194 



PRINCIPLES OF ELECTRICAL DESIGN 



consider the joint losses in connection with the entire cooling 

 surface. The reader is referred to Art. 11, Chap. II, where the 

 heating of magnet coils was discussed. The problem of keeping 

 the temperature rise of field coils within safe limits (40 to 45C.) 

 is complicated by the fact that the fanning action of the rotating 

 armature will have an effect upon the cooling coefficient; but 

 this has been taken into account in the curves of Fig. 76. The 



0.011 



1000 2000 3000 4000 5000 



Peripheral Speed of Armature - Feet per Minute 



FIG. 76. Cooling coefficients for field coils of dynamos. 



6000 



curve marked A applies to machines with wide spacing between 

 poles, and good ventilation, while the curve B should be used 

 when the main poles are close together, or when commutating 

 poles interfere with the free circulation of air round the main 

 windings. The coefficient obtained from the curves, being 

 waits per square inch per degree rise, is the reciprocal of the coef- 

 ficient k used in the chapter on magnet design; but the cooling 

 surface considered is the same, namely, the total external surface 

 of the winding, including the two ends and also the inner surface 



