Electrical Machines
The residual magnetism of a self-excited dc generator is lost. To build up its emf again

 the field winding must be replaced
 field winding must be excited by low voltage dc supply
 the field winding connection must be reversed
 the armature connection must be reversed

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Electrical Machines
The armature mmf of a DC machine has

 stepped distribution and rotates at the speed of armature.
  trapezoidal space distribution and is stationary in space.
  triangular space distribution and rotates at the speed of armature.
  triangular space distribution and is stationary in space.

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