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

# DC generator principle; function of carbon brush

Electrical Machines · 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_machines/dc-generator-principle-brush-function
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/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 induces AC emf in rotating armature conductors; commutator and brushes rectify to DC. Carbon brushes provide sliding contact to collect current, short-circuit coils during commutation for current reversal, and are self-lubricating to reduce commutator wear and sparking.

## Answer

A DC generator converts mechanical energy into electrical energy using **Faraday's law of electromagnetic induction**: when a conductor cuts magnetic flux, an emf is induced in it, $e = Blv$ volts (direction by Fleming's right-hand rule).

**Working:**
1. The field poles produce a flux $\Phi$ across the air gap.
2. The prime mover rotates the armature, so the armature conductors cut this flux and an emf is induced in each conductor.
3. In a single coil the induced emf is alternating, because each coil side passes under a north pole and then a south pole.
4. The **commutator** and **brushes** act as a mechanical rectifier. Each time the coil emf reverses, the segments reverse their connection to the brushes, so the brush terminals give a **unidirectional** (DC) voltage.
5. With a load connected, current flows out through the positive brush.

#### Functions of carbon brushes

1. They provide a **sliding contact** with the rotating commutator and **collect the current** from the armature to the external circuit (or supply it, in a motor).
2. They connect the armature coils to the load at the right moment, so that coils undergoing commutation are short-circuited by the brush and the current in them reverses.
3. Carbon is used because it is **self-lubricating**, has a high contact resistance (which helps commutation and reduces sparking), and is softer than copper, so the brush wears away instead of the commutator.

The brushes are set on the **magnetic neutral axis**, where the emf in the short-circuited coil is nearly zero, so sparking is minimum.

## Check yourself

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

1. The main function of carbon brushes is to
   A. Produce the field flux
   B. Make sliding contact with the commutator and carry current between armature and external circuit
   C. Rectify the emf by themselves
   D. Cool the armature
   Answer: B. Brushes press on the rotating commutator to collect the armature current and deliver it to the load. (Key point: Brush main function)

2. Carbon is used for brushes because it
   A. Is harder than copper
   B. Is self-lubricating, has high contact resistance and wears instead of the commutator
   C. Has zero resistance
   D. Is magnetic
   Answer: B. Being softer than copper it protects the commutator, and its contact resistance helps commutation and reduces sparking. (Key point: Why carbon)

3. Brushes are placed on the magnetic neutral axis because
   A. The flux is maximum there
   B. The emf in the short-circuited coil is nearly zero, so sparking is minimum
   C. The current is maximum there
   D. It increases the output voltage
   Answer: B. At the neutral axis the coil cutting no flux is shorted by the brush with almost no emf, giving sparkless commutation. (Key point: Brush position)

## Related in Electrical Machines

- [DC generator principle and emf equation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-generator-principle-emf) — Electrical Machines, chapter 3, asked 4×
- [Working principle of dc generator with diagram](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-generator-principle-diagram) — Electrical Machines, chapter 3
- [Function of commutator in DC generator](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Acommutator-function-dc-generator) — Electrical Machines I, chapter 3
- [Commutator function and DC generator emf equation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Acommutator-function-emf-equation) — Electrical Machines I, chapter 3
- [EMF of 25 kW short shunt generator with brush drop](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashort-shunt-25kw-250v-brush-drop) — Electrical Machines I, chapter 3
- [DC motor principle and role of commutator and brushes](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Adc-motor-principle-commutator-brushes) — Electrical Machines I, chapter 4

## Related in other subjects

- [DC generator principle; brush functions](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Adc-generator-principle-brush-function) — Electrical Machine, chapter 3
- [Commutator and carbon brush functions](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Acommutator-brush-functions) — Electrical Machine, chapter 3
- [Working principle of DC generator](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Adc-generator-working-neat-diagram) — Electrical Machine, chapter 3