18 ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 
Ss 
regard to the slots for the coils. The bolts used to hold 
the armature core during the piling and pressing should 
be somewhat smaller in size than the holes, but after this 
operation is completed, each bolt is replaced by another 
bolt which fits its hole tightly. The holes should be 
reamed out to provide for a tight fitting bolt. 
In the case of an armature divided into several parts, a 
perfect joint is essential between the adjoining surfaces 
of the armature core, and the designer should be earefu! 
to take into account the magnetie resistance between these 
core joints. The number of segments must be in aeccord- 
ance with the number of slots, as well as the number of 
partitions or ducts. If necessary. half segments can be 
applied in the design next to the joints. 
36. Form and Number of Slots.—The form and number 
of armature slots has not only an influence on the working 
or performance of the generator, but influences the choice 
of the insulating material, as well as that of the magnet 
poles. Generally, from the standpoint of electrical 
design, shallow and broad slots are favored, as they give 
smaller leakage, but for mechanical reasons the teeth 
should never be less than 0.1 ineh in width. 
The teeth must be kept tight by the thick end plates on the 
side faces, in order that they should not work loose 
and cause humming. Generally speaking, the ratio be- 
tween the depth and width of a slot should not exceed 3. 
The use of a closed or half closed hole implies the use of 
hand wound coils, or in low tension work, the application 
of copper bars. The application of closed holes means 
an increase in the cost of winding; but on the other hand 
it permits the employment of solid pole pieces, which is 
a great advantage, as explained in Par. 47. 
37. Open Slots.—With open slots the armature coils may be 
maehine wound, better insulated and easily exchanged. 
