Electrical Machines
The polarity of a DC generator can be reversed by

 any of above.
 increasing field current.
 reversing field electric current as well as direction of rotation.
 reversing the field current.

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Electrical Machines
Where the shunt generators are widely used?

 Where voltage variation is needed over a wide load range.
 Where the load is very high.
  Where a constant voltage is required over a narrow load range.
 Where often load fluctuation is occurred.

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