REPULSION MOTOR 281 



in Fig. 171. OF denotes the direction of the flux impressed on tin- 

 rotor I iy tin- stator. Two sets of brushes are used, B B m and B 'B,,/, 

 \\liii !i HIV independently short-circuited. The brushes BO and BO 

 are always maintained in fixed positions along the diameter whose 

 direction coincides with that of the impressed field OF. The 

 remaining brushes B m and B w ' are arranged to be movable. Now, 

 since the magnetic axis of the rotor is given by the bisector of the 

 angle between t\\o brushes not belonging to the same pair (Bo and 

 B m ', or B ' and B, n ), it follows that a displacement of the brushes 

 !'. , !'/ through a given angle will shift the bisector of the angle 

 between B and B m ' (or B ' and B m ) i.e. the magnetic axis of the 

 rotor through half the angle of displacement of the brushes. In 

 the simple arrangement of Fig. 170, on the other hand, the magnetic 

 ;ixis of the rotor is displaced through an angle equal to the angle of 

 brush displacement. From this it immediately follows that with the 



Fio. 171. De"ri's Arrangement of FIG. 172. Latour's Arrangement of 



Brushes for Repulsion Motor. Brushes for Repulsion Motor. 



arrangement of Fig. 171 a given displacement of the brushes will 

 have less effect in altering the speed, and speed regulation will be 

 easier. 



Another modification in the arrangement of the brushes, due to 

 Latour, is shown in Fig. 172.* There are, as in Deri's arrangement, 

 two sets of brushes. All the brushes are, however, arranged to move 

 simultaneously. The difference between this and the simple pair of 

 brushes in Fig. 170 is that only a portion of the winding is short- 

 circuited instead of the entire winding. The advantage gained by 

 adopting the double short-circuit of Fig. 172 is a decreased tendency 

 to spark. In order to explain how the improvement is brought 

 about, we may refer to Fig. 170. The component of the field which 

 gives rise to the sparking difficulty is OB. The short-circuited coils, 

 \\ith the arrangement of Fig. 170, are subject to the full inductive 

 action of this component. By adopting the double short-circuit of 



* EleldrolecKnitohe Ztitschrift, vol. xxv. \>. 952 (1904). 



