212 PRINCIPLES OF ELECTRICAL DESIGN 



Item (65): Temperature Rise Refer Art. 34, page 107. The 

 radiating coefficient for the cylindrical surfaces is, by formula 



(54), 



1 500 [ 3 070 

 For the outside surface w c = - - = 0.0457 



lUUjUUU 



For the inside surface w c = ' * nn = 0.03 



1UU, 



In the case of the ducts, it should be noted that the radial 

 depth of the armature stampings is large because of the wide 

 polar pitch, and instead of taking the velocity of air through 

 the vent ducts as one-third of the outside peripheral velocity, we 

 shall assume a lower value, making v d = 850. Thus, the cool- 

 ing coefficient for the ducts and ends will be, by formula (56), 



1.2 X 850 



Wd = -JOMOO- = - 0102 



The procedure is exactly as followed in the example on page 111, 

 and the temperature rise is found to be 37C., which is within 

 the specified limit of 40. 



Items (66) and (67): Equivalent Air Gap. Refer Art. 36, 

 page 117. 



The permeance of the air gap over one slot pitch at the center 

 of the pole face where the actual clearance is 5 = Y in. may be 

 written, 



= 98 

 and the equivalent air gap, as given by formula (58), will be 



= 6.45XUXl.076 = 078cm 



= 0.307 in. 



Item (68): Drawing of Equivalent Flux Lines. Refer Art. 41, 

 page 129. 



Before completing the drawing of the pole shoe as shown in Fig. 

 78, it is well to estimate the cross-section of the pole core, as 



