26 ELECTRICAL ENGINEERING PROBLEMS 



6. If the flux be 3 X 10 6 , the reluctance of the whole circuit be 

 0.001 and there be ten back turns, required the total number of 

 ampere-turns on the field necessary if the armature be giving 48.6 

 amperes. (1 min.) 



7. A bipolar smooth-core machine has an air gap of f inch, a 

 gap density of 12,000, a polar angle of 135 degrees and 200 con- 

 ductors. How much current could be taken from the machine 

 without reversing the field under the pole tips? (4 min.) 



Note. On account of their greater simplicity most of the 

 problems involving length of air gap are given as applying to the 

 old-style smooth-core machines. The flux conditions in the gap 

 and teeth of a slotted core dynamo are too complicated to lend 

 themselves readily to problems of this kind. The principals 

 involved are, however, similar. 



8. Given the following data of a 2-pole machine: the number of 

 conductors is 320, the gap density is 6000; the length of gap (two 

 sides) is 1.6 cm.; 80% of the surface of the armature is covered 

 by the poles. If the cross ampere-turns are not to exceed 0.6 of 

 the part of the field ampere-turns applied in the gap, how much 

 current can the above armature carry? How much if only 75% 

 of the surface of the armature be covered by the poles? (3 min.) 



9. A 110-volt shunt dynamo has 1130 field turns with a shunt 

 resistance of 55 ohms. There are 120 armature conductors and 

 a 20-degree angle of lead. The polar angle is 120 degrees. The 

 machine is feeding 132 15-watt lamps. Required the cross, back 

 and effective ampere-turns. (3 min.) 



10. Determine the number of cross ampere-turns per pole, back 

 ampere-turns per pole and series turns per pole to balance the 

 back turns in the case of a dynamo having the following data: 

 Conductors 500, total current 200, 4-pole, 2-path winding, polar 

 angle 60 degrees, angle of lead 10 degrees, 0.9 of the total current 

 flowing through the series turns; leakage coefficient, 1.3. (4 min.) 



11. Given a 4-pole smooth-core dynamo, with the polar angle 

 60 degrees, angle of lead 12 degrees, 240 conductors, current (total) 

 824, 4-path winding, leakage coefficient 1.25; required the series 

 turns per pole necessary to balance the back turns; also the 

 minimum thickness, in inches, of the air gap without possible 

 reversal of the field under the pole tip, if the gap density be 10,000. 

 If this machine were bipolar with i the total current and double 

 the polar angle, what would the above quantities be? (4 min.) 



