Electrical Machines
Open circuit in a transformer is prefered with

 rated transformer voltage
 direct current
 high frequency supply
 rated transformer current

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Electrical Machines
Armature reaction in a DC machine

 is in direct opposition to the main poles.
 makes an angle with the main pole axis which is load dependent.
 is in the same direction as the main poles.
  makes an angle of 90° with the main pole axis.

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