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
 2, 3, 4
 1, 2, 3
 2, 3, 4, 5

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Electrical Machines
The armature mmf of a DC machine has

 stepped distribution and rotates at the speed of armature.
  triangular space distribution and is stationary in space.
  trapezoidal space distribution and is stationary in space.
  triangular space distribution and rotates at the speed of armature.

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