---
title: "Function of commutator in DC generator"
subject: "Electrical Machines I"
chapter: 3
key: "semester_4/electrical_machines_i:commutator-function-dc-generator"
source: https://www.bctengineer.com/q/semester_4/electrical_machines_i/commutator-function-dc-generator
site: BCT Engineering
---

# Function of commutator in DC generator

Electrical Machines I · Chapter 3: DC Generator

- Asked in: 2070 Bhadra, 4 marks
- Video: [Commutator in dc machine | Need of commutator in dc generator | Role of commutator | Mruduraj](https://www.youtube.com/watch?v=RFCXKKePS8Y) — Learning Vibes
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines_i/commutator-function-dc-generator
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines_i/chapter-3

## Question

Explain the functions of commutator segments in dc generator.

## Short answer

The commutator converts alternating coil emf to unidirectional output by reversing connections at the instant of emf reversal. It also collects current via brushes, connects coils in series for additive emf, and with many segments reduces ripple.

## Answer

The **commutator** is a cylinder of hard-drawn copper segments, insulated from one another by mica, fixed on the armature shaft. Armature coil ends are soldered to the segments and stationary carbon brushes rest on it.

The emf induced in an armature conductor is **alternating**, since each conductor moves alternately under N and S poles. The commutator converts this into DC at the terminals.

#### Functions
1. **Mechanical rectifier:** at the instant the emf in a coil reverses, the coil's connection to the brushes is also reversed, so the brushes always see emf of one polarity. The output current is unidirectional.
2. **Current collection:** together with the brushes, it provides a sliding contact between the rotating armature and the stationary external circuit.
3. **Series addition of coil emfs:** segments join the coils so that their emfs add between brushes in each parallel path (lap or wave).
4. **Smooth output:** with many coils and segments, the rectified emf has very little ripple.

```text
 Coil emf (AC)        Brush output (DC)
   /\                   /\    /\
  /  \                 /  \  /  \
 /    \    /          /    \/    \
       \  /
        \/
```

## Check yourself

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

1. How does the commutator produce DC?
   A. It switches the coil connection to the brushes as the coil emf reverses
   B. It filters the AC with inductance
   C. It uses diodes
   D. It increases speed
   Answer: A. Reversing coil connection at the instant of emf reversal keeps the brush polarity fixed. (Key point: Mechanical rectifier)

2. How are segments built?
   A. Soft iron segments joined by solder
   B. Hard-drawn copper segments insulated from each other by mica
   C. Steel rings with rubber insulation
   D. Carbon segments
   Answer: B. Copper segments are separated by mica and mounted on the shaft. (Key point: Construction)

3. What gives smooth output with little ripple?
   A. Few coils and segments
   B. A larger air gap
   C. Many coils and segments
   D. Higher brush pressure
   Answer: C. With many segments the rectified emf is nearly constant. (Key point: Smooth output)

## Related in Electrical Machines I

- [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
- [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
- [EMF equation of DC generator](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Adc-generator-emf-equation) — Electrical Machines I, chapter 3

## Related in other subjects

- [Minimum number of commutator segments](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Adc-minimum-commutator-segments) — Electric Machine Design, chapter 5, asked 2×
- [Commutator, brushes; start shunt generator no load](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Acommutator-brush-function-no-load-start) — Electrical Machines, 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
- [DC generator principle; function of carbon brush](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-generator-principle-brush-function) — Electrical Machines, chapter 3