Electrical Machines
The air gap between the yoke and armature is dc machine is kept very small

All of these
 to avoid locking of the armature.
 to achieve a stronger magnetic field.
 to avoid over heating.

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Electrical Machines
Flashing of DC generator means

  increasing flux density by providing extra ampere-turns in field.
None of these
  creation of residual magnetism by a DC source.
 neutralization of residual magnetism.

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