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, 4
 2, 3, 4, 5
 1, 2, 3
 2, 3 ,5

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Electrical Machines
In speed control of DC Series motor, speed lower than the rated speed can be obtained by

 Any of the above.
 Connecting an additional resistance in stator circuit.
 Connecting an additional resistance in the armature circuit.
 Connecting an additional resistance in the field circuit.

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