126 ALTERNATING CURRENTS 



The power factor of an induction motor depends very largely on 

 the length of air-gap, and it is important to keep the air-gap as small 

 as mechanical considerations will allow. The extreme shortness of 

 the air-gap of induction motors is an important constructional feature, 

 which forms a striking contrast to the relatively very long air-gap of 

 the continuous-current type of motor. Thus, taking the case of a 

 railway motor, the air-gap of the continuous-current type will 

 generally lie between 3 and 7 mm. ; whereas an induction motor will 

 have a gap ranging from 1 to 3 mm. Even a 250-h.p. modern 

 induction motor may have a gap as small as 1'5 mm. 



Owing to the shortness of the air-gap, exact centering of the 

 rotor relatively to the stator is a matter of great importance, as a 

 slight amount of eccentricity may result in a strong side-pull, tending 

 to bend the shaft. The bearings must be of very ample proportions, 

 so that the wear is inappreciable. The effect of a slight amount of 

 eccentricity in producing a side-pull has formed the subject of several 

 recent investigations. The following very simple formula is given 

 by J. K. Sumec : * 



p=r - l 



- ( * 



where P = total side-pull, in dynes ; B 2 = mean square of magnetic 

 induction around the rotor periphery, on the assumption that the 

 rotor is exactly coaxial with the stator ; S = total cylindrical surface 

 of rotor, in sq. cms. ; S = (single) air-gap ; t = eccentricity (distance 

 between axes of stator and rotor). 



* Zeitschrift fur EleJdrotechnilc (Wien), vol. xxii. p. 727 (1904). 



