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

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

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Electrical Machines
In a transformer

 both Oc and SC tests are conducted on HV side
 OC test is conducted on LV side and SC test is conducted on HV side
 both OC and SC tests are conducted on LV side
 OC test is conducted on HV side and SC test is conducted on LV side

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