Electrical Machines
A commutator in DC machines can 1. provide half-wave rectification 2. Provide full-wave rectification 3. convert ac to dc 4. Convert dc to ac 5. provide controlled full-wave rectification From these, the correct answer is

 2, 3 ,5
 1, 2, 3
 2, 3, 4, 5
 2, 3, 4

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Electrical Machines
Armature reaction in a DC machine

 makes an angle with the main pole axis which is load dependent.
 is in direct opposition to the main poles.
  makes an angle of 90° with the main pole axis.
 is in the same direction as the main poles.

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