20 ELECTRICAL ENGINEERING PROBLEMS 



shunt field 61 ohms. What e.m.f. will have to be generated at 

 full load in order to get the 250 volts at the terminals? If the 

 flux were kept constant in this machine from no load to full load, 

 what would be the regulation expressed in per cent at the rated 

 voltage at full load? By what per cent must the flux density be 

 increased to maintain a constant voltage? (2 min.) 



18. The armature of a 30-kw., 110-volt, 4-pole machine is 

 wound multiplex with two bar windings of 156 conductors each. 

 By changing the method of connection to the commutator and 

 without other change this may be made a single winding. What 

 will then be the e.m.f., current capacity and power rating of the 

 armature? (1 min.) 



19. A maker uses the same armature core for 125 and 500- volt 

 machines of the same watt capacity. At 125 volts there are 144 

 turns of No. 8 wire. Specify the number of turns and the B. & S. 

 number for the 500-volt machine of the same speed. The winding 

 is to occupy the same space neglecting additional space taken by 

 the insulation. (1 min.) 



20. A 4-path armature has 42 slots with wire space | X J inch, 

 and each slot contains 8 No. 8 wires. The e.m.f. is 120 volts. 

 This armature is rewound with No. 14 wire, putting in as many 

 conductors in a slot as possible. Required the e.m.f. in the second 

 case and the power in both cases, using 600 circular mils per 

 ampere. The speed remains constant. (4 min.) 



21. If a unipolar dynamo runs at 6000 r.p.m. and the strength 

 of field is 16,000 gausses, what must be the diameter of the disk to 

 generate 5 volts, the inside diameter being 4 inches. (3 min.) 



22. A certain 110-volt shunt generator when run at 0.8 normal 

 speed gives 65 volts. What percentage change has taken place 

 in the flux? When run at 20% above normal speed the voltage 

 is 145; what is the change in the flux? (4 min.) 



23. A 4-pole generator has a speed of 1500; the length of the 

 armature is 15 cm. and its diameter is 25 cm. The pole covers 

 60 degrees. The armature is 4-path and has 31 slots each con- 

 taining 8 conductors of No. 10 wire. The gap density is 10,000. 

 Required the e.m.f. generated and the output in kilowatts with 

 400 circular mils per ampere allowed in the armature winding. 

 (4 min.) 



