Electrical Machines
The brush-axis of a bipolar dc motor is rotated by 90°. The effect of this rotation on the back emf Eband the torque developed Tdwould be such that

 both Eb and Td are unchanged
 Eb is unchanged, but Td is zero
 both Eb and Td are zero
 Eb is zero, but Td is unchanged

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Electrical Machines
When the supply terminal of a DC shunt motor are interchanged

 The motor will run much faster in the same direction.
  The motor will run at its normal speed in the same direction as before.
 The motor will stop.
 The direction of rotation will reverse.

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Electrical Machines
Compensating windings are used in DC generators

  to provide path for the circulation of cooling air.
  to neutralize the cross magnetizing effect of the armature reaction.
None of these
  mainly to reduce the eddy currents by providing local short circuit.

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