106 PRINCIPLES OF ELECTRICAL DESIGN 



large amount of heat, and the higher the peripheral velocity of 

 the armature, the better will be the cooling effect. 



When forced ventilation is adopted, a fan or centrifugal blower 

 may be provided at one end of the armature. This may as- 

 sist the action of radial ventilating ducts, or it may draw air 

 through axial ducts. When axial air ducts are provided, the 

 radial ventilating spaces are omitted, and the gross length of the 

 armature may therefore be reduced. The ventilation is through 

 holes punched in the armature plates which, when assembled, 

 will provide a number of longitudinal openings running parallel 

 with the armature conductors. These openings may be circular 

 in section and should preferably not be less than 1 in. in diameter, 

 especially when the axial length of the armature is great, because 

 they will otherwise offer too much resistance to the passage of 

 the air, and will also be liable to become stopped up with dirt. 



One advantage of axial ducts which, however, can only be 

 used with forced ventilation is that the heat from the body 

 of the armature can travel more easily to the surface from which 

 the heat is carried away than in the case of radial ducts. When 

 the cooling is by radial ducts, the heat due to the hysteresis and 

 eddy-current losses in the core must travel not only through the 

 iron, which is a good heat conductor, but also through the paper 

 or other insulation between laminations, which is a poor conductor 

 of heat. As a rough approximation, it may be said that the 

 thermal conductivity of the assembled armature stampings is 

 fifty times greater in the direction parallel to the plane of the 

 laminations than in a direction perpendicular to this plane. For 

 this reason, radial vent ducts, to be effectual, must be provided 

 at frequent intervals. The thickness of any one block of stamp- 

 ings between radial vent ducts rarely exceeds .3 in. 



The coefficients for use in calculating temperature rise are 

 based on data obtained from actual machines, and owing to 

 variations in design and proportions they are at the best un- 

 reliable. When forced ventilation is adopted whether with 

 radial or axial vent ducts it is possible to design the fans 

 or blowers to pass a given number of cubic feet of air per second, 

 and the quantity can readily be checked by tests on the finished 

 machine. The design of such blowers does not come within the 

 scope of this book, neither is it possible to discuss at length the 

 whole subject of ventilation and temperature rise. For a more 

 complete study of this problem, the reader is referred to other 



