What is Commutation?
In an armature conductor of a hetero-polar machine a.c. voltages
are induced as the conductor moves under north and south pole polarities alternately. The
frequency of this induced emf is given by the product of the pole pairs and the speed in
revolutions per second. The induced emf in a full-pitch coil changes sign as the coil crosses the magnetic-neutral axis. In order to get maximum d.c. The coil voltage in the external circuit should be shifted to the negative group. This process of switching is called commutation.
During a short interval when the two adjacent commutator segments get bridged by the
brush, the coils connected in series between these two segments get short-circuited. Thus in
the case of ring winding and simple lap winding 2p coils get short-circuited. In a simple wave
winding in a 2p pole dc machine, 2 coils get short-circuited. The current in these coils becomes
zero and gets reversed as the brush moves over to the next commutator segment. Thus brush
and commutator play a vital role in commutation. Commutation is the key process that converts the induced a.c. voltages in the conductors into d.c.
Sparking at Contacting Brushes:
If the reversal of Current happens in commutation then it is called ideal or linear commutation. Still, if the reversal of current does not happen in commutation time then it is called delayed or nonlinear commutation and due to this sparking is produced at the contacting brushes which damages the brush surface.
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| Coil Undergoing Commutation |