DIRECT-CURRENT MACHINERY 167 



and the number of ampere turns required is 



-- (202) 



Thus the ampere turns required per pair of poles for a density of 

 63,000 lines per square inch for the two air gaps is 0.4028 X 0.3133 

 X 63,000 = 7968. 



The teeth form the next section; they are projections of the 

 armature core and are made of sheet steel. 



The length of the path through the teeth per pair of poles is 

 twice the depth of a slot = 2 X 1.75 = 3.5 ins. 



The section of the path through the teeth is taken as the mean 

 iron section of one tooth multiplied by the number of teeth under 

 one pole; the mean width of a tooth is 0.62 in.; the over-all length 

 of the armature core is 8 ins. but from this must be subtracted 3 

 vent spaces -fy in. in width and an allowance of 10 per cent must 

 be made as a stacking factor, since the armature is built up of thin 

 sheets insulated with varnish ; thus, the length of iron in the tooth 

 is (8 3 X T 5 sO X 0.9; the number of teeth under one pole 

 allowing for fringing is 11; therefore the iron section of the path 

 through the teeth is 



11 X 0.62 X (8 - 3 X A) X 0.9 = 43.4 sq. ins. 



The flux passing through the teeth is the same as that through 

 the air gap = 6,300,000 lines, and the flux density in the teeth is 



fiO ^00 000 



- = 145,200 lines per square inch; this requires 1500 am- 



pere turns per inch length and a total of 1500 X 3.5 = 5250 ampere 

 turns per pair of poles. 



If the flux density in the teeth is above 100,000 lines per square 

 inch it is necessary to take account of the fact that the path through 

 the teeth is paralleled by an air path consisting of the slots, the 

 vent ducts and the insulation between punchings; this path has 

 usually a larger section than the path through the teeth and 

 consequently at high densities, where the permeability of the 

 iron is low, a considerable amount of the flux will follow this 

 path. 



In this case the area of the parallel air path is 58 sq. ins. and 

 from Fig. 59 the permeability of the iron for a flux density of 



