112 PRINCIPLES OF ELECTRICAL DESIGN 



3. The ventilating duct surface, including the two ends of 

 the armature core, of area A 3 = 5 (32 2 23 2 ) (5+1) = 4,680 



sq. in. 



The radiating coefficients to be used are calculated by formulas 

 (54) and (56); thus, for surface (1), 



B 1,500 +3,360 

 100,000 



and the watts that can be dissipated per degree rise of temperature 

 are 



= 0.0486 X 1,510 = 73.4 

 For surface (2), 



1,500 + (3,360 X !|) 

 w c = TT^T^T; = 0.0391 



and the watts that can be dissipated per degree rise of temperature 

 are 



W* = w c A 2 



= 0.0391 X 1,085 = 42.5 

 For surface (3), 



' 



and the watts that can be dissipated per degree rise of temperature 

 are 



Wz = w d A s 



= 0.0134 X 4,680 = 63 



The total watts that can be dissipated per degree rise of tem- 

 perature are 73.4 + 42.5 + 63 = 178.9; whence the rise in 

 temperature to be expected will be 



7 ' 000 _ OR QOp 



17p 



Temperature Rise of Machines with Forced Ventilation. When 

 a machine is designed for forced ventilation, suitable ducts 

 whether radial or axial must be provided in the armature, and 

 the frame must be so arranged as to provide proper passages for 

 the incoming and outgoing air. The fan or blower may be out- 

 side or inside the enclosing case. It is usual to allow 100 cu. ft. 

 of air per minute for every kilowatt lost in heating the arma- 



