64 PROBLEMS IN ALTERNATING CURRENT MACHINERY 



What is the power factor? What current does the motor 

 take? 



39. A 2-phase, 10,000-volt synchronous motor has a rated 

 capacity of 4500 h.p. The armature has an effective resistance 

 of 0.64 ohm and a synchronous reactance of 23.7 ohms per phase. 

 At no load with an excitation greater than normal the motor 

 takes 74 kw. at 10,000 volts and the line current is 41 amperes. 



With the field excitation unchanged what current will the 

 motor take when it is delivering its rated output? At what 

 power factor will the motor be operating? 



40. At the end of a 3-phase transmission line are induction 

 motors which take a total load of 1200 kw. at 0.87 power factor. 

 An additional load of 600 h.p. should be provided for. Find the 

 kilovolt-ampere capacity of a synchronous motor that will 

 supply this load and will at the same time make it possible to 

 adjust the power factor of the entire load to unity. Assume that 

 the efficiency of the synchronous motor is 92 per cent. 



41. An induction motor load at the end of a 3-phase trans- 

 mission line takes 6000 kw. at 0.90 power factor. A synchronous 

 motor is operating in parallel with the induction motors in order 

 to improve the power factor. The motor has a full-load capacity 

 of 6500 h.p. when operating at unit power factor from a 6600-volt 

 circuit. The armature windings, which are connected in Y, have 

 an effective resistance of 0.081 ohm per phase. The synchronous 

 impedance is 3.57 ohms per phase. The friction and windage 

 losses are 62.8 h.p. and the core losses at normal voltage are 86 

 kw. The latter may be assumed to be constant. The synchro- 

 nous motor is operated so that the resultant power factor of the 

 load is unity, and the line voltage is 6600 volts. If the current 

 is limited to 125 per cent, of its full-load value what is the 

 greatest power that the synchronous motor can supply? What 

 is the necessary excitation voltage of the synchronous motor at 

 this time? 



42. Induction motors at the end of a 3-phase transmission 

 line deliver a total power of 3500 h.p. and operate at a resultant 

 efficiency and power factor of 91.6 per cent, and 0.903 respectively. 

 The line voltage is 13,200 volts. A 1640-kv.-a. synchronous 

 motor is operated in parallel with these motors to improve the 

 power factor of the load, and to supply an additional load of 

 1000 h.p. The armature windings of this synchronous motor 

 are connected in Y and have an effective resistance of 1.52 ohms 



