48 PROBLEMS IN ALTERNATING CURRENT MACHINERY 



synchronous reactance are respectively 0.067 and 2.64 ohms per 

 phase. Assume that the rotational losses, both core loss and 

 friction, are constant and equal. 



(a) If these alternators are delivering 2000 kw. at unit power 

 factor and their rated voltage, by what amount can the load be 

 shifted from one to the other if their field excitations are adjusted 

 so that there is an interchange current equal to the full-load 

 current, viz., 240 amperes? 



(b) If these alternators are delivering 1500 kw. at 0.75 power 

 factor and their rated voltage, by what amount can the load be 

 shifted from one to the other if their field excitations are adj usted 

 so that there is an interchange current equal to the full-load cur- 

 rent, viz., 240 amperes. 



90. The alternators described in problem 89 are operating in 

 parallel with non-inductive resistances of 0.8 ohm inserted in 

 each phase of each alternator. 



(a) If the alternators are jointly delivering 2000 kw. at unit 

 power factor and their rated voltage to a load by what amount 

 can this load be shifted from one to the other if the excitations 

 are adjusted so that there is an interchange current equal to the 

 full-load current, viz., 240 amperes? 



(b) If the alternators are jointly delivering 1500 kw. at 0.75 

 power factor and their rated voltage to a load, by what amount- 

 can this load be shifted from one to the other if the excitations 

 are adjusted so that there is an interchange current equal to 

 the full-load current, viz., 240 amperes? 



91. Two identical 3-phase alternators, rigidly coupled together 

 so that their excitation voltages are in phase, are driven by 

 a shunt motor. The effective armature resistance and the 

 synchronous reactance of each alternator are respectively 0.302 

 and 4.36 ohms per phase. The terminals of the alternators are 

 electrically connected as they would be for parallel operation but 

 no external load is supplied. When the field excitations are ad- 

 justed so that the excitation voltages are respectively 200 and 

 300 volts per phase, what is the armature current? If the 

 armature windings are connected in delta what is the terminal 

 voltage? What is the electrical output of the alternator 

 which is acting as a generator? If the rotational losses are 

 supplied by the shunt motor what is the mechanical output of 

 the alternator which is acting as a motor? If the rotational 

 losses are 1620 watts what power does the shunt motor supply? 



