Working principle of dc generator with diagram

Principle

A DC generator works on Faraday's law of electromagnetic induction. Whenever a conductor moves in a magnetic field so that it cuts flux, an emf is induced:

$$e = Blv\ \text{volts}$$

The direction of the induced emf is given by Fleming's right-hand rule (thumb = motion, forefinger = field, middle finger = emf).

Working with the figure

        N pole            S pole
      +---------+       +---------+
      |    ^    |       |         |
      |  B -->  |       |         |
      |  ______ |_______|______   |
      | / loop (armature coil) \  |
      +-+-----------------------+-+
        | split-ring commutator |
     brush A ----[ load ]---- brush B

1. The stator poles (N and S) set up a field. The armature coil is rotated by a prime mover (turbine, engine or motor).
2. Sides of the coil cut the flux, so an emf is induced in the coil. When a coil side is under the N pole the emf is in one direction; under the S pole it is in the opposite direction, so the coil emf is alternating.
3. The coil ends are joined to a split-ring commutator (two half rings insulated from each other). The brushes press on the commutator.
4. At the instant the coil emf reverses, the commutator segments also swap brushes. Brush A always touches the segment connected to the coil side under the N pole, so brush A stays positive and brush B stays negative.
5. The voltage across the brushes is therefore unidirectional but pulsating. A practical machine uses many coils and many commutator segments, so the output is nearly steady DC.

Key point: the generator itself produces AC in the armature conductors; the commutator rectifies it.