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

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

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Electrical Machines
In a synchronous generator with constant steam input supplies power to an infinite bus at a lagging power factor. If the excitation is increased

 both power angle and power factor decrease
 both power angle and power factor increase
 power angle decrease and power factor increase
 power angle increase and power factor decrease

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