40 ELECTRICAL ENGINEERING PROBLEMS 



resistance would have to be put in series with the armature to 

 reduce the speed to 565 r.p.m.? If, with this resistance in circuit, 

 the current be next reduced one-half, what will the speed become? 

 (7 min.) 



7. A 2-pole, 110- volt motor with a full-load armature current 

 of 90 amperes, has an armature resistance of 0.08 and 300 con- 

 ductors; and the total flux is 3.4 X 10 6 . Required the no-load 

 speed, the full-load speed, and the per cent that the drop in speed 

 is of the full-load speed (the regulation). The brushes are midway 

 between the poles, and the no-load current is 3 amperes. (4 min.) 



8. In problem 7 if the armature resistance were doubled, what 

 would be the speeds and the per cent regulation? Also what would 

 be the speeds and regulation if by reducing the field current the 

 flux were reduced to 3 X 10 6 ? (5 min.) 



9. If in problem 7 a resistance of 0.2 ohm be placed in the 

 armature circuit for the purpose of reducing the speed, what will 

 the no-load and the full-load speeds become? What per cent of 

 the power used will be consumed in the resistance at full load? 

 (5 min.) 



10. A 4-pole, 2-path, 220-volt, 24-h.p. motor has a full-load 

 current of 95 amperes. The field current is 5 amperes, the arma- 

 ture resistance is 0.03, the flux is 4 X 10 6 and the conductors, 160. 

 The brushes are midway between the pole pieces and the no-load 

 current is 4 amperes. Required the no-load and full-load speeds, 

 and the per cent regulation. (5 min.) 



11. What will the regulation in problem 10 become if the 

 brushes are drawn back so as to produce back ampere-turns which 

 reduce the flux 2%? What would the flux have to be made in 

 order to reduce the full-load speed 10%? (2 min.) 



12. How much resistance would have to be put in the armature 

 circuit of problem 10 in order to reduce the speed one half? What 

 would the regulation from no load to full load become then? 

 What per cent of the power supplied would be lost in the resist- 

 ance? (6 min.) 



13. In problem 10 how much starting resistance would have 

 to be put in series with the armature if the starting current were 

 not to exceed the full-load current? If the motor were carrying 

 half load current, what speed would it reach with this resistance 

 in the circuit? How much resistance could be cut out, allowing 

 the current to again reach full-load value? (7 min.) 



