ARMATURE DROP 19 



makes 1500 r.p.m.; the flux is 2 X 10 6 . Required the current 

 if the total resistance is 3 ohms. (1 min.) 



10. A series machine has a resistance of 0.2 ohm. For 5 

 amperes the terminal e.m.f. is 99 volts; increasing the current to 

 10 amperes increases B from 10,000 to 15,000; required the e.m.f. 

 at the terminals of the machine for a current of 10 amperes. 

 (1 min.) 



11. An armature core is 15 cm. in diameter; the shaft is 3 cm. 

 in diameter; the net iron armature length is 15 cm.; the armature 

 density is 12,000 and there are 100 conductors; required the e.m.f. 

 at 2400 r.p.m. (2 min.) 



12. A dynamo has 64 conductors; the length of the armature 

 is 30 cm. and its diameter is 25 cm. The pole pieces cover three- 

 fourths of the armature, and the density in the air gap is 10,000. 

 Required the e.m.f. at 1000 r.p.m. (2 min.) 



13. A 50-kw., 4-pole, 120-volt machine, designed to run at 

 680 r.p.m., is to have its speed reduced to 340 to permit of direct 

 coupling to an engine. Its armature winding is made up of 128 

 bars, each 5 X 10 mm. What will be the values of the e.m.f., 

 current capacity and power after this speed reduction? State 

 what change must be made in the winding to bring the e.m.f. back 

 to 120 volts, and what will then be the current capacity and power. 

 The winding is to occupy the same space in both cases, neglecting 

 insulation. (3 min.) 



14. In the machine of problem 9, Chap. VI, it is required to 

 increase the e.m.f. at full load to 135 volts. What per cent increase 

 must be made in the flux in order to accomplish this? (1 min.) 



15. A 250- volt, 40-kw., 4-pole generator has a 4-path winding 

 of 272 conductors. By leaving out two of these conductors this 

 winding may be changed to a 2-path winding. What would then 

 be the e.m.f., current capacity and power of the machine? How 

 much should the speed be increased to make up for the loss of the 

 two conductors? (2 min.) 



16. A 150-kw., 6-pole, 250-volt, 450-r.p.m. generator has 496 

 armature conductors, wound 6-path. If this were changed to a 

 2-path winding, what would its e.m.f., current capacity and power 

 rating become? What would be the e.m.f. generated in one con- 

 ductor? (2 min.) 



17. The resistance of the 6-path armature, including brush 

 drop of problem 16, is 0.0081 of the series coil 0.00083, and of the 



