Electrical Machines
The armature shaft must be able to withstand

 any unbalanced magnetic pull on the armature core.
All of these
 twisting strains due to transmission of torque.
 bending moment due to weight of the armature.

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Electrical Machines
For a given torque, reducing divertor resistance of a DC series motor

  Decrease in speed but armature current remains the same.
  Increase in speed but armature currnet remains the same.
  Decrease in speed demanding less armature current.
  Increase in speed demanding more armature current.

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