Electrical Machines
To avoid formation of grooves in the commutator

  brush position has no effect on the commutator grooving.
None of these
  the brushes of opposite polarity should track each other.
 the brushes of same polarity should track each other.

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Electrical Machines
Ward-Leonard system for controlling the speed of a DC motor is used where

 Very fine speed control over the whole range from zero to normal speed is required.
 The accuracy in speed control is required.
 The speed in both the direction is required.
 All of above.

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