LEWIS INDUCTION MOTOR 



121 



shown in Fig. 94, which is due to Kahlenberg, and is used by Siemens 

 and Halske. It will be noticed that the neutral point of the resistance 

 is represented by a single movable arm, whose contact is of sufficient 

 width to cover three segments. As the arm moves over the contacts, 

 it cuts out a section of the resistance in one phase only at a time ; 

 this destroys the balance of the rotor windings, which, however, is 

 not an important point. 



Contact with the slip-rings is maintained by means of carbon 

 brushes. In order to do away with the heating which occurs at the 



A ^ S 3 



FIG. 94. Kahlenberg'e Starting Resistance. 



contact surfaces when once the motor has been run up to full speed, 

 it is usual to provide an internal short-circuiting arrangement. 

 This consists essentially of a sleeve capable of sliding along the 

 shaft. By means of a lever the sleeve is forced into contact with 

 three springs, which form the ends of the rotor windings, thereby 

 short-circuiting the rotor independently of the brushes. The brushes 

 are now no longer required, and may be lifted off the slip-rings ; 

 arrangements for doing this easily are frequently provided. 



64. Lewis Induction Motor 



The use of wound rotors with slip-rings introduces numerous 

 complications an external starting resistance, and the somewhat 

 intricate internal short-circuiting and brush-lifting devices. Attempts 

 have, therefore, been made to design motors which combine the 

 advantage of simplicity possessed by a rotor having no slip-rings 



