PARTS OF A DYNAMO-ELECTRIC MACHINE 



69 



The pole shoe serves to increase the area of the air gap 

 so as to reduce the density in this part of the magnetic 

 circuit. Tabulating our data as before, and using the 

 magnetization curves, we have: 



Total ampere-turns required 



4S65 

 Ampere-turns per pole = 



4865 

 2433 



These two examples will serve to show how the number 

 of field ampere-turns is roughly determined. In an actual 

 design other factors have to be taken into account; the 

 eft'ect of the armature m.m.f. on the field strength has to 

 be considered and the problem is somewhat more com- 

 plicated than we have indicated. 



Proper Size of Wire. The size of wire to be used in 

 winding the field coils is now to be determined. Generally, 

 all of the field coils of the machine are connected in series 

 and then to some source of e.m.f. from which the field 

 current is to be taken. The average length of one turn of 

 the field coil is estimated from the known size of the field 

 pole and the assumed depth of the coil. Suppose that the 

 Icimth of one turn close to the field pole was 10 inches and 

 that we assume a winding depth of one inch; the length of 



