Electrical Machines
Satisfactory commutation of DC machine requires

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

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Electrical Machines
The following statements relate to compensating winding (CW) and interpole winding (IW) of a dc machine : 1. CW neutralizes the armature reaction under a pole pitch 2. IW is connected in series with armature circuit 3. IW produces mmf in the commutating zone only 4. CWmmf is directed along the brush axis 5. Mmf produced by IW is equal to armature mmf From these, the correct statements are

 1, 2, 3, 4 and 5
 1, 3, 4, 5
 2, 3, 4
 2, 3, 4, 5

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