---
title: "Working principle of dc generator with diagram"
subject: "Electrical Machines"
chapter: 3
key: "semester_4/electrical_machines:dc-generator-principle-diagram"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/dc-generator-principle-diagram
site: BCT Engineering
---

# Working principle of dc generator with diagram

Electrical Machines · Chapter 3: DC Generator

- Asked in: 2071 Magh, 3+3 marks
- 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-diagram
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-3

## Question

Explain the working principle of dc generator with neat diagram.

## Short answer

A DC generator works on Faraday's law: a rotating coil in a magnetic field induces an AC emf. A split-ring commutator and brushes rectify it to DC by reversing connections each half-cycle, giving unidirectional output. The diagram shows a simple loop generator with N-S poles, coil, commutator, and brushes.

## Answer

#### 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

```text
        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.

## Check yourself

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

1. The emf induced in a single rotating coil of a DC generator is
   A. Direct (unidirectional)
   B. Alternating, as a coil side passes under N and then S pole
   C. Zero
   D. Constant at all times
   Answer: B. Each coil side meets opposite polarity poles in turn, so the induced emf alternates. (Key point: Nature of coil emf)

2. How does the split-ring commutator give DC at the brushes?
   A. It reverses the connection of the coil to the brushes each time the emf reverses
   B. It blocks half the current
   C. It filters the ripple
   D. It reverses the field
   Answer: A. Brush A always touches the segment connected to the coil side under the N pole, so it stays positive and brush B stays negative. (Key point: Split-ring commutator)

3. With a single coil, the output across the brushes is
   A. Pure steady DC
   B. Alternating
   C. Unidirectional but pulsating
   D. Zero
   Answer: C. A practical machine uses many coils and commutator segments to make the output nearly steady. (Key point: Output with one coil)

4. The direction of the induced emf is found with
   A. Fleming's left-hand rule
   B. Fleming's right-hand rule
   C. Right-hand screw rule
   D. Ampere's circuital law
   Answer: B. For generators, thumb = motion, forefinger = field, middle finger = induced emf. (Key point: Direction of emf)

## Related in Electrical Machines

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## Related in other subjects

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- [DC generator construction, working, emf](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Adc-generator-construction-emf) — Electrical Machine, chapter 3
- [DC generator working and emf equation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Adc-generator-principle-emf-equation) — Electrical Machine, chapter 3
- [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