THE INDUCTION MOTOR. 



cheaper to wind the coils on forms and to use open slots. The cost 

 of labor being considerably smaller in Europe, some of the leading 

 builders of polyphase motors have preferred to use closed slots and 

 to wind by hand. Coils wound outside of the machine require more 

 insulating material, and as the room is always somewhat scant in 

 polyphase motors, the European method offers some advantages. 

 On the other hand, we have to bear in mind that the greater ease 

 with which machine-wound coils can be exchanged, should not be 

 underrated, and might even be bought with the loss of another ad- 

 vantage. A design is more or less of a compromise, and the best de- 

 sign is that which combines the greatest number of advantages with 

 the least number of drawbacks. 



NUMBER OF SLOTS PER POLE. 



31. A little consideration teaches us that we obtain the least amount 

 of fluctuation in the field the more slots we have per pole. Theoretically, 

 then, it would be advisable to have as many slots as possible. From 

 a commercial standpoint, however, it is advisable to have as few as 

 possible. Either extreme is impracticable ; we must try to strike the 

 golden mean. 



32. The influence of the number of slots upon the leakage can also 

 readily be seen. The more conductors we have in a slot the larger 

 will be the leakage field surrounding the slot. The active field is 

 with enough accuracy for our present consideration the same 

 whether we distribute the same number of cbnductors in a few slots 

 or in many. The e. m. f. induced by this field upon these conductors 

 may, therefore, be set constant independently of the number of slots. 

 Let us take a concrete case. If we have, for instance, 100 conductors 

 arranged in 5 slots, there are in each slot 20 conductors. The leakage 

 field per slot is produced by these 20 conductors, hence proportional 

 to their number. The e. m. f. induced by the leakage field per slot in 

 the 20 conductors in each slot, is proportional to 20 times 20. Hence 



22 



