182 ELECTRICAL ENGINEERING 



and there is a greater tendency to spark than before. If the 

 load is reduced the neutral line falls behind the brushes and a 

 voltage assisting commutation is generated in the coil. 



If the load is all removed from a generator when the brushes 

 are not on the no-load neutral there will be sparking due to the 

 fact that the short-circuited coil has an e.m.f . generated in it which 



i 2 . 

 builds up a current i and stores energy L 75 in the field. This 



Zi 



energy appears as a spark. 



Voltage commutation is also limited in its application and as 

 the current in the armature is increased a point is reached (usually 

 about twenty-five per cent overload) beyond which sparkless com- 

 mutation is impossible, since when the current is increased a 

 stronger field is required to reverse it, but the stronger current in 

 the armature increases the m.m.f. of the armature and moves the 

 neutral line ahead of the brushes and at the same time decreases 

 the flux. The brushes have to be advanced further and the de- 

 magnetizing effect is increased. When the armature m.m.f. is 

 large enough to overbalance the field m.m.f. the flux at the pole 

 tip is wiped out and voltage commutation is impossible. Moving 

 the brushes further ahead only decreases the flux. 



To take full advantage of voltage commutation it would be 

 necessary to vary the position of the brushes with varying load 

 but this is not practicable, and therefore the brushes must be set 

 to give good commutation at some intermediate load and the 

 resistance of the brush contact must be relied on to prevent spark- 

 ing above and below this point. Modern machines are designed 

 to give good commutation at all loads from no load to twenty- 

 five per cent overload with fixed brush position. 



111. Interpoles. Interpoles are small poles placed midway 

 between the main poles of either motors or generators. They are 

 magnetized by a winding connected in series with the armature 

 and carrying the load current. It has been seen that the brushes 

 of motors must be moved back under load to obtain satisfactory 

 commutation. This is possible with motors which run only in one 

 direction, but with all reversible motors the brushes must be fixed 

 on the no-load neutral points. Perfect commutation under these 

 conditions was not possible until the introduction of the interpole 

 or commutating pole. Fig. 147 shows an interpole motor or 

 generator. The brushes are fixed on the no-load neutral points. 



