MOTORS 



41 



14. A 4-pole, 220-volt, 25-h.p. shunt motor is wound with a 

 2-circuit winding of 344 conductors in 43 slots, and runs ligi 

 800 r.p.m. The dimensions of the conductors are 90 X 200 mils. 

 It is desired to use this frame for a 400-r.p.m. 110-volt m 

 Specify two windings which will accomplish this. The field i- 

 wound with 2200 turns per spool of wire 46 mils in diameter. 

 Specify the changed winding necessary. (4 min.) 



15. The resistance of the armature (unchanged) of problem 14 

 is 0.067 ohm. The efficiency of the motor being 91% and t In- 

 field current 1.7 amperes, what will be the speed with rated load? 

 (7 min.) 



16. The magnetization curve for a 440-volt, 10-h.p., 4-pole, 

 800-r.p.m. (at no load) shunt motor, is given by the following data: 



Ampere-turns 1000 2000 3000 4000 



Maxwells -=- 10 6 0.53 0.97 1.24 1.48 



The normal ampere-turns are 2500 for 440 volts. At what speed 

 will the motor run with no load on a 330-volt circuit, and also on a 

 550-volt circuit? Neglect changes in the resistance of the shunt 

 winding. (7 min.) 



17. The normal resistance of the shunt motor in problem 16 at 

 60 C. is 25 ohms. At what speed will the motor run when started 

 up out of doors at 20 C.? (4 min.) 



18. The motor of problem 16 has its field rewound with win* 

 of double the area so as to make it available for variation of speed 

 by the field rheostat. What must be the resistances in the rheo- 

 stat when the motor is running at 600 and 1000 r.p.m.? (4 min.) 



19. If in problem 16 the armature resistance is 1.4 ohms, at 

 what speed will the motor run with rated load on the 440-volt 

 circuit? (8 min.) 



20. A 6-pole, 500-volt shunt motor with a 6-path armature has 

 a full load of 300 amperes. The armature resistance is 0.032 

 ohm, and the total flux at no load is 8.96 X 10 8 . The armature 

 has 978 conductors, and the field excitation is 15,000 ampere-turns 

 per pair of poles. Assuming the brushes to be midway bet 



the poles, required the speed in r.p.m. for currents of 5 and 300 

 amperes in the armature. (4 min.) 



21. A 440-volt, 850-h.p., 100-r.p.m., 14-pole motor has a 

 parallel-wound armature, a full-load current of 1550 amperes, and 



