56 PROBLEMS IN ALTERNATING CURRENT MACHINERY 



normal speed is 7.6 kw. The core loss due to rotation is 22.3 

 kw. at 5460 volts and may be assumed to vary as the square of 

 the generated armature voltage. What is the efficiency of 

 this motor under the conditions described in problem 2? 



Calculate the field current (a) by the general method, (b) by 

 the synchronous-impedance method, and (c) by the magneto- 

 motive-force method. 



10. Concerning the synchronous motor described in problem 

 3 the following additional data are given. The field circuit has 

 a resistance of 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 

 may be assumed to vary as the square of the generated armature 

 voltage. What is the efficiency of this motor under the conditions 

 described in problem 3? 



Calculate the field current (a) by general method, (b) by the 

 synchronous-impedance method, and (c) by the magnetomotive- 

 force method. 



11. A 3-phase, 2200-volt synchronous motor is rated to 

 deliver 1000 h.p. when operating at unit power factor. The 

 armature windings, which are connected in Y, have an effective 

 resistance of 0.172 ohm per phase. The resistance of the field 

 circuit is 0.576 ohm. The friction, windage and core losses are 

 17.2 kw. and may be assumed constant. The open- and short- 

 circuit characteristics are given by the following data : 



What is the efficiency of this motor when it delivers 960 h.p. and 

 is operating at 0.83 power factor from a 2200-volt circuit (a) if 

 the excitation is greater than normal, (b) if the excitation is less 

 than normal? 



Calculate the field current by the synchronous impedance 

 method. 



12. A 2-phase, 2200-volt synchronous motor has a full-load 

 capacity of 5000 h.p. when operating at unit power factor. The 

 armature has an effective resistance of 0.0255 ohm per phase. 



