SYNCHRONOUS MOTORS 55 



(a) Calculate the field current by the synchronous-impedance 

 method. 



(b) Calculate the field current by the magnetomotive-force 

 method. 



6. The full-load capacity of a 3-phase, 13,500-volt, synchro- 

 nous motor is 2200 h.p. The armature windings, which are 

 connected in Y, have an effective resistance of 1.52 ohm per 

 phase. The open- and short-circuit characteristics are given by 

 the following data: 



Calculate the field current by the synchronous impedance 

 method when this motor receives 1640 kv.-a. at 0.75 power 

 factor, (a) with excitation greater than normal, (b) with excita- 

 tion less than normal. 



7. The synchronous motor described in problem 6 receives 

 1250 kw. at 0.75 power factor. 



Calculate the field current by the magnetomotive-force 

 method (a) with excitation greater than normal, (b) with excita- 

 tion less than normal. 



8. Concerning the alternator described in problem 1 the fol- 

 lowing additional data are given: The field current is supplied 

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

 normal speed. The core loss is 280 watts, 480 watts, and 610 

 watts for generated armature voltages of 199 volts, 240 volts, 

 and 275 volts respectively. 



What is the efficiency of this generator when it is running as 

 an overexcited synchronous motor and receives 15 kw. at 0.8 

 power factor? 



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

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

 motive-force Illl-tll...!. 



9. Conn-ruing the synchronous motor described in proMmi J 

 the following MoMitional data are given: Tin- resistance of tin- 

 field circuit is 0.82 ohm. The friction and windage loss at 



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