n6 



ALTERNATING CURRENTS 



The simplest form of rotor winding is that known as the squirrel- 

 cage winding. It consists of a number of copper rods or bars arranged 

 in holes or slots around the rotor periphery, and connected at each 

 end to a ring of copper. One example of a squirrel-cage winding is 

 given in Fig. 92. In this, the rotor is provided with a number 

 of equidistant circular holes (open at the top) close to its periphery, 

 and through these holes pass copper rods. The projecting ends of the 

 rods are slotted, and into the slots, on each side, is fitted a copper ring, 

 which is then soldered to the rods. In Fig. 95 is shown a slightly 

 different arrangement. The conductors are, in this case, of rectangular 

 cross-section, and fit into rectangular semi-closed slots in the rotor 



COPPER RODS 

 WORT-CIRCUITING RING' 



FIG. 92. Squirrel-cage Rotor. 



core. The projecting ends of the copper bars are bolted to end- 

 rings. 



The squirrel-cage winding is at once the simplest and cheapest 

 form of rotor winding, and is almost exclusively used for small 

 motors up to about 5 h.p. In the United States, it is used for very 

 large motors as well. It possesses, however, the disadvantages of 

 taking a heavy starting current and exerting a comparatively feeble 

 starting torque. It is thus incapable of starting against a heavy load, 

 and must be run up to full speed on a loose pulley. For starting 

 motors with squirrel-cage rotors, auto-transformers ( 58) are 

 employed. 



In order that an induction motor may be capable of exerting a 

 large torque at starting, its rotor circuits must have a comparatively 



