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

 supply voltage and field current at constant excitation.
 supply voltage and excitation current at constant load.
 power factor and supply voltage during hunting.
  power factor and DC excitation at constant load.

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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.
  by which electric current in a coil is reversed.
 how alternate flux affects the main field flux.
  in which magnetic field is set up.

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