THE INDUCTION MOTOR. 



43. There remains to be answered the second question, How is the 

 leakage factor dependent upon the pitch of the poles ? 



THE INFLUENCE OF THE POLE-PITCH UPON THE LEAKAGE FACTOR. 



44. Before entering upon the experiments made to clear up this point, 

 I shall attempt to show deductively how the leakage factor may be 

 expected to vary with the pole-pitch. 



Fig. 19 gives a view of the slots in a polyphase motor. The broken 

 lines mark the leakage flux threading each slot. Now, let us assume 

 that we have a motor with 48 slots in the field, which we want to 

 wind as a two-pole, four-pole, or eight-pole motor, the armature 

 being provided with a squirrel-cage winding, thus being suitable for 

 any number of poles. 



If the number of ampere-conductors per slot remains the same 

 for any number of poles, the leakage flux per slot also remains con- 

 stant. 



The total number of ampere-turns spread over the circumference 

 of the field is then constant whether the motor has two, four, or eight 

 poles. 



45. But the number of ampere-turns per pole is inversely proportional 

 to the number of poles; hence, the induction produced by these am- 

 pere-turns in the air-gap is also inversely proportional to the number 

 of poles. In other words, the magnetic field per pole, being propor- 

 tional to the product of the induction in the air-gap into the pole- 

 pitch, varies inversely with the square of the pole-pitch. 



46. The leakage field, as we have seen, is constant for each slot. 

 Hence, the total amount of leakage the sum of all the leakage fields 

 pertaining to each lot is also constant. The number of leakage lines 

 per pole is proportional to the number of slots per pole. 



47. The ratio of leakage field -=- main field is therefore inversely pro- 

 portional to the pole-pitch for the same number of ampere-turns per 

 slot. 



34 



