---
title: "DC generator principle; brush functions"
subject: "Electrical Machine"
chapter: 3
key: "semester_4/electrical_machine:dc-generator-principle-brush-function"
source: https://www.bctengineer.com/q/semester_4/electrical_machine/dc-generator-principle-brush-function
site: BCT Engineering
---

# DC generator principle; brush functions

Electrical Machine · Chapter 3: DC Generator

- Asked in: 2072 Asoj
- Video: [Working Principle of DC Generator | [Electric Machine #1]](https://www.youtube.com/watch?v=mq2zjmS8UMI) — chrvoje_engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machine/dc-generator-principle-brush-function
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-3

## Question

Explain the operation principle of dc generator. What are main functions of carbon brush in dc generator?

## Short answer

A DC generator operates on Faraday's law: armature conductors rotated in a magnetic field cut flux and induce an alternating emf. The commutator rectifies this to DC output. Carbon brushes transfer current from the rotating commutator to the external circuit, are self-lubricating for low friction, and their high contact resistance aids commutation by reducing circulating current.

## Answer

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.

```text
 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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. On which law does a DC generator work?
   A. Faraday's law of electromagnetic induction
   B. Ampere's circuital law
   C. Ohm's law
   D. Coulomb's law
   Answer: A. An emf is induced in a conductor that cuts magnetic flux, which is Faraday's law. (Key point: Principle)

2. Which rule gives the direction of the emf induced in the armature conductor of a generator?
   A. Fleming's right-hand rule
   B. Fleming's left-hand rule
   C. Right-hand screw rule
   D. Lenz's rule applied to force only
   Answer: A. For a generator, thumb = motion, forefinger = field, middle finger = emf/current: Fleming's right-hand rule. (Key point: Direction of emf)

3. Why is the emf induced in the armature coil alternating?
   A. The prime mover speed alternates
   B. The field current alternates
   C. The conductor passes alternately under N and S poles, so the emf direction reverses
   D. The brushes reverse the field
   Answer: C. Under a N pole the emf is one way and under a S pole the opposite way. (Key point: Why emf is alternating)

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