COMMUTATION 



151 



zone has been found where the fringe of flux from the leading 

 pole tip is sufficient to neutralize the flux due to the armature 

 windings. l With the excitation of the leading main-pole tending 

 to send flux through the armature core from right to left, and the 

 armature e.m.f. tending to produce a flux distribution gener- 

 ally as indicated in Fig. 57, the resulting flux distribution in the 

 commutating zone will be somewhat as shown in Fig. 58. Here 

 the flux cut by the conductors during commutation is represented 

 by eight lines only, the direction of this commutating flux 

 being such as to maintain the current during the earlier stages 

 of commutation and reverse it during the later stages. At a 

 point midway between the two extreme positions the conductor 

 is cutting no flux, and the current is therefore zero. It should be 

 observed that the correct position for the brush is in advance of 

 the "apparent" neutral zone; that is to say, the position of the 

 neutral field on the surface of the armature does not correspond 

 with the correct position for the center of the brush. That is 

 because the slot flux must enter through the upper part of the 

 teeth if it is not to be cut by the conductors during commutation. 



Thus the conductor, which at the instant of time t = -| must be 



in a neutral field, is actually below a point on the armature 

 periphery where flux is entering or leaving the teeth, and this 

 condition occurs even when, as in the present instance, the effect 

 of the end connections is entirely negligible. This flux, which 

 enters the teeth comprised between the two extreme positions 

 of the short-circuited conductors, is neither more nor less than 

 the slot flux (or equivalent slot flux, as the case may be). It is 

 represented by 12 lines in the diagram Fig. 58, and it must 

 be provided by the leading pole shoe if brush shift is resorted 

 to, or by the commutating interpole when this method of 

 cancelling the armature flux is adopted. 



47. Effect of End Flux. When the effect of the end connec- 

 tions of the armature coils cannot be neglected, the armature 

 flux cut by this portion of the short-circuited coil must be 

 cancelled in the same manner as the slot flux; that is to say, an 



1 The reader is supposed to understand the phrase "a flux neutralizing 

 a flux " to be equivalent to the more scientific but less convenient expression 

 "the magnetic force due to one magnetizing source being of such magnitude 

 and direction as to neutralize the magnetic force due to a second magnetizing 

 source, thus causing the resultant flux of induction to be of zero value." 



