Electrical Machines
The ventilating ducts in most of DC machine are

 none of these.
 radial.
 radial as well as longitudinal.
 longitudinal.

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Electrical Machines
Commutation in a DC machine may be explained as a process

 how alternate flux affects the main field flux.
 by virtue of which emf is induced in the armature conductors.
  by which electric current in a coil is reversed.
  in which magnetic field is set up.

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