A scheme that is frequently used to neutralize this emf of slef induction and the effect of the armature mmf is the inclusion of small poles.Those small poles are located on the brush axis and excited by a coil carrying the armature current.Because of their location and fuction, these poleas are callsed interpoles or commutating poles.Use of armature current excitation is dicatated by the fact that both the self-induction of emf in the short-circuited coil and the flux in the brush axix.They are produced by the armature mmf, which is proportional to armature current. For generator action, the polarith of the interpole must be the same as that of the field pole into which the coil is moving.
The most effective way to prevent flashover condition is to neutralize the armature reaction and this causes the severe peaking of air-gap flux density which will lead to breakdown.This is achieved the best by use of a compensating winding which is embedded in slots that are distributed along the pole faces.By allowing armature current to flow through this pole-face winding with a polarity opposite to that of the armature winding and a suitable number of ampere-turns, the armature reaction could be completedly cancelled.The major disadvantages of eliminating or reducing the commutation problem is high expense on pole-face windings.
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Emily