Electrical Machines
To avoid formation of grooves in the commutator

  the brushes of opposite polarity should track each other.
  brush position has no effect on the commutator grooving.
None of these
 the brushes of same polarity should track each other.

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Electrical Machines
Armature reaction in a DC machine

  makes an angle of 90° with the main pole axis.
 is in direct opposition to the main poles.
 makes an angle with the main pole axis which is load dependent.
 is in the same direction as the main poles.

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