this displacement of the field poles is reduced to zero 

 what will be the reduction in the armature copper loss of each 

 alternator? The load delivered is still 1500 kw. at 0.87 power 

 factor and their rated voltage. 



96. Two identical, 2-phase, 3750-kv.-a. water-wheel genera- 

 tors, operating in parallel, are driven by prime movers which have 

 the same speed-load characteristic. The effective armature resis- 

 tance and the synchronous reactance of these alternators are 

 respectively 0.0254 and 1.12 ohms per phase. The alternators 

 are jointly delivering 7500 kw. at unit power factor and their 

 rated terminal potential difference of 2200 volts. The excita- 

 tions are equal. 



Instantaneous records show that due to hunting the maximum 

 displacement of their excitation voltages is 30 degrees. What is 

 the synchronizing power at the time of maximum displace- 

 ment? What is the maximum value of the effective armature 

 current? 



97. If the alternators described in problem 96 are jointly 

 delivering 5250 kw. at 0.7 power factor and their rated voltage 

 to an inductive load, what is the synchronizing power at the 

 time the displacement of their excitation voltages is 30 degrees? 

 What are the effective armature currents at this time? 



98. If the alternators described in problem 96 are jointly 

 delivering 5250 kw. at 0.7 power factor and their rated voltage 

 to a condensive load, what is the synchronizing power at the 

 time the displacement of their excitation voltages is 30 degrees? 

 What are the effective armature currents at this time? 



99. Two identical 3-phase, Y-connected alternators, each of 

 which is rated to deliver 1640 kv.-a. at 13,500 volts are operating 

 in parallel. The speed-load characteristics of the prime movers 

 are the same and the load requires 70 amperes from each alter- 

 nator at 0.83 power factor and the rated voltage. The effective 

 armature resistance and the synchronous reactance of the alter- 

 nators are respectively 1.52 ohms and 37.4 ohms per phase. 

 Oscillograph records show that due to hunting the greatest 

 value of the effective armature current is 87 amperes. 



