348 THE INDUCTION MOTOR. 



the armature conductors in the same way as the revolving 

 tidd of an alternator induces currents in the conductors of 

 its armature. 



If an alternator armature carries no current, the 

 electromotive force induced in the conductors exerts no 

 retarding force on the field. When the armature circuit 

 is closed, so that the conductors have a current formed in 

 them, there is at once set up a force tending to retard the 

 revolving field and to reduce the speed of the relative 

 motion of armature and field. This force increases in 

 direct proportion to the current carried by the armature 

 conductors. 



In the induction motor exactly the same action occurs, 

 and, if the electromotive force generated in the armature 

 conductors is allowed to produce a current, this current 

 will produce a force acting on the rotating field opposing 

 the relative motion of the field and armature, and tending 

 at the same time to retard the rotating field, and to cause 

 the armature to revolve with it. 



Since the field always rotates at a fixed speed, 

 depending only on the frequency of the current supplied 

 to it, and the armature is mounted on a shaft, the force 

 cannot retard the field, but may produce a revolution of 

 the armature in the same direction as that of the field. 

 The force producing the motion of the armature of an 

 induction motor is, consequently, essentially the same as 

 that which is sustained by the stationarj r armature of a 

 revolving-field alternator, and its value is calculated in a 

 similar manner. 



Thus the torque of the motor is proportional to the 

 product (field strength) x (armature current). 



In an induction motor the armature is usually called 

 the rotor, since it is the rotating member, while the 

 circular magnet system is called the stator, since it forms 

 the stationary part. These terms will in future be used. 



Production of Current in Rotor Winding. The diagram 

 (Fig. 174) indicates the outline of the stator and rotor of 

 an induction motor, a few conductors being shown to 

 indicate the rotor winding. The diagonal arrow indicates 

 the direction of the rotating field at the moment under 

 consideration, and the curveH arrow shows the direction of 

 its movement. 



