66 PROBLEMS IN ALTERNATING CURRENT MACHINERY 



46. A 1200-h.p. synchronous motor is operated at the end of 

 a 3-phase transmission line which has a resistance of 3.3 ohms 

 and a reactance of 3.7 ohms per conductor. The line voltage 

 at the generating station is 5000 volts, and the motor delivers 

 energy to a constant load of 1000 h.p. The rotational losses of 

 the motor are 29.8 kw. and may be assumed constant. The 

 armature windings are connected in Y and have a resistance of 

 0.40 ohm and a synchronous reactance of 15.4 ohms per phase. 



If the maximum allowable line current is 135 amperes what is 

 the greatest possible terminal voltage at the motor? What is 

 the necessary excitation voltage of the motor? 



47. At the end of a transmission line which has a resistance of 

 12.2 ohms and a reactance of 17.4 ohms per conductor there is a 

 synchronous motor which delivers a constant load of 1300 h.p. 

 The armature windings of the motor, which are connected in Y, 

 have an effective resistance of 0.94 ohm and a synchronous 

 reactance of 56 ohms per phase. At no load with an impressed 

 voltage of 11,000 volts the motor takes a current of 1.52 amperes 

 when the field current is adjusted for unit power factor. The 

 line voltage at the generating station is maintained constant at 

 11,500 volts. 



If the line current is limited to 120 per cent, of its full-load 

 value, which is 62.5 amperes, what is the least additional react- 

 ance that should be inserted in each line so that a terminal poten- 

 tial difference of 12,000 volts may be obtained at the motor? 

 What is the necessary excitation voltage of the motor in this 

 case? 



48. A 13,800-volt, 3-phase synchronous motor has a full-load 

 capacity of 1350 h.p. The armature windings are connected 

 in Y and have an effective resistance of 2.18 ohms and a syn- 

 chronous reactance of 92 ohms per phase. At no load with an 

 impressed voltage of 13,800 volts the motor takes 28.2 kw. at 

 unit power factor when the excitation is normal. This motor 

 is operated at the end of a transmission line which has a resist- 

 ance of 22 ohms and a reactance of 28 ohms per conductor. 

 The line voltage at the generating station is maintained con- 

 stant at 14,000 volts. 



When the motor is delivering 1200 h.p. what is the greatest 

 potential difference at the motor if the armature current is 

 limited to 125 per cent, of its full-load value? What is the neces- 

 sary excitation voltage of the motor? 



