S YNCHRONO US GENERA TORS 49 



92. If the alternators describe* 1 in problem 91 are mechanically 

 coupled together so that their excitation voltages differ in phase 

 by 30 degrees what will be the armature current when the 

 excitation voltages are each 300 volts? What is their 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? What power does the shunt motor 

 supply if the rotational losses are 1620 watts? 



93. Two identical, Y-connected, 60-cycle alternators are 

 rigidly coupled together and are driven at their rated speed of 

 1200 rov. per min. The alternators have revolving fields and 

 the coupling is so made that the north poles of the first are 10 

 degrees (mechanical) ahead, i.e., in the direction of rotation, of 

 the corresponding north poles of the second. The corresponding 

 terminals are connected through non-inductive resistances of 

 1.5 ohms each. The effective armature resistance and the 

 synchronous reactance of each alternator are respectively 0.302 

 and 4.36 ohms per phase. The field currents are adjusted so 

 that the excitation voltages are respectively 200 and 300 volts 

 per phase. What is the current? What is the electrical out- 

 put of the alternator which is acting as a generator? If the 

 rotational losses are supplied by the driving motor what is the 

 mechanical output of the alternator which is acting as a motor? 

 If the rotational losses are 1620 watts what is the output of the 

 driving motor? 



94. Two identical, 3-phase, Y-connected alternators, rigidly 

 coupled to the same prime m.\ -r, are operating in parallel and 

 supply 1500 kw. at 0.83 power factor and a terminal potential 

 difference of 5000 volts. The efl'ertive armature resistance and 

 the synchronous reactan< h alternator are respectively 

 0.42 and 15.4 ohm- p -r phase. The mechanical coupling is so 

 made that the excitation voltages are in phase, and the field 

 < MI r rents are adjusted so that one of these voltages is 50 per cent, 

 greater than the other. What is the output of each alternator? 



95. Two 3-p ; ;i Y-coimeeti-d alternators, each of which is 

 rated to deliver 760 kv.-a. at 2200 volts, are rigidly coupled to the 

 -anie prime IMMYIT. Kach of these altrrnalor-; has 111 field poles. 

 an effective arm at un- resistance of 0.172 ohm and a synchronous 

 reactance of 2.12 ohms per phase. With equal field excitations 

 they operate at full load with undue heating and on examination 



