CHAPTER X 

 ARMATURE REACTIONS 



1. A bipolar armature has 420 conductors, and its field has a 

 polar angle of 120 degrees. When it is giving 100 amperes from 

 the brushes, how many cross and how many back ampere-turns 

 will be present, supposing that the brushes are placed opposite 

 the pole tips? (1 min.) 



2. The pole pieces of a bipolar generator subtend 108 degrees 

 each, and the armature has 63 slots with 4 conductors each. If 

 the brushes are opposite the pole tips, how many cross and how 

 many back turns will be present? If the machine is giving 40 

 amperes, how many cross and back ampere-turns will there be? 

 (1 min.) 



3. A 4-pole, 55-kw., 125-volt, 4-path (parallel- wound) generator 

 has poles subtending 60 degrees each. There are 100 slots with 

 two conductors each. What will be the back and cross ampere- 

 turns when the machine is giving its full load if the brushes are 

 opposite the pole tips? If the connections of this armature be 

 changed so as to use it in a 2-pole field having the same total area 

 of pole face, what will the cross and back ampere-turns become? 

 The same current density is used in the conductors. (2 min.) 



4. A 100-kw., 250-volt, 8-pole generator has an armature cir- 

 cumference of 360 cm.' The width of each pole face is 36 cm., 

 there are 167 slots with two conductors each, and they are con- 

 nected as a 2-path (series) winding. How many cross and back 

 ampere-turns are present at full load with the brushes opposite the 

 pole tips? How many would there be if the same conductors used 

 at the same current density could be connected as an 8-path 

 (parallel) winding? How many if the same conductors were 

 reconnected and used in a 2-pole field with the same total polar 

 area? (2 min.) 



5. If in problem 1 the gap density be 8000 and the density due 

 to the cross field must not exceed 75% of the gap density, what 

 will be the smallest permissible length of gap? How much could 

 this be reduced if the machine were 4-pole? (1 min.) 



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