---
title: "DC generator construction, working, emf equation"
subject: "Electrical Machines"
chapter: 3
key: "semester_4/electrical_machines:dc-generator-construction-emf"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/dc-generator-construction-emf
site: BCT Engineering
---

# DC generator construction, working, emf equation

Electrical Machines · Chapter 3: DC Generator

- Asked in: 2071 Bhadra, 8 marks
- Video: [EMF equation of dc generator with animation | Derive EMF equation of dc generator | Mruduraj](https://www.youtube.com/watch?v=m7lxUqVyAyg) — 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/dc-generator-construction-emf
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-3

## Question

Describe the construction and working principle of a dc generator with neat diagram. Also derive the emf equation of a dc generator.

## Short answer

Main parts: yoke, pole cores with field windings, laminated armature core with slots, armature winding, commutator, and brushes. Working: armature rotated in magnetic field induces AC emf, commutator rectifies to DC. Emf equation: E_g = (Φ Z N P)/(60 A) V.

## Answer

A DC generator converts mechanical energy into DC electrical energy by electromagnetic induction.

#### Construction

```text
   +----------- Yoke (frame) ------------+
   |  [Pole core + shoe]    [Pole core]  |
   |   field winding         field wdg   |
   |        ( Armature core + winding )  |
   |        Commutator -- Brushes        |
   +---- Shaft, bearings, end covers ----+
```

1. **Yoke (frame):** cast iron or cast steel outer frame. It gives mechanical support and carries the return path of the magnetic flux.
2. **Pole core and pole shoe:** laminated steel bolted to the yoke. The shoe spreads the flux evenly over the air gap and holds the field coils.
3. **Field winding:** copper coils on the pole cores that produce the main flux when excited.
4. **Armature core:** cylindrical, made of thin laminated silicon steel (to reduce eddy-current loss) with slots on the outside to hold conductors.
5. **Armature winding:** insulated copper conductors in the slots, connected as lap or wave winding.
6. **Commutator:** cylinder of hard-drawn copper segments insulated by mica. It rectifies the armature emf and collects the current.
7. **Brushes and brush gear:** carbon brushes in holders press on the commutator and carry current to the external circuit.
8. **Shaft, bearings and end covers:** support and rotate the armature.

#### Working principle

When the armature is rotated by a prime mover in the field of the poles, its conductors cut flux and an emf $e = Blv$ is induced (Faraday's law). The emf in each coil is alternating; the commutator and brushes convert it to a unidirectional output at the terminals.

#### EMF equation

Let
- $\Phi$ = flux per pole (Wb), $P$ = number of poles
- $Z$ = total armature conductors, $A$ = number of parallel paths
- $N$ = speed in rpm

In one revolution a conductor cuts the flux of all poles, $P\Phi$ weber, in time $60/N$ second.

$$
\text{average emf per conductor} = \frac{d\phi}{dt} = \frac{P\Phi}{60/N} = \frac{P\Phi N}{60}\ \text{V}
$$

The $Z$ conductors form $A$ parallel paths, so each path has $Z/A$ conductors in series. The generated emf equals the emf of one path:

$$
E_g = \frac{P\Phi N}{60}\times\frac{Z}{A} = \frac{\Phi Z N P}{60A}\ \text{V}
$$

- Lap winding: $A = P$, so $E_g = \dfrac{\Phi Z N}{60}$
- Wave winding: $A = 2$, so $E_g = \dfrac{\Phi Z N P}{120}$

In terms of angular speed, $E_g = \dfrac{ZP}{2\pi A}\Phi\omega = K_a\Phi\omega$.

## Check yourself

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

1. The yoke of a DC machine is made of cast iron or steel because it
   A. Carries the armature current
   B. Gives mechanical support and carries the return path of the flux
   C. Rectifies the emf
   D. Reduces eddy-currents
   Answer: B. The yoke is the outer frame that supports the poles and completes the magnetic circuit. (Key point: Yoke function)

2. Why is the armature core made of thin laminated silicon steel?
   A. To reduce eddy-current loss
   B. To increase brush contact
   C. To increase speed
   D. To insulate the commutator
   Answer: A. Laminations break up eddy-current paths in the rotating core, reducing the eddy-current loss. (Key point: Armature core lamination)

3. The commutator consists of
   A. Carbon segments separated by air
   B. Hard-drawn copper segments insulated from each other by mica
   C. Laminated steel rings
   D. Aluminium bars welded together
   Answer: B. Copper segments carry current and mica insulates them; the commutator rectifies the armature emf. (Key point: Commutator construction)

4. Eg = Phi Z N P/(60A). If the flux per pole doubles with other quantities fixed, Eg
   A. Stays the same
   B. Doubles
   C. Halves
   D. Becomes four times
   Answer: B. Eg is directly proportional to flux per pole, so doubling Phi doubles Eg. (Key point: EMF equation result)

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