---
title: "Speed of 4 and 6 pole alternators for 50 Hz"
subject: "Electrical Machines"
chapter: 6
key: "semester_4/electrical_machines:alternator-rpm-4-6-pole-50hz"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/alternator-rpm-4-6-pole-50hz
site: BCT Engineering
---

# Speed of 4 and 6 pole alternators for 50 Hz

Electrical Machines · Chapter 6: Three Phase Synchronous Machines

- Asked in: 2078 Baisakh, 4 marks
- Video: [How RPM and number of poles affect frequency](https://www.youtube.com/watch?v=7kfcpXhO19Q) — The Electric Academy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines/alternator-rpm-4-6-pole-50hz
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-6

## Question

What should be the rpm of a 4 pole and 6 pole alternators to produce a frequency of 50 Hz.

## Short answer

Synchronous speed N_s = 120f/P. For 4-pole, N_s = (120×50)/4 = 1500 rpm. For 6-pole, N_s = (120×50)/6 = 1000 rpm.

## Answer

The frequency of an alternator is related to speed and poles by
$$
f = \frac{PN_s}{120} \quad\Rightarrow\quad N_s = \frac{120f}{P}
$$
For $f = 50$ Hz:

- **4-pole:** $N_s = \frac{120\times50}{4} = 1500$ rpm
- **6-pole:** $N_s = \frac{120\times50}{6} = 1000$ rpm

**Answer:** 1500 rpm for the 4-pole alternator and 1000 rpm for the 6-pole alternator.

## Check yourself

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

1. Which formula gives the speed for a given frequency?
   A. N = 120f/P
   B. N = 60P/f
   C. N = fP/120
   D. N = 120P/f
   Answer: A. f = PN/120. (Key point: Formula)

2. Speed for 4 poles at 50 Hz?
   A. 1000 rpm
   B. 1500 rpm
   C. 3000 rpm
   D. 750 rpm
   Answer: B. 120 x 50/4 = 1500 rpm. (Key point: 4-pole)

3. Speed for 6 poles at 50 Hz?
   A. 1500 rpm
   B. 750 rpm
   C. 1000 rpm
   D. 500 rpm
   Answer: C. 120 x 50/6 = 1000 rpm. (Key point: 6-pole)

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