46 PROBLEMS IN ALTERNATING CURRENT MACHINERY 



synchronous reactance of each alternator are respectively 0.42 

 and 15.4 ohms per phase. 



82. Two identical 3-phase alternators, connected in parallel, 

 are driven by prime movers that have dissimilar speed-load 

 characteristics. When the excitations of the alternators are 

 equal the first delivers 100 amperes at 0.9 power factor (lagging) 

 and the second, 75 amperes at 0.7 power factor (lagging). 



(a) What per cent, of the total load does each alternator 

 deliver? 



(b) What is the power factor of the load? 



(c) If the field excitations are adjusted so that both alter- 

 nators operate at the same power factor what current will each 

 deliver? 



(d) If the field excitations are adjusted so that the total 

 armature copper toss is reduced to its least value at what power 

 factor will each alternator operate? 



83. Two identical 2-phase alternators, connected in parallel, 

 are driven by prime movers which have somewhat dissimilar 

 speed-load characteristics. The first alternator delivers 3200 

 kw. at 2210 volts and has an armature current of 760 amperes, 

 while the second delivers 3700 kw. at 2210 volts and has an 

 armature current of 1100 amperes. The effective armature 

 resistance and the synchronous reactance of each alternator are 

 respectively 0.0256 and 1.12 ohms per phase. The excitations 

 of the alternators are now adjusted so as to reduce the total 

 armature copper loss to its least value, but the terminal voltage 

 is maintained at 2210 volts. 



(a) At what power factor will each alternator be operating? 



(b) What is the reduction in the total armature copper loss? 



(c) What is the change in the copper loss of each alternator? , 



84. Two identical 3-phase alternators, operating in parallel 

 on a balanced load, are driven by prime movers with different 

 speed-load characteristics. The power output of each alter- 

 nator is measured by two wattmeters. Show that when the 

 differences between the wattmeter readings for each alternator 

 are the same the total armature copper loss is reduced to its least 

 value for the given load. 



85. Two dissimilar Y-connected alternators, operating in 

 parallel, supply a load of 1500 kw. at 0.85 power factor and a 

 terminal potential difference of 5000 volts. The alternators 

 being of the same capacity are adjusted to deliver equal loads. 



