COMMUTATION 



145 



neutral point may be considered as straight lines, the resultant 

 flux cut by the two coil-sides joined in series by the end con- 

 nections may be represented by the shaded area in Fig. 55, 

 where positive values are measured above, and negative values 

 below, the horizontal axis. Intervals of time are measured 

 horizontally from left to right, and the straight line BB' rep- 

 resents the flux distribution in the commutating zone. The 

 direction of this flux is such as to develop in the short-circuited 

 coil, at every instant of time during the period of commutation, 

 an e.m.f. tending to produce a current in the required direction; 

 that is to say, from the commencement of short-circuit, when 



FIG. 55. Flux distribution in commutating zone of ideal armature coil. 



t 0, until the middle of the commutation period, when both 

 flux and current are of zero value, the small amount of flux cut 

 by the short-circuited conductors is of the same kind as that 

 previously cut by the conductors, while from the time t = t c /2 

 until the end of commutation (t = t c ) the flux is of the opposite 

 kind, being such as will cause the current to flow in the oppo- 

 site direction. The amount of the flux required to bring about 

 this condition is only a small percentage 1 of the flux cut by a coil 

 under the main poles in the same interval of time, because the 

 resistance of the armature windings is always low in comparison 

 with the resistance of the external circuit, and, as a matter of 

 fact, it is the average value of the flux entering the armature 

 over the commutating zone with which the designer is usually 

 concerned. If the brushes are so placed as to bring the short- 

 circuited conductors in a neutral field, satisfactory commutation 

 will result. 



1 The flux density where the coil-side enters or leaves the commutating 

 zone (the positions t = and t = t c of Fig. 55) would be about 2.5 per cent, 

 of the average density under the main poles, because this is the ratio of the 

 armature IR drop to the developed voltage in a well-designed dynamo of 

 moderate size say, 50 to 100 kw. 

 10 



