THE INDUCTION MOTOR. 



the same disadvantages as the regulation of a direct- 

 current motor by a resistance in its armature circuit, while 

 the degree of variation which is possible is much smaller 

 than with a direct-current motor. 



A method of enabling two motors to be run either at full 

 speed or half speed is that known as the cascade system. 

 It will be remembered that the periodicity of the currents 

 in the rotor circuit of an induction motor is equal to the 

 slip. Usually this frequency is extremely small. If, 

 however, the motor runs at a low speed, the slip will be 

 high and the periodicity of the rotor currents m&y be 

 sufficient to drive a second motor if connected to the rotor 

 circuit of the first. In the cascade arrangement, the 

 stator of one motor is connected to the supply, while the 

 windings of its rotor, instead of being connected to 

 the usual starting resistance, are connected to the 

 stator of the second motor, the rotor of which may be 

 either short-circuited or connected to a starting resistance. 

 The speed of the second motor will be approximately 

 equal to the slip of the first motor, while the speed of the 

 first motor will be equal to the frequency of the supply 

 minus the slip. If the motors are coupled together or 

 compelled to run at equal speeds, we shall have the 

 condition 



(frequency - slip of motor 1) 



= speed of motor 2 

 = slip of motor 1 



hence the speed of both motors will be approximately 

 equal to half the frequency of supply, and would be 

 accurately half this speed but for the slip of the second 

 motor. 



Braking and Return of Power to Line. A 3-phase motor is 

 reversed by interchanging any pair of supply connections, 

 which has the effect of reversing the direction of rotation 

 of the rotating field. 



A reversing switch is usually made in the form of a 

 3-pole throw-over switch, the inter-connections being so 

 arranged that with the switch thrown over to one side the 

 motor receives current and runs in one way, and with the 

 switch in the opposite position the motor runs in the 

 opposite direction. 



If, while the motor is running at full speed, the 

 reversing switch be suddenly thrown over, so as to reverse 



