COMMUTATION 



169 



on an iron core in order that the time constants of the main and 

 shunt circuits may be approximately equal. If this is not done, 

 the interpole winding will not take its proper share of the total 

 current when the change of load is sudden, and this may lead to 

 momentary destructive sparking. 



Among the advantages of commutating poles may be men- 

 tioned the fixed position of the brushes and the fact that fairly 

 heavy overloads can be taken from the machine without de- 

 structive sparking, because of the building up of the commutat- 

 ing flux with increase of load. The limiting factor in this con- 

 nection is the saturation of the iron (mainly of the interpole 

 itself) in the local circuit, and this is aggravated by the large 

 percentage of leakage flux due to the proximity of main and com- 

 mutating poles. Deeper armature slots may be used than in 

 the case of machines without interpoles, and the specific load- 

 ing (ampere-conductors per unit length of armature periphery) 

 may be increased, thus allowing of greater output notwithstand- 

 ing the slight reduction in width of main poles necessary to 

 accommodate the interpoles. The maximum output of the 

 machine with commutating poles is usually determined by the 

 heating limits, the ventilation being less effective than in the 

 case of non-interpole machines. The PR loss in interpole 

 windings is to some extent compensated for by a reduction of 

 the ampere-turns on the main poles when shorter air gaps are 

 used. With the brushes on the geometric neutral and an air 

 gap which is small relatively to the space between pole tips, 

 field distortion per se has nothing to do with commutation, 

 whether interpoles are used or not; if the fringe from the leading 

 pole tip is not to be used for counteracting the effects of end flux 

 and slot flux on the coil undergoing commutation, the unequal 

 flux distribution under the main poles due to cross-magnetization 

 does not affect the field at a point midway between two main 

 poles. It is not suggested that field distortion is unobjectionable 

 when the brushes are on the geometric neutral or when inter- 

 poles are used. The concentration of flux at one side of the main 

 pole may lead to flashing over the commutator surface (an effect 

 often attributed to unsatisfactory commutation, though rarely 

 due to this cause),* but the chief objection to a large number of 

 armature ampere-turns per pole is the fact that the flux in the 

 zone of commutation due to this m.m.f. must be compensated for 

 somehow if satisfactory commutation is to be obtained. It 



