Electrical Machines
For satisfactory parallel operation of two transformers which of the following conditions is most essential,

 voltage ratio of all transformers must be same.
  polarities of all transformers must be same.
 percentage impedance of all transformers must be same.
 MVA rating of all transformer inversely proportional to impedance.

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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 speed in both the direction is required.
 The accuracy in speed control is required.

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