---
title: "Synchronous generator working principle"
subject: "Basic Electrical and Electronics Engineering"
chapter: 2
key: "semester_1/basic_electrical_and_electronics_engineering:synchronous-generator-principle"
source: https://www.bctengineer.com/q/semester_1/basic_electrical_and_electronics_engineering/synchronous-generator-principle
site: BCT Engineering
---

# Synchronous generator working principle

Basic Electrical and Electronics Engineering · Chapter 2: Electrical Machines

- Asked in: 2081 Kartik (new course), 2 marks
- Video: [Synchronous Generator (Working Principle) Electrical machines (1st year engineering) BEE/EEE](https://www.youtube.com/watch?v=TRQ9Mb4U-Xo) — Gautam Varde
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_1/basic_electrical_and_electronics_engineering/synchronous-generator-principle
- Whole chapter: https://www.bctengineer.com/md/semester_1/basic_electrical_and_electronics_engineering/chapter-2

## Question

Explain working principle of a synchronous generator.

## Short answer

A DC excited rotor creates a rotating magnetic field. According to Faraday's law, this flux cutting the stationary three-phase armature winding induces alternating emfs,120° apart. Frequency f = PNs/120, where Ns is synchronous speed.

## Answer

A synchronous generator (alternator) works on Faraday's law of electromagnetic induction: an emf is induced in a conductor when there is relative motion between the conductor and a magnetic field.

**Working:**
1. The rotor carries a field winding supplied with DC through slip rings, which creates alternate N and S poles.
2. A prime mover (turbine or engine) rotates the rotor at constant synchronous speed $N_s$.
3. The rotating field flux cuts the stationary three-phase armature conductors in the stator slots, inducing an alternating emf in each phase.
4. The three phase windings are placed $120^\circ$ apart in space, so the emfs are $120^\circ$ apart in time, giving a three-phase output.
5. The frequency depends on the speed and the number of poles:

$$
f=\frac{PN_s}{120}
$$

For 50 Hz and 4 poles, the speed is 1500 rpm. The rms emf per phase is $E=4.44f\Phi T_{ph}K_w$. Because the rotor speed is locked to the frequency, the machine is called synchronous.

## Check yourself

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

1. On which law does a synchronous generator work?
   A. Faraday's law of electromagnetic induction
   B. Ohm's law
   C. Coulomb's law
   D. Gauss's law
   Answer: A. Relative motion between the field and the armature conductors induces an emf. (Key point: Governing law)

2. Why is the output three-phase?
   A. The rotor has three poles
   B. The prime mover has three cylinders
   C. Three phase windings are placed 120° apart in space, so emfs are 120° apart in time
   D. Three slip rings are used for the load
   Answer: C. Windings displaced by 120° electrical give emfs displaced by 120° in time. (Key point: Three-phase output)

3. What speed does a 4-pole alternator need to give 50 Hz?
   A. 3000 rpm
   B. 1500 rpm
   C. 750 rpm
   D. 1000 rpm
   Answer: B. f = P Ns/120 gives Ns = 120 x 50/4 = 1500 rpm. (Key point: Frequency)

## Related in Basic Electrical and Electronics Engineering

- [Synchronous generator constructional diagram](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_and_electronics_engineering%3Asynchronous-generator-diagram) — Basic Electrical and Electronics Engineering, chapter 2

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