Electrical Machines
Satisfactory commutation of DC machine requires

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

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Electrical Machines
In a DC generator, the armature reaction results in

 magnetization of leading and demagnetization of trailing pole tip.
  magnetization of inter poles.
  demagnetization of leading & magnetization of trailing pole tip.
 demagnetization of centers of poles.

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