260 



ALTERNATING CURRENTS 



there be introduced a lap-wound continuous-current armature, wound 

 for a 2 P-pole field. Instead of the usual two sets of brushes per 

 pole-pair which would be used in connection with a continuous-current 

 machine, let there be provided three sets of brushes per pole-pair, 

 spaced f of the pole-pitch ( = I period) apart, the brush sets occupy- 

 ing similar positions relatively to the stator field being connected 

 together. Further, let the brushes be placed in such positions rela- 

 tively to the stator windings that when three-phase currents are 

 fed into the rotor through the brushes, the rotating field due to them 



is at every instant coincident in 

 position with the stator field. 

 Lastly, let the three groups of 

 rotor brushes be connected across 

 the mains, so that the stator and 

 rotor windings are in parallel with 

 each other, and that their ampere- 

 turns are at every instant added 

 arithmetically when the windings 

 are both traversed by currents 

 coming from the mains. 



The arrangement of connec- 

 tions will be easily understood by 

 reference to Fig. 159, where, for the 

 sake of simplicity, a two-pole field 

 is shown. The windings are not 

 indicated, but only the three points 

 of connection to the stator wind- 

 ings, and the three rotor brushes, 

 spaced 120 apart. The directions of the two windings are supposed 

 to be such that if each of them is supplied with currents from the 

 mains, the fields to which such currents give rise are at every instant 

 coincident as regards both position and direction.* 



FIG. 159. Connections of Compensated 

 Induction Motor. 



155- Equivalent of Compensated Motor 



A moment's consideration will show that, in so far as the action 

 of the motor is concerned, the arrangement of Fig. 159 is practically 

 identical with that of Fig. 160, where the rotor is shown disconnected 



* So that if in Fig. 159 we assume the stutor winding to be an ordinary right-handed 

 Gramme or ring winding, the rotor ring winding would have to be a left-handed one if 

 the brushes are in the positions shown in the figure. With both windings either right- 

 or left-handed, the brushes would have to be rotated through 180 (or, generally, through 

 a pole-pitch or a distance corresponding to period in a multipolar field). Otherwise, 

 the condition may be expressed by saying that two conductors lying along the same 

 radius on opposite sides of the air-gap should convey currents identical in phase. 



