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

 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.
 how alternate flux affects the main field flux.

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Electrical Machines
In synchronous generator, inverted V curve is drawn between

 field current on X-axis and power factor on Y-axis
 field current on Y-axis and power factor on X-axis
 field current on X-axis and armature current on Y-axis
 field current on Y-axis and armature current on X-axis

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