Electrical Machines
An inverted V-curve of synchronous motor shows the variation of

 supply voltage and field current at constant excitation
All of these
 power factor and dc excitation at constant load
 power factor and supply voltage during hunting

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

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

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