42 



ELECTEICAL MACHINERY 



for insulation. Sometimes thin paper is put between every 

 few laminations to increase the insulation. 



Form of Laminations. In small armatures the lamina- 

 tions are solid discs, merely having a hole in the center for 

 the shaft. In multipolar machines it is unnecessary to 

 use solid discs because in a large core made up of solid 

 laminations, there would be more iron than is necessary 

 for the magnetic flux. The paths of the flux in a multi- 

 polar machine is shown in Fig. 17. It is seen that the iron 



FIG. 17. Hollow Discs are Used for Large Armature Cores. The 

 magnetic flux would not penetrate very deep into the core even 

 if it were solid. 



inside the dotted circle is useless as far as the magnetic 

 circuit is concerned. 



The discs for large machines, therefore, are generally 

 made ring-shaped; the iron which is punched from the 

 inside of the disc may be used for the armature of a smaller, 

 machine. If the armature is large (say more than 2 feet 

 in diameter) the rings will not be in one piece; each lamina- 

 tion is made up of several pieces. In building up an 

 armature core of such laminations, they are so assembled 



