2 5 8 



INDUCTION MOTORS. 



t F.xr. 



Each of these members is built of laminated iron and has on 

 it a winding disposed in slots. These windings are termed the 

 primary and the secondary, for the induction motor is a form 

 of transformer. The primary receives current directly from the 

 line, while the secondary is short-circuited upon itself and has 

 no electrical connection with the line or with the primary, the 

 current in it being set up by induction, as in a transformer. 



Usually, the primary winding is on the stator and the second- 

 ary is on the rotor. This gives the simplest form of motor; the 

 primary can be connected directly to the supply line and there 

 is no need of brushes or slip rings. Sometimes, however, the 



secondary is on the stator and 

 the primary is wound on the 

 rotor, the current from the 

 line being in this case intro- 

 duced through slip rings. The 



principle of operation is the 



V* 

 same in the two cases. 



3. Rotating Field. The 



operation of an induction 

 motor will be most readily 

 understood by a consideration 

 of its rotating magnetic field. 

 FIG. i. Stator of a 3-phase motor Let us consider a polyphase* 



with 4 poles per phase, often termed a . , . 



4-pole motor. (This diagram is illustra- motor in which the pn 

 tive and does not represent construction.) . g s t a tj onar y 4- 



Fig. I illustrates the primary or stator Qf a 3-phase motor in 

 which there are four poles per phase. When the current in 

 Phase I. is a positive maximum, AA form north poles and A'A r 



* (3a). For a single-phase motor, see 56. 



t (3b). In this case, with the primary stationary, the idea of the rota- 

 ting field is most simply seen. The relation between primary and secon- 

 dary, however, and the torque produced, will be the same, irrespective of 

 which winding is stationary and which is moving. 





