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

# IM sync speed, slip, rotor frequency

Electrical Machine · Chapter 5: Three Phase Induction Machines

- Asked in: 2068 Magh, 8 marks
- Video: [slip in induction motor | slip and frequency in hindi | concept of slip in 3 phase induction motor](https://www.youtube.com/watch?v=SRkuxKuY0pY) — Mind of Engineer 
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machine/im-208v-60hz-4pole-1755rpm
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-5

## Question

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

## Short answer

Synchronous speed N_s = 120×60/4 = 1800 rpm. Slip = (1800−1755)/1800 = 0.025 (2.5%). Rotor frequency = 0.025×60 = 1.5 Hz.

## Answer

**Given:** 208 V, 60 Hz, 4-pole, 3-phase induction motor; full-load speed N = 1755 rpm.

#### (i) Asynchronous (synchronous) speed
N_s = 120 f / P = 120 × 60 / 4 = **1800 rpm** (188.5 rad/s)

#### (ii) Slip
s = (N_s − N)/N_s = (1800 − 1755)/1800 = 45/1800 = **0.025 (2.5 %)**

#### (iii) Rotor frequency
f₂ = s f = 0.025 × 60 = **1.5 Hz**

| Quantity | Value |
|---|---|
| Synchronous speed | 1800 rpm |
| Slip | 0.025 |
| Rotor frequency | 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 60 Hz and 4 poles?
   A. 1500 rpm
   B. 1800 rpm
   C. 3600 rpm
   D. 1755 rpm
   Answer: B. Ns = 120 x 60 / 4 = 1800 rpm. (Key point: Synchronous speed)

2. What is the slip at 1755 rpm?
   A. 0.025
   B. 0.975
   C. 0.25
   D. 0.045
   Answer: A. s = (1800 - 1755)/1800 = 0.025. (Key point: Slip)

3. How is rotor frequency found?
   A. f2 = f/s
   B. f2 = s x f = 1.5 Hz
   C. f2 = 60 - 1755/60
   D. f2 = 60 Hz
   Answer: B. f2 = 0.025 x 60 = 1.5 Hz. (Key point: Rotor frequency)

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