THE DYNAMO. 67 



The following disadvantages of the multipolar construction 

 outweigh the above advantages when the number of poles is 

 increased beyond the proper limit. 



(a) Magnetic leakage. A considerable portion of the mag- 

 netic flux, which is forced through the field magnet cores by the 

 field winding, crosses from pole tip to pole tip without passing 

 through the armature. This flux, called the leakage flux, does 

 not contribute to the generation of electromotive force in the 

 armature conductors. When a great number of field magnet 

 poles surround an armature of given diameter, the tips of adja- 

 cent poles which are of opposite polarity are very close together 

 and the magnetic leakage is excessive. The effect of this exces- 

 sive magnetic leakage is to reduce the useful magnetic flux, and 

 hence to reduce the electromotive force generated in the arma- 

 ture, unless the field excitation is very greatly increased at the 

 expense of an increase of field copper, or an increase of power 

 expended in field excitation. The percentage of armature surface 

 actually covered by the pole faces is usually made smaller as the 

 number of poles is increased, in order to avoid excessively short 

 distances between adjacent pole tips. 



(b) Multiplicity of parts. A multipolar machine usually has 

 as many brush sets as poles, and as many field coils as poles. 

 Therefore the cost of manufacture increases with the number of 

 poles because of the increase in the number of parts required. 



37. Smooth core and slotted armatures compared. Dynamo 

 armatures were formerly made with smooth cores built up of 

 circular disks of sheet iron, the armature windings being placed 

 upon the armature surface and bound with bands of phosphor- 

 bronze or steel wire. With this type of armature there is neces- 

 sarily a thick layer of non-magnetic material, air and copper, 

 between the pole face and the armature core. The armature 

 wires are exposed to mechanical injury, and the sidewise drag on 

 the wires due to the side push of the magnetic field between the 

 pole face and armature core not infrequently causes the armature 



