258 ALTERNATING CURRENTS 



108. The Induction-motor Air-gap. The air-gaps of direct- 

 current generators and motors, and of alternators, are much 

 groat or than is necessary for mechanical clearance. This is due 

 to the fact that with too short an air-gap, the effect of an na- 

 ture reaction becomes too great, that is, the field is relatively 

 weak as compared with the armature. On the other hand, the 

 air-gap of the induction motor is made just as short as mechanical 

 clearance will permit. The back emf. of the stator varies only a 

 few per cent, from no load to full load. This back emf. is in- 

 duced by the air-gap flux cutting the stator conductors. As the 

 speed of the rotating field is constant, the flux in the gap must 

 be substantially constant from no load to full load. Therefore, 

 in a given motor, the magnetizing current is practically constant 

 at all loads. If the length of the air-gap be increased, the reluct- 

 ance of the magnetic circuit is also increased. As the back emf. 

 does not change except slightly, the flux changes also but slightly. 

 Therefore, with a fixed flux the greater air-gap reluctance will 

 necessitate a greater magnetizing current. This increased mag- 

 netizing current lowers the power-factor (see Fig. 233, page 244). 



Large slot openings increase the reluctance of the air-gap and 

 so lower the power-factor. Therefore, from the standpoint of the 

 magnetizing current it is desirable to use semi-closed slots or 

 open slots with magnetic wedges. The disadvantage of closing 

 the slot too much is that both the stator and the rotor induct- 

 ances increase and the break-down and starting torques are 

 reduced (see page 247, equation 67). The increase of inductance 

 also tends to lower the power-factor. 



The small mechanical clearance between the rotor and the 

 stator makes it necessary to have a heavier shaft and heavier 

 and stiffer bearings in the induction motor than are required in 

 other types of rotating machinery of the same speed and size. 



109. Speed Control of Induction Motors. The speed of the 

 rotor of an induction motor is given by 



f v 120 

 N 2 = - ^p- (1 ~ *) (Page 236, equation 65) 



where N 2 is the rotor speed in revolutions per minute, / is the 

 frequency of supply in cycles per second, P is the number of poles 

 and s is the slip. 



Obviously, there are three factors, frequency, slip and 



