---
title: "Salient pole vs cylindrical rotor alternators"
subject: "Electrical Machines"
chapter: 6
key: "semester_4/electrical_machines:salient-vs-cylindrical-alternator-justify"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/salient-vs-cylindrical-alternator-justify
site: BCT Engineering
---

# Salient pole vs cylindrical rotor alternators

Electrical Machines · Chapter 6: Three Phase Synchronous Machines

- Asked in: 2067 Mangsir (old course), 4 marks
- Video: [Difference between Salient Pole and Cylindrical Pole Rotor](https://www.youtube.com/watch?v=OF2chMtJ36Q) — Come4Concepts
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines/salient-vs-cylindrical-alternator-justify
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-6

## Question

Justify the statement: Salient pole alternators are suitable for low speed whereas cylindrical pole alternators for high speed.

## Short answer

Frequency f = PNs/120 requires many poles for low speed and few poles for high speed. Salient-pole rotors with projecting poles suit large-diameter low-speed hydro turbines. Cylindrical rotors have small diameter and distributed windings to withstand high centrifugal forces at 1500/3000 rpm, ideal for steam turbines.

## Answer

The frequency of an alternator is $f = \frac{PN_s}{120}$. For $f = 50$ Hz the speed is $N_s = \frac{6000}{P}$. A low speed therefore needs many poles, and a high speed needs few poles.

#### Salient-pole alternator (low speed)
- Used with hydraulic turbines (100 to 400 rpm), engines, and wind turbines, which need many poles (e.g. $P = 24$ at 250 rpm).
- Many poles must fit on the rotor, so it has a **large diameter and short axial length**, with projecting poles.
- At low speed, the centrifugal force and windage loss are small, so the projecting poles are mechanically safe.

#### Cylindrical (non-salient) alternator (high speed)
- Used with steam or gas turbines at 1500 or 3000 rpm, with 2 or 4 poles.
- At high speed, the large-diameter projecting poles would suffer large centrifugal force and high windage loss and noise.
- A **smooth cylindrical rotor of small diameter and long axial length** (slotted forged steel with distributed winding) withstands the centrifugal stresses and gives low windage loss and a better sinusoidal flux distribution.

So the number of poles needed and the mechanical limits decide the type: salient poles for low-speed, many-pole machines, and cylindrical rotors for high-speed, 2- or 4-pole turbo-alternators.

## Check yourself

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

1. For a 50 Hz alternator, how are speed and number of poles related?
   A. Ns = 6000/P, so low speed needs many poles
   B. Ns = 6000/P, so low speed needs few poles
   C. Ns = 120P/f, so high speed needs many poles
   D. Speed is independent of the poles
   Answer: A. From f = P Ns/120, Ns = 6000/P at 50 Hz; low speed therefore means many poles. (Key point: Pole count and speed)

2. Why does a low-speed salient-pole alternator have a large diameter and short axial length?
   A. To reduce windage at high speed
   B. To make the rotor smooth
   C. To reduce slip-ring losses
   D. To accommodate many poles on the rotor
   Answer: D. A large number of projecting poles needs a large circumference; at low speed centrifugal force is small so this is safe. (Key point: Salient rotor shape)

3. Why is a smooth cylindrical rotor used at 1500 or 3000 rpm?
   A. Projecting poles of large diameter would suffer large centrifugal force and windage; a small-diameter long slotted rotor withstands the stress
   B. It has more poles
   C. It needs lower mechanical strength
   D. It produces a trapezoidal flux
   Answer: A. A forged-steel slotted rotor of small diameter and long length resists centrifugal stress and has low windage loss. (Key point: Cylindrical rotor at high speed)

## Related in Electrical Machines

- [Speed of 4 and 6 pole alternators for 50 Hz](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aalternator-rpm-4-6-pole-50hz) — Electrical Machines, chapter 6
- [4-pole motor: asynchronous speed, slip, rotor frequency](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aim-208v-60hz-1755rpm) — Electrical Machines, chapter 5
- [Two reaction model of salient pole machine](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Atwo-reaction-model-salient-pole) — Electrical Machines, chapter 6
- [Power of salient pole machine, power angle curve](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Asalient-pole-power-angle-characteristics) — Electrical Machines, chapter 6
- [8-pole motor: speed at max torque (50 Kg-m)](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aim-8pole-50kgm-starting-torque) — Electrical Machines, chapter 5, asked 4×
- [8-pole motor: speed at max torque (50 N-m, j4)](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aim-8pole-50nm-starting-torque-j4) — Electrical Machines, chapter 5, asked 4×

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