Electrical Machines
Satisfactory commutation of DC machine requires

 smooth, concentric commutator properly undercut.
All of these
 brushes should be of proper grade and size.
 brushes should smoothly run in the holders.

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Electrical Machines
Armature reaction in a DC machine

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

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