---
title: "4-pole motor: asynchronous speed, slip, rotor frequency"
subject: "Electrical Machines"
chapter: 5
key: "semester_4/electrical_machines:im-208v-60hz-1755rpm"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/im-208v-60hz-1755rpm
site: BCT Engineering
---

# 4-pole motor: asynchronous speed, slip, rotor frequency

Electrical Machines · Chapter 5: Three Phase Induction Machines

- Asked in: 2068 Magh, 8 marks
- Video: [Induction Motor #4 - Electrical frequency at the Rotor](https://www.youtube.com/watch?v=6XuNQKTD53M) — ELECTRICAL IS EASY
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines/im-208v-60hz-1755rpm
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-5

## Question

A 208 V, 60 Hz, 4 pole, 3-phase induction motor has a full-speed of 1755 rpm. Calculate: (i) asynchronous speed, (ii) the slip and (iii) rotor frequency.

## Short answer

Synchronous speed = 120×60/4 = 1800 r pm. Slip = (1800-1755)/1800 = 0.025 (2.5 %). Rot or fre quency = 0.025×60 = 1.5 Hz.

## Answer

**Given:** 208 V, 60 Hz, 4-pole, full-load speed 1755 rpm.

#### (i) Asynchronous (synchronous) speed
$$
N_s = \frac{120 f}{P} = \frac{120\times 60}{4} = 1800\ \text{rpm}
$$
#### (ii) Slip
$$
s = \frac{N_s - N_r}{N_s} = \frac{1800-1755}{1800} = 0.025 = 2.5\%
$$
#### (iii) Rotor frequency
$$
f_r = sf = 0.025\times 60 = 1.5\ \text{Hz}
$$

**Answer:** (i) 1800 rpm; (ii) 0.025 (2.5 %); (iii) 1.5 Hz.

## Check yourself

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

1. What is Ns for a 4-pole, 60 Hz machine?
   A. 1800 rpm
   B. 1500 rpm
   C. 3600 rpm
   D. 1755 rpm
   Answer: A. Ns = 120 x 60/4 = 1800 rpm. (Key point: Synchronous speed)

2. What is the slip at 1755 rpm?
   A. 2.5 %
   B. 1.5 %
   C. 4.0 %
   D. 45 %
   Answer: A. s = (1800-1755)/1800 = 0.025. (Key point: Slip)

3. What is the rotor frequency?
   A. 60 Hz
   B. 1.5 Hz
   C. 2.5 Hz
   D. 45 Hz
   Answer: B. f_r = s f = 0.025 x 60 = 1.5 Hz. (Key point: Rotor frequency)

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