Electrical Machines
The polarity of a DC generator can be reversed by

 increasing field current.
 reversing field electric current as well as direction of rotation.
 any of above.
 reversing the field current.

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Electrical Machines
Flashing the field of DC generator means

  making the magnetic losses forces parallel.
 increasing flux density by adding extra turns of winding.
  neutralising residual magnetism.
  creating residual magnetism by a DC source.

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