PROBLEMS. 475 



Ans. () 67 per cent ; (I)) 6,400 ampere-turns per magnetic circuit ; 

 (c) 12,800 ampere-turns per pole. 



118. A certain 6-pole, 25o-kilowatt, 5 5O-volt railway generator 

 has a six-path armature winding with 1,200 conductors. The 

 commutator has 600 segments, and at full load the brushes are 

 shifted forwards through an angle represented by 8 commutator 

 segments. Find : (a) the angle of lead of the brushes in degrees ; 

 and (&) the armature demagnetizing ampere-turns per magnetic 

 circuit at full-load, neglecting the shunt current. Ans. (a) 4.8 ; 

 (&) 2,424 ampere-turns. 



119. The pole faces of the generator specified in problem 1 18 

 cover 70 per cent, of the periphery of the armature, and the radial 

 length of the air gap between the pole faces and the armature 

 core is 0.527 inch. Find: (a) the cross-magnetizing ampere- 

 turns per pole on the armature at full-load ; (b) the intensity of 

 the magnetic field under the pole tips due to the armature current 

 alone ; (c) the intensity of the magnetic field under the leading 

 and trailing pole tips due to the combined action of field coils and 

 armature, the field intensity in the air gap due to field coils alone 

 being 6,500 gausses at full-load. Ans. (a) 10,640 ampere -turns 

 per pole ; (fr) 5,000 gausses ; (c) 1,500 gausses and 1 1,500 gausses. 



NOTE. The radial length of the air gap in this dynamo is in fact 0.313 inch, but 

 the effect of the high degree of magnetic saturation of the teeth of this armature core 

 is equivalent to a greatly increased radial length of air gap. 



120. A bipolar generator has a drum armature with 420 con- 

 ductors and each pole face subtends an angle of 120. The dis- 

 tance across the gap space is O.6 centimeter and the field intensity 

 in the gap space due to the field winding alone is 8,000 gausses. 

 What is the greatest permissible current in the armature of the 

 generator if it is assumed that sparkless running requires the 

 resultant field intensity under the leading pole tips to be at least 

 25 per cent, of the field intensity due to the field winding alone? 

 Ans. 81.8 amperes. 



121. The drum armature specified in problem 120 is arranged so 

 as to be suitable for a four-pole field magnet, the pole faces cover- 



