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

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

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Electrical Machines
Short circuit test is performed on a transformer with a certain impressed voltage at rated frequency. If the short circuit test is now performed with the same magnitude of impressed voltage, but at frequency higher than the rated frequency then

 The magnitude will both increase.
 The magnitude of circuit as well as the power factor will decrease.
 The magnetude of current will increase, but power factor will decrease.
 The magnitude of current will decrease but the power factor will increase.

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