Methods of Improving Commutation
To make the commutation satisfactory we have to make sure that the current flowing through the coil completely reversed during the commutation period attains its full value.There are three main methods of improving commutation. These are
- Resistance commutation
- E.M.F. commutation
- Compensating windings
Resistance CommutationIn this method of commutation we use high electrical resistance brushes for getting spark less commutation. This can be obtained by replacing low resistance copper brushes with high resistance carbon brushes.
We can clearly see from the picture that the current IC from the coil C may reach to the brush in two ways in the commutation period. One path is direct through the commutator segment b and to the brush and the 2nd path is first through the short-circuit coil B and then through the commutator segment a and to the brush. When the brush resistance is low, then the current IC from coil C will follow the shortest path, i.e. the 1st path as its electrical resistance is comparatively low because it is shorter than the 2nd path.
When high resistance brushes are used, then as the brush moves towards the commutator segments, the contact area of the brush and the segment b decreases and contact area with the segment a increases. Now, as the electrical resistance is inversely proportional to the contact area of then resistance Rb will increase and Ra will decrease as the brush moves. Then the current will prefer the 2nd path to reach to the brush.
Thus by this method of improving commutation, the quick reversal of current will occur in the desired direction.
ρ is the resistivity of the conductor.
l is the length of the conductor. A is the cross-section of the conductor (here is this description it is used as contact area).
E.M.F. CommutationThe main reason of the delay of the current reversing time in the short circuit coil during commutation period is the inductive property of the coil. In this type of commutation, the reactance voltage produced by the coil due to its inductive property, is neutralized by producing a reversing emf in the short circuit coil during commutation period.
The voltage rise in the short circuit coil due to inductive property of the coil, which opposes the current reversal in it during the commutation period, is called the reactance voltage.
We can produce reversing emf in two ways
- By brush shifting.
- By using inter-poles or commutating poles.