DC generator principle; brush functions

A DC generator works on Faraday's law of electromagnetic induction: when a conductor moves in a magnetic field so that it cuts the flux, an emf is induced in it. It converts mechanical energy into electrical energy.

Operating principle

1. A prime mover rotates the armature conductors inside the magnetic field produced by the field poles.
2. The conductors cut the flux, so an emf e = B l v is induced in each. Its direction is given by Fleming's right-hand rule (thumb: motion, forefinger: field, middle finger: emf/current).
3. Under a N pole the emf is in one direction and under a S pole in the opposite direction, so the induced emf in the coil is alternating.
4. The commutator and brushes change this alternating emf to a unidirectional (DC) emf at the output terminals.
5. When the load is connected, current flows through it, and the electrical power obtained is the mechanical power input less losses.

 Prime mover --> shaft --> ARMATURE in field of N-S poles
                              |  emf = B l v  (Faraday)
                          commutator + brushes
                              |  DC
                            LOAD

The magnitude of the emf is E = PΦZN / (60A) volts, where P = number of poles, Φ = flux per pole, Z = total conductors, N = speed in rpm and A = number of parallel paths.

Main functions of the carbon brushes

• They collect the current from the rotating commutator and deliver it to the external circuit.
• Carbon is self-lubricating, so friction and commutator wear are low.
• Its higher contact resistance improves commutation by limiting the circulating current in a coil short-circuited by the brush.
• Being softer than copper, the brush wears first, and it is cheap to replace.
• The brushes are set on the magnetic neutral axis so that sparking is minimum.