THE INDUCTION MOTOR. 369 



the direction of the rotating field, the rotor will at the 

 moment after reversal rotate in the opposite direction to 

 the rotating field, and the slip will be practically 200 per 

 cent., and large currents will be induced in the rotor, 

 since this will be running with the starting resistance cut 

 out. It is, consequently, not permissible to brake any 

 except small motors with squirrel-cage rotors by reversing 

 the field, although in case of serious danger it might be 

 resorted to for larger motors. 



It is not possible to brake an induction motor by dis- 

 connecting it from the line and short-circuiting the 

 terminals through a resistance, as is frequently done with 

 direct-current motors. As soon as the stator of an 

 induction motor is disconnected from the supply, the 

 rotating field ceases to exist, and no force exists to retard 

 the motor, which will continue to rotate until gradually 

 stopped by the friction of the shaft or gearing. 



It is important to notice what happens if, while the 

 rotor is driven by external means, the motor remains 

 connected to the supply, and the rotating field conse- 

 quently continues to rotate. 



If the speed of the rotor is below that of synchronism, 

 currents will be induced in the rotor in the usual manner, 

 and will flow in such a direction that the motor tends to 

 rotate with the field, and the external driving force will be 

 less than that required to drive the motor without current. 

 As the speed of the rotor is increased the currents 

 induced in it will fall until at the synchronous speed no 

 currents will be formed, and the external force required to 

 maintain the rotation of the rotor will exactly balance the 

 friction and other forces opposing the rotation. 



If the speed is now still further increased the rotor 

 conductors will begin to cut the lines of the rotating field 

 in the opposite direction. This will cause rotor currents 

 to flow in the reverse direction, and their action will be 

 to oppose instead of to assist the rotation. Under these 

 conditions the machine becomes a generator and supplies 

 current to the line through the primary winding. It 

 may thus be made to act as a brake, preventing the 

 speed of the rotor rising much above synchronism. 



No-load Curves of an Induction Motor. No-load curves may be 

 obtained on an induction motor under either of two con- 

 ditions, viz. : (1) with the armature shaft free to rotate, so 



