ROTOR WINDINGS 117 



high resistance.* But high resistance in the rotor circuits would 

 involve large heating loss and poor efficiency under normal running 

 conditions. Thus, although a squirrel-cage rotor could easily be 

 constructed to give a powerful starting torque (by the use of high- 

 resistivity material for its conductors), its efficiency under normal 

 running conditions would be extremely low. Large starting torque 

 and high efficiency are, in fact, incompatible features, and the only 

 way of securing both lies in the use of a rotor the resistance of whose 

 windings at starting is high, and under normal running conditions, 

 low. This variation of resistance is easily obtained by the use of an 

 external rheostat, which may be connected in series with the rotor 

 windings at starting, then gradually short-circuited as the motor 

 gains speed, and finally cut out altogether. Now, a squirrel-cage 

 winding does not readily adapt itself to the insertion of external 

 resistances. It is for this reason that most of the larger European 

 induction motors are provided with so-called wound rotors as dis- 

 tinguished from squirrel-cage or short-circuited rotors. The winding 

 of these rotors is connected to slip-rings, which allow of the insertion 

 of suitable starting resistances. 



62. Rotor Windings 



Two types of three-phase windings, both of them star-connected, 

 are commonly used in the case of rotors provided with stalling 

 resistances. One of these windings is similar to the form of armature 

 winding for a three-phase generator already described ( 43 and 

 Fig. 76), while the other closely resembles the wave winding of a 

 continuous-current armature, but differs from it in not being entirely 

 symmetrical. This second kind of winding is frequently described as 

 a bar winding, in order to distinguish it from the first type, which 

 is spoken of as a coil winding. In the smaller sizes, coil windings 

 are generally used, while the bar winding is commonly employed in 

 large motors. In order to explain clearly how such a bar winding is 

 carried out, we shall consider a particular case that of a four-pole 

 rotor having 96 conductors or bars arranged in 48 slots, so that there 

 are 2 conductors per slot. 



It will be sufficient to consider the winding of one phase, since all 

 three phases are similarly wound. The number of conductors per 

 phase is 9 ^ = 32, and since the motor is a four-pole one, the number 

 of conductors per pole per phase is ;i ^ = 8. The winding of each 

 phase must, therefore, clearly consist of four equally spaced groups of 



The reason for this will be understood later, when we coine to study the theory of 

 buch motors in detail (Chapter XII.). 



