PARTS OF A DYNAMO-ELECTRIC MACHINE 35 



to a greater extent in the bipolar machine than in the 

 multipolar machine of the same capacity. There are 

 other advantages of the multipolar frame which cannot 

 be discussed here. 



Pole Shoes. Nearly all machines have the pole pieces 

 fitted with pole shoes. These shoes are sometimes made 

 in the form of cast iron collars which fit around that 

 end of the pole next to the armarture, but are generally 

 made as part of the pole itself. The laminations of which 

 the pole is built up are so shaped that, when the pole is 

 assembled, the lugs on the laminations form a pole shoe. 

 This is shown in Fig. 12 in which an assembled pole is 

 shown. The two projections on such a pole are called 

 the pole tips; that one which a point on the armature 

 first comes under as the armature revolves is called the 

 leading pole tip, and the other is called the trailing pole 

 tip. The term pole tip is used even when the pole has 

 no shoe; the edges of the pole itself constitute the pole 

 tips in this case. 



The functions of the pole shoe are to increase the 

 effective area of the air gap, and to hold the field coil in 

 place. The air gap (that part of the magnetic circuit 

 between the pole face and armature core) constitutes 

 the greater part of the reluctance of the magnetic path; in 

 actual machines the air gap reluctance constitutes about 

 four-fifths of the total reluctance and so four-fifths of the 

 magnetomotive force of the field coil is used up in forcing 

 the flux through the air gap. By lessening the reluctance 

 of the gap the size of the field coil may be much 

 reduced. 



Now the reluctance of the air gap = -, where Z = the 



[LA 



length of the air gap (parallel to direction of magnetic lines) 

 and A=the cross-sectional area of the air gap. For air, \L, 

 the permeability, is equal to one. 



The minimum length of the air gap being fixed from 



