COMMUTATION 



149 



to picture the actual distribution of the magnetic flux, it seems 

 preferable to consider a machine with toothed armature because 

 this is the case which has generally to be dealt with by the 

 practical designer, and moreover it is exactly this question of 

 teeth, or what is known as the slot flux, which sometimes leads 

 to confusion of ideas, if not to inaccurate conclusions, and it 

 should therefore not be disregarded in any modern theory of 

 commutation. 



46. Effect of Slot Flux. In Fig. 57 an attempt has been made 

 to represent, by the usual convention of magnetic lines, the flux 

 due to the armature current alone, which enters or leaves the 

 armature periphery in the interpolar space when the field mag- 

 nets are unexcited. The position chosen for the brushes is the 

 geometric neutral i.e., the point midway between two (sym- 

 metrical) poles and the magnetic lines leaving the teeth will 

 cross the air spaces between armature surface and field poles 

 and so close the magnetic -circuit. The brush is supposed to 

 cover an angle equal to twice the slot pitch: the current in the 

 conductor just entering the left-hand end of the brush is +/., 

 the current in the conductor under the center of brush is zero, 

 and the current in the conductor just leaving the brush on the 

 right-hand side is 7 C . The armature is supposed to be rotat- 

 ing, and it will be seen that the conductors in which the current 

 is being commutated are cutting the flux set up by the armature 

 as a whole. It is important to note that the flux cut by a con- 

 ductor while travelling between the two extreme positions during 

 which the short-circuit obtains is not only the flux passing into 

 the air gaps from the tops of the teeth included between these 

 extreme po'sitions of the conductor, but includes also the flux 

 due to the currents in the short-circuited conductors, which 

 crosses the slot above the conductor 1 and leaves the armature 

 surface by teeth which are not included in what at first sight may 

 seem to be the commutating zone. In other words, the portion 

 of the armature flux cut by a conductor undergoing commuta- 

 tion when no reversing flux is provided from outside is that 

 which passes up through the roots of the teeth included between 

 the two extreme positions of the short-circuited coil. This 

 picture of the conductor cutting the field set up by the armature 



1 For the sake of simplicity, a single conductor is shown at the bottom of 

 each slot and the whole of the slot flux is supposed to link with it. The 

 calculation of the "equivalent" slot flux will be taken up later. 



