CHAPTER VIII 



61. Induction motors. Squirrel-cage rotor 62. Rotor windings 63. Starting 

 resistances for rotors. Internal short-circuiting and brush-lifting gear 64. Lewis 

 induction motor 65. Example of induction motor. Variation of torque with 

 position of rotor 66. Asymmetry of hemi-tropic stater winding with odd number 

 of pole-pairs. Method of obtaining symmetry 67. General characteristics of 

 induction motor. Air-gap length. Effect of rotor eccentricity. 



61. Induction Motors. Squirrel-cage Rotor 



IN connection with polyphase systems of distribution, a type of 

 motor is employed which is known as the induction motor. The 

 action of such a motor depends on the possibility ( 20) of producing 

 rotating waves of magnetic flux by means of polyphase currents. 



Let a laminated stator be provided with polyphase windings (two- 

 or three-phase) similar in every respect to the armature windings of 

 a polyphase generator, and let the rotor consist of a solid cylinder 

 of iron (cf. Fig. 27). When polyphase currents are sent through 

 the stator windings, they give rise to rotating waves of flux 

 commonly spoken of as a rotating field and as these waves sweep 

 across the rotor they induce currents in it. The direction of these 

 currents is, in accordance with Lenz's law, such as to oppose relative 

 motion of the rotor and the rotating field. A driving torque is 

 thereby exerted on the rotor, which (assuming that the resistances to 

 its motion are not excessive) will run up to a speed only slightly 

 below that of the rotating field. The difference between the speed of 

 the rotating field and that of the rotor, on which depends the magni- 

 tude of the induced currents, is spoken of as the slip. Since with 

 increasing load a larger driving torque, and hence larger induced 

 currents, are required to keep the rotor running, it follows that the 

 slip will increase with increase of load. 



The rudimentary form of motor considered, in which the rotor 

 is a solid cylinder of iron, although mechanically simple, would be 

 unsatisfactory from the point of view of efficiency, as the loss in the 

 rotor would be excessive. In practice, therefore, the rotor takes the 

 form of a laminated iron cylinder having copper conductors embedded 

 in it. 



