DYNAMO DESIGN 



77 



If the speed of rotation (N) is not specified, it is necessary 

 to make some assumptions regarding the peripheral velocity of 

 the armature. This velocity lies between 1,200 and 6 000 

 ft. per minute; the lower values corresponding to machines 

 of which the speed of rotation is low, while the higher values 

 would be applicable to belt-driven dynamos, or to direct-coupled 

 sets of which the prime mover is a high-speed engine or high-head 

 waterwheel. When the generator is coupled to a steam turbine, 

 the speed is always exceptionally high, and the surface velocity of 

 the armature may then attain 2 or 3 miles per minute. The 

 discussion of steam-turbine-driven generators, in so far as the 

 electrical problems differ from those of the lower-speed machines, 

 will be taken up in connection with alternator design. 



The peripheral velocity in feet per minute is, 



irDN 



v = 



whence 



12 



irD 



Inserting this value of N in formula (44) we get 



LDv 

 kw. output = Q~4Q/) (45) 



Relation of l a to D. The output equation (43) shows that there 

 is a definite relation between the volume of the armature and the 

 output, provided the quantities represented by the symbols 

 B g , q, and r, can be estimated. In order to determine the rela- 

 tion between the length l a and the diameter D, certain further 

 assumptions must be made. Thus, 



l a = ^ (46) 



pole arc 

 where p = the number of poles, and k is the ratl armature i en gth' 



It is desirable to have the pole face as nearly square as possible 

 because this will lead to the most efficient field winding. If the 

 section of the pole core departs considerably from the circular or 

 square section, the length per turn of field winding increases 

 without a proportionate increase of the flux carried by the pole. 

 For a square pole face, k = 1 and 



Y- = = 0.45p (approximately) 



la HT 



The ratio D/l a usually lies between the limits of 0.35p and 0.65p. 



