DC Machines Commutation.
Improvement of Commutation.
There are two methods of improving commutation i.e., of making the current in the short circuited coil attain its full value in the reverse direction by the end of the short circuit period. They are known as
- resistance commutation and
- emf commutation.
1. Resistance Commutation.
(i) high contact resistance resulting in good commutation,
(ii) lubricating and polishing the commutator as it rotates and
(iii) in the event of sparking, reducing the damage to the commutator by the brushes.
The carbon brushes,have some drawbacks also as enumerated below:
(i) Due to high contact resistance, a loss of about 2 Vis caused. Hence these are not much suitable for small machines where this voltage loss forms an appreciable The carbon brushes, however, have some
percentage loss.
(ii) Because of the above large loss, the commutator has to be made somewhat larger than with copper brushes so as to the dissipate heat efficiently without appreciable rise in temperature.
(iii) Because of lower current density, large brush holders are needed.
2. EMF Commutation.
In this method of improving commutation an arrangement is made to neutralize the reactance voltage by producing a reversing emf in the short-circuited coil under commutation.
This is accomplished by placing a small nd), pole, called the interpole or the commutating pole, over the short-circuited coil i.e., the location of commutating poles must be midway between the main poles.
Commutating poles neutralize the reactance voltage automatically and make the commutation the spark less and neutralize the cross magnetizing effect of armature reaction.
Commutating poles are so effective in producing good and commutation that the majority of all dc machines built nowadays are provided with them.
In non-commutating pole machines satisfactory and commutation is produced by shifting the brushes from the mechanical neutral so as to lie in the magnetic neutral plane.
Since the amount of brush shift required varies with the load supplied by the machine, so brushes are required to be shifted with each change in load. Hence this method of reducing sparking at the brushes is satisfactory for the machines operating a constant loads only.
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