SYNCHRONOUS MOTORS AND GENERATORS 95 



imum; with this new line e.m.f. what further change in motor 

 e.m.f. would reduce the current to still another minimum; deter- 

 mine also the value of the current for each case. (20 min.) 



14. For the motor of problem 6, having a resistance of 1.2 and 

 a reactance of 5.8 ohms and excited to 1100 volts, construct the 

 circle diagrams for 10, 25 and 40 amperes with 1200-volts line 

 e.m.f. and from these determine the power transformed to mechani- 

 cal energy by the motor for each case. Find also the maximum 

 load the motor will carry, and the current for that load. (20 min.) 



15. Repeat problem 14, with a motor e.m.f. of 1200 volts. 

 (10 min.} 



16. Repeat problem 14, with the resistance doubled. (10 min.) 



17. Repeat problem 14, with the resistance and reactance both 

 doubled. (10 min.) 



18. Find the maximum value that the motor e.m.f. can have 

 with a line e.m.f. of 1200 for each of the impedances of problems 

 14, 16 and 17. (5 min.) 



19. A synchronous motor running on a 220- volt line has a 

 resistance of 0.18 and a reactance of 0.98, and is generating 210 

 volts. Construct the parallelogram diagram for an input of 4 

 kw. and determine the power factor of the line current. With 

 the same power and line e.m.f. find the motor e.m.f.'s to give unity 

 and 86.6% (leading) power factor. (20 min.) 



20. With the same motor as in problem 19 construct the 

 parallelogram diagram for 4 kw. with the current in opposition to 

 the motor e.m.f. and determine the necessary e.m.f. of the motor. 

 Note that this gives maximum output of the motor for a given 

 current and motor e.m.f. (10 min.) 



21. For the motor of problem 14 plot excitation characteristics 

 for 10 and 30 kw.; use motor e.m.f. 's for abscissae and currents as 

 ordinates; show on the curves the range within which the motor 

 will run. Use the circular diagram to obtain the points. (45 

 min.) 



22. A motor having a resistance, 0.04 ohm, and a reactance, 

 0.25 ohm, is excited to 200 volts and run on 220 volts. Draw the 

 circle diagrams for the case where the motor is taking no power and 

 where it breaks from step, also for J the latter load; determine 

 from these the current and power in each case; also what would 

 be the minimum generator e.m.f. on which the motor could run 

 with the least load; also with a generator e.m.f. of 220 volts what 



