330 PRINCIPLES OF ELECTRICAL DESIGN 



tance might have been obtained by using a smaller width and 

 greater depth of copper conductor; but, seeing that the width 

 of tooth will probably be sufficient, the proposed design of slot 

 (as shown in Fig. 135) has the advantage that the eddy-current 

 loss in the armature inductors, from both causes referred to in 

 Art. Ill (page 318), will be very small. 



The width of copper strip was selected to fit into the 1-in. 

 slot, because this seems to provide a suitable cross-section for 

 the stator tooth. Thus, a section halfway down the tooth, or 

 (say) 2 in. from the top, will have a diameter of 44 in., and the 



average width of tooth will be ^-jg -- 1 = 1.88 in. On the 



basis of B g = 6,000 gausses, and a sinusoidal flux distribution 

 over the pole pitch, the " apparent" tooth density (item (27)) 



., , 7r D Xk TT X 6,000 X 2.62 X 51 

 would be *.- = 2 X L88 X 46.8 14 ' 30 gaUSS6S 



which is not too high (see Art. 76, page 251). 



Items (28) to (32). Assuming a flux density of 8,500 gausses 

 in the armature core (see Art. 88, Chap. XII), the net radial depth 

 of stampings below the slots will be 



62 ' 2 X 10 ' . 12.1 in. 



2 X 8,500 X 6.45 X 46.8 



The actual radial depth should be greater than this to allow for 

 the reduction of section due to the presence of axial vent ducts. 

 In this particular machine it is proposed to ventilate, if possible, 

 with axial ducts only, and a fairly large cross-section of air 

 passages must therefore be allowed. An adequate supply of air 

 will probably be obtained if the total cross-section of air duct 

 through the body of the stampings (in square inches) is not less 

 than 0.005 X cubic inches of iron in stator below slots. In this 

 case the volume of iron in the stator ring will be approximately 

 7r(48.25 + 12.1) X 12.1 X 46.8 = 107,500 cu. in.; and the total 

 cross-section of air ducts in the stampings should be 0.005 

 X 107,500 = 537.5 sq. in. 



The actual radial depth of stamping below the teeth can be 

 calculated by assuming that the air ducts reduce the gross depth 



537.5 , , 537.5 



by an amount equal to average circumference > or (say) ^^ = 



in. Let us make the depth R d (item (29)) = 14 in., and 

 provide vent ducts arranged generally as shown in Fig. 136, 



