Methods of Improving Commutation
In order to obtain sparkles commutation, the current should be completely reversed at the end of the commutation. There three methods for improving commutation in DC machines.
- Resistance Commutation
- EMF Commutation
- Compensating Winding
Resistance Commutation: In this method of improving commutation we replace copper brushes with carbon brushes. Carbon brushes have high resistance than copper brushes. This high contact resistance has the tendency to force the current in the short-circuited coil to change according to the commutation requirements.
From the figure it is clear that there are two paths for current I1 to reach the brush. One is through “C2b” and another is “c2B1a”. When the brush resistance is low i.e. copper brushes are used, then the current I1 will follow the path “C2b”. No current will flow through the coil “B” because the current always flow the low resistance path.
When the brush resistance is high i.e. carbon brushes are used, then some portion of current will flow through the coil “B” . The magnitude current flowing through the coil “B” depends on the contact resistance of commutator segment “a” and “b”. When the armature moves, the area of contact between commutator “a” and brush increases. With the increase contact area between brush and commutator “a” the resistance decrease because the resistance is inversely proportional to the area.\\\;\\\;\;\;\;\;\;R = \rho\frac{L}{A}
As the resistance of path “21a” decrease the more current will flow through the short circuited coil. Thus by this method of improving commutation, the quick reversal of current will occur in the desired direction.
EMF commutation: The main reason of sparking during is incomplete reversal of current in the short circuited coil due to reactance voltage. In this method of commutation, a voltage is induced in the short circuited coil undergoing commutation to neutralize the reactance voltage.
Two methods are used to induce voltage opposite to that of reactance voltage are – Brush shifting and Using of inter-poles.\\Brush Shifting: In this method of commutation the brush are given sufficient forward lead (In case generator) or back lead (In case of Motor) to bring the coil undergoing commutation in the influence of next pole which has opposite polarity. Since the coil undergoing commutation is in the influence of reversing field, the reverse voltage will be induced in the coil which cancels the reactance voltage. This method has some drawbacks.
- The reactance voltage depends upon armature current. Therefore, the brush shift will depend on the magnitude of armature current which keeps on changing. This necessitates frequent shifting of brushes.
- The greater the armature current, the greater must be the forward lead for a generator. This increases the demagnetizing effect of armature reaction and further weakens the main field.
Using InterPoles or Compoles: Using of interpoles or compoles is the best way to neutralize the reactance voltage and improve the commutation. The interpoles are the same poles having few turns between the main poles. The polarity of these poles are the polarity of the main pole ahead in the direction of rotations in case of generators. They are connected in series with the armature so that they carry full armature current.
The interpoles perform two main functions.
- They help in producing the sparkless commutation by neutralizing the reactance voltage. As the polarity of the interpole is same as the main pole ahead they induce emf in the coil undergoing commutations opposite to that of the reactance voltage and neutralise the reatance volatge.
- The m.m.f. of the compoles neutralizes the cross-magnetizing effect of armature reaction in small region in the space between the main poles. It is because the two m.m.f.s oppose each other in this region
Compensating Winding: Compensating winding the winding place in the slots of the main pole faces. They are connected in series with the armature winding but the direction of current is opposite to that of armature current. The produce mmf that is equal and opposite to the armature MMF. The compensating winding demagnetizes or neutralizes the armature flux produced by the armature conductors. The flux per pole is then undisturbed by the armature flux regardless of the load conditions. The major drawback with the compensating windings is that they are very costly.
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