COMMUTATION IN DC MACHINES



  •     In dc machines the induced emf is an alternating.
  •      This alternating emf is converted into unidirectional emf by means of using commutators.
  •     The emf induced in the armature coil is zero when it is at the magnetic neutral axis and reverses as it passes through it.
  •     Therefore brushes riding over the commutator segments are placed at the magnetic neutral axis so as to short circuit the coil in which emf induced is zero and effect reversal of direction of emf.
  •     This reversal of emf and current taking place in the short circuited coil when it crosses the magnetic neutral axis is called commutation.
  •     The time taken by the short circuited coil to move across the brush short circuiting, it is called commutation time or period of commutation.


SPARKING AT THE BRUSHES DURING COMMUTATION


  
  • When armature conductors placed in armature slots carry current and this will acquire self inductance.
  •     Therefore when current reversal takes place in the coil during commutation an emf is induced by the self inductance.
  •     This self induced emf opposes the reversal of current in the conductors.
  •     Therefore under-commutation takes place and produces sparking at the brushes.

Sparking produces the following effects

(i) It increases the wear and tear of the brushes and commutator.

(ii)Damage the commutator surface and this will progressively
     worse and leads to short circuit of entire commutator.

(iii)Heats up the commutator and there by limits the capacity of the
machine.

           To avoid sparking, commutation must be improved.




Methods of improving commutation

1. Using high resistance carbon brushes instead of copper brushes.




2. By fixing interpoles or commutating poles.
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