124 ALTERNATING CURRENTS 



casting, which carries two end-shields containing the bearings. In 

 order to improve the ventilation, each end-shield has four windows, 

 closed by expanded metal gratings. The bearings are of the usual 

 self-oiling type, each having two oiling rings, which ride loose on the 

 shaft and dip into oil-wells. The rotor core-plates are mounted on a 

 spider, to which they are secured by means of a key. The rotor 

 conductors are bolted to the short-circuiting rings. It will be noticed 

 that whereas there are forty-eight slots in the stator, the number of 

 rotor slots is twenty-nine, so that the two numbers have no common 

 factor. This arrangement is invariably adopted in the case of motors 

 having squirrel-cage rotors, and its object is to prevent variations in 

 the torque with varying position of the rotor ; such variations occur 

 to a marked extent when the number of stator and that of rotor slots 

 have a common factor. These variations are due to the fact that in 

 certain positions of the rotor the reluctance of the magnetic circuit 

 is less than in others, and the rotor will always tend to pass from a 

 position of higher to one of lower reluctance. If, e.g., the stator and 

 rotor have the same number of slots, then on exciting the stator the 

 (open-circuited) rotor will tend to move so as to bring about coinci- 

 dence of the teeth and slots in the two cores. 



With a wound three-phase motor, however, it is impossible to 

 prevent the stator and rotor slots from having a common factor. For 

 since there must be at least three slots per pole per phase (correspond- 

 ing to the three phases), the stator and rotor slot numbers must both 

 be multiples of six. It is found, accordingly, that the starting torque 

 of such motors has a larger value in certain positions of the rotor 

 than in others. 



66. Asymmetry of Hemi= tropic Stator Winding 

 with Odd Number of Pole-pairs. Method of 

 obtaining Symmetry 



When the hemi-tropic type of winding ( 43) with alternately 

 straight and bent coils (Fig. 77) is adopted for the stator, a slight 

 asymmetry is introduced into the arrangement of the coils in cases 

 where there is an odd number of pairs of poles. This will be readily 

 seen by considering a six-pole motor. With a hemi-tropic winding 

 there will be three coils per phase (one coil per pair of poles), or a 

 total of nine coils. If we proceed to arrange the coils on the stator 

 core, making them alternately straight and bent, we are left with an 

 odd coil, and in order to fit this in between a bent coil on one side 

 and a straight coil on the other, one half of it must be " straight " 

 and the other " bent." The shape of the projecting ends of the odd 



