44 PROBLEMS IN ALTERNATING CURRENT MACHINERY 



75. ( 'oncoming the alternator described in problem 59 the 

 following additional data are given. The resistance of the field 

 circuit is 0.549 ohm at 25 C. The temperature of the field 

 under load conditions is 68 C. The friction and windage loss 

 is 38 kw. The core loss is given by: 



What is the efficiency of this generator for the load described in 

 problem 59? 



76. Two alternators of the same design are operating in 

 parallel. The first delivers 980 kw. at 0.95 power factor, and 

 the second 720 kw. at 0.73 power factor. What adjustments 

 should be made to have these alternators operate under the best 

 conditions? When these have been made what power will each 

 deliver and at what power factor will it operate? 



77. Two 3-phase alternators connected in parallel are driven 

 by shunt motors whose speed load characteristics for particular 

 field excitations are given by the following data: The speed of 

 the first motor falls uniformly from 600 rev. per min. at no load 

 to 530 rev. per min. at full load of 100 kw. on the alternator. 

 The speed of the second motor falls uniformly from 590 rev. per 

 min. at no load to 550 rev. per min. at full load of 100 kw. on the 

 alternator. 



(a) For what load will the alternators divide the load equally? 



(b) What will be the load on each alternator when their com- 

 bined load is 200 kw.? 



(c) What is the greatest load that can be delivered without 

 overloading either alternator? 



78. A 3-phase, 2200-volt alternator which is rated to deliver 

 760 kv.-a. is connected in parallel through transformers with a 

 3-phase, 5500-volt alternator which is rated to deliver 1500 

 kv.-a. The first alternator has 64 poles and is driven by an 

 engine whose speed falls from 94 rev. per min. at no load to 

 91 rev. per min. at full load on the alternator. The second al- 

 ternator has 72 poles and is driven by an engine whose speed falls 

 from 83 rev. per min. at no load to 79 rev. per min. at full load 

 on the alternator. 



