---
title: "Construction and operation of synchronous generator"
subject: "Electric Machines II"
chapter: 1
key: "semester_5/electric_machines_ii:synchronous-generator-construction-principle"
source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/synchronous-generator-construction-principle
site: BCT Engineering
---

# Construction and operation of synchronous generator

Electric Machines II · Chapter 1: Three Phase Synchronous Generator

- Asked in: 2076 Asoj, 7 marks
- Video: [alternator working principle | synchronous generator working principle | synchronous generator](https://www.youtube.com/watch?v=AhJh8nEGovI) — Mind of Engineer 
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/synchronous-generator-construction-principle
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-1

## Question

Discuss the construction and principle of operation of a three phase synchronous generator.

## Short answer

A synchronous generator has a stationary three-phase armature winding in a laminated core and a rotating field excited with DC. The prime mover drives the rotor at synchronous speed, and the rotating field cuts armature conductors, inducing alternating emf (e=Blv) at frequency f=PNs/120. Salient-pole rotors suit low speeds, cylindrical rotors suit high speeds.

## Answer

A **three-phase synchronous generator (alternator)** converts mechanical energy into three-phase AC electrical energy at a frequency fixed by its speed, $f = \dfrac{PN_s}{120}$.

#### Construction
```text
           stator frame
     +------------------------+
     |  laminated stator core |
     |  with 3-ph armature    |
     |  winding in slots      |
     |      +----------+      |
     |      |  rotor   |      |  DC field via
     |      |  N    S  |------+--- slip rings
     |      +----------+      |  and brushes
     +------------------------+
          shaft <- prime mover
```

**Stator (armature)** – stationary part
- Frame (cast iron/welded steel) for support and ventilation.
- Core of 0.35–0.5 mm silicon-steel laminations with slots, to reduce iron losses.
- Three-phase, double-layer, distributed, short-pitched winding, displaced 120° electrical, usually star connected.

**Rotor (field)** – rotating part, excited by DC from an exciter through slip rings (or brushless exciter):
1. **Salient-pole rotor**: projecting poles on a spider, large diameter, short length, many poles; used for low-speed hydro turbines. Damper bars in pole faces.
2. **Cylindrical (non-salient) rotor**: smooth forged-steel cylinder, small diameter, long axial length, 2 or 4 poles; used for high-speed steam/gas turbines.

**Why stationary armature, rotating field?** High voltage (11 kV and above) armature is easier to insulate when stationary; output is taken directly without slip rings; only low-voltage DC (100–400 V) is fed through slip rings; the rotor is lighter and better for high speed; the stator can be cooled better.

#### Principle of operation
1. The field winding is excited with DC, producing alternate N and S poles on the rotor.
2. The prime mover drives the rotor at synchronous speed $N_s$.
3. The rotating magnetic field cuts the stationary armature conductors. By **Faraday's law** of electromagnetic induction, an emf is induced in them: $e = Blv$.
4. As N and S poles pass a conductor alternately, the emf alternates; one cycle is produced per pair of poles passing. Hence $f = PN_s/120$.
5. The three phase windings are 120° electrical apart in space, so three emfs equal in magnitude and 120° apart in time are produced.
6. The rms emf per phase is $E = 4.44\,k_p k_d f\Phi T$.
7. When a load is connected, current flows; the armature mmf (armature reaction) and the impedance drop change the terminal voltage, which is corrected by adjusting the field current.

## Check yourself

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

1. The frequency of generated emf is
   A. f = 120 Ns/P
   B. f = P Ns/120
   C. f = P/(120 Ns)
   D. f = Ns/P
   Answer: B. One cycle per pair of poles passing. (Key point: Frequency)

2. Why is the armature stationary and the field rotating?
   A. Because DC cannot be rotated
   B. To raise the frequency
   C. Easy to insulate high-voltage winding and avoids high-power slip rings
   D. Because the armature is lighter
   Answer: C. Only low-power DC passes through slip rings. (Key point: Stationary armature)

3. The emf is induced by
   A. self-induction of the field
   B. the load current
   C. mutual attraction of poles
   D. Faraday's law, field cutting stationary armature conductors e = Blv
   Answer: D. Rotating flux cuts armature conductors. (Key point: Principle)

4. Three-phase emfs result because
   A. phase windings are 120° electrical apart in space
   B. the rotor has three poles
   C. the field is AC
   D. the speed varies
   Answer: A. Space displacement gives time displacement. (Key point: Three phases)

## Related in Electric Machines II

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## Related in other subjects

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- [Hunting; loaded operation of synchronous motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-motor-hunting-loaded-operation) — Electrical Machines, chapter 6
- [Synchronous generator working; armature reaction](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-gen-working-armature-reaction) — Electrical Machines, chapter 6