REPULSION MOTOR 



279 



1 1' a series-wound motor is intended to operate on both continuous- 

 aiul alimiating-current circuits, then since, when using continuous 

 current, it is desirable to work with a stronger field, the field winding 

 may be divided into two sections, which are connected in series with 

 r;uli other for continuous-current operation, and in parallel for single- 

 l>lia<e current. 



166. Repulsion Motor 



Another type of single-phase commutator motor which has under- 

 gone considerable development of late is the repulsion motor. This 

 was originally invented by Professor Elihu Thomson in 1887. In 

 its simplest form, it is shown in Fig. 170. The brushes, it will bo 

 noticed, are entirely disconnected from the field winding, and are 

 short-circuited by a short length of flexible conductor. They are dis- 

 placed from the neutral zone through a certain angle. The general 

 principle of action of such a motor is not diffi- 

 cult to understand. The flux impressed on the 

 armature by the field, and represented in Fig. 170 

 by the vector OF, may be resolved into two 

 components : one OA being along the line of 

 brushes, and the other OB at right angles to 

 this line. The first component will obviously 

 give rise to an e.m.f. which will produce currents 

 in the armature winding flowing through the 

 short-circuited brushes. The armature winding 

 behaves towards this component of the flux like 

 the short-circuited secondary of an ordinary 

 transformer. The e.m.f.'s induced in the arma- 

 ture winding by the alternations of the second 

 component OB will at every instant balance each 

 other, so that there will be no current due to 

 the two equal and opposite e.m.f.'s.* But since 

 the magnetic axis of the armature is along the 

 line of brushes, while the component OB is at right angles to this 

 line, a torque will be exerted on the armature. When the armature 

 is running, there will, in addition to the two e.m.f.'s in its windings 

 already considered one of which, that due to the alternations of the 

 flux component OB, is inoperative, and may therefore be left out of 

 account be two other e.m.f.'s, due to the rotation of the armature 

 in the field components OA and OB. The former e.m.f. is easily 

 seen to be inoperative, while the latter is compounded with the e.m.f. 



* These e.m.f.'s would give rise to a current if a pair of brushes were provided at 

 right angles to the lirst pair. 



FIG. 170. Repulsion 

 Motor. 



