42 PROBLEMS IN ALTERNATING CURRENT MACHINERY 



windings are connected (1) for an open-coil single-phase winding 

 and (2) for a 3-phase star winding. 



68. In problem 66 compare the rated outputs on the same 

 basis when the windings are connected (1) for a 2-phase winding 

 and (2) for a 3-phase star winding. 



69. Concerning the alternator described in problem 51 the 

 following additional data are given. The field current is sup- 

 plied at 110 volts. The friction and windage loss is 310 watts at 

 normal speed, and the core loss due to rotation is 480 watts for 

 an armature generated voltage of 240. Assume that the core 

 loss is constant. What is the efficiency of this alternator at the 

 load described in problem 51? (1) Calculate the field current 

 by the synchronous impedance method, (2) by the magneto- 

 motive-force method. 



70. Concerning the alternator described in problem 52 the 

 following additional data are given. The resistance of the field 

 circuit is 0.516 ohm. The friction and windage loss is 6.2 kw. 

 at normal speed. The core loss due to rotation is 11.0 kw. at 

 2200 volts and may be assumed to vary as the square of the 

 generated armature voltage. 



(a) What is the efficiency of this generator at the load de- 

 scribed in problem 52? 



(b) What is the efficiency of this generator when delivering 

 the same current at 0.80 power factor? 



Calculate the field current (1) by the general method and (2) 

 by the magnetomotive-force method. 



71. Concerning the alternator described in problem 53 the 

 following additional data are given. The resistance of the 

 field circuit is 0.376 ohm. The friction and windage loss is 8.4 

 kw. The core loss due to rotation is 20.2 kw. at 5500 volts, and 

 should be assumed to vary as the square of the generated arma- 

 ture voltage. 



(a) What is the efficiency of the generator at the load de- 

 scribed in problem 53? 



(b) What is the efficiency of this generator when delivering 

 the same power at unit power factor? 



Calculate the field current (1) by the synchronous impedance 

 method, and (2) by the magnetomotive-force method. 



72. The full-load capacity of a 3-phase, 25-cycle alternating- 

 current generator is 850 kv.-a. at 5000 volts. The armature 

 windings are connected in Y and have an effective resistance of 





