Electrical Machines
The heat transfer by convection in a transformer least depends on

 air velocity.
 surface area.
 temperature difference.
  color of the paint.

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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.
 All of above.
 The accuracy in speed control is required.
 The speed in both the direction is required.

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