---
title: "3-phase IM principle; speed less than sync"
subject: "Electrical Machine"
chapter: 5
key: "semester_4/electrical_machine:three-phase-im-principle-slip"
source: https://www.bctengineer.com/q/semester_4/electrical_machine/three-phase-im-principle-slip
site: BCT Engineering
---

# 3-phase IM principle; speed less than sync

Electrical Machine · Chapter 5: Three Phase Induction Machines

- Asked in: 2075 Baisakh, 6+2 marks
- Video: [Why 3ph Induction Motor Cannot Run at Synchronous Speed| How Induction Motor Works|](https://www.youtube.com/watch?v=DycOsIslFAM) — Electrical With Abhishek Joshi
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machine/three-phase-im-principle-slip
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-5

## Question

Explain the operating principle of three phase induction motor with neat sketches. Why rotor speed is always less than synchronous speed. Justify.

## Short answer

A balanced three-phase supply to the stator creates a rotating magnetic field at synchronous speed N_s. This field induces rotor currents, producing torque. Rotor speed must be less than N_s because at N_s relative speed would be zero, so induced emf, current, and torque would vanish, and the motor would stop.

## Answer

#### Operating principle
A three-phase induction motor works on the principle of a **rotating magnetic field** and **electromagnetic induction**.

```text
   Stator (3-phase winding)         Rotor (short-circuited bars)
        R Y B                             ______
   +--------------+                    __/ |  | \__
   |  rotating    |    air gap        |    bars   |   -> torque
   |  field Φ at  |  ------------->   |___________|
   |  speed Ns    |
   +--------------+
```
1. A balanced three-phase supply is given to the stator winding, whose coils are displaced by 120° in space. The three currents, 120° apart in time, produce a **rotating magnetic field of constant magnitude (1.5 Φ_m)** rotating at the synchronous speed N_s = 120 f / P rpm.
2. This field sweeps across the stationary rotor conductors. By Faraday's law, emf is induced in the rotor conductors.
3. The rotor circuit is closed (short-circuited bars or rings), so rotor currents flow.
4. The rotor currents lie in the stator field, so each conductor experiences a force (F = B I l, Fleming's left-hand rule). Together these forces give a torque.
5. By **Lenz's law**, the rotor tries to oppose the cause of the induced currents, i.e. the relative motion between the field and the rotor, so the rotor starts turning in the direction of the rotating field.

#### Why the rotor speed is always less than the synchronous speed
- The induced emf and current in the rotor exist only because of the **relative speed** between the rotating field and the rotor (slip speed N_s − N).
- If the rotor were to reach N_s, the relative speed would be zero; no flux would be cut, so the rotor emf, rotor current and torque would all become zero.
- With no torque the rotor would slow down due to friction and windage; the relative motion would reappear, and torque would develop again.
- Therefore the rotor runs at a speed N slightly less than N_s, just enough to induce the current needed for the load torque. The difference is expressed as slip:

**s = (N_s − N) / N_s**, typically 2 to 5 % at full load.

For this reason the motor is also called an **asynchronous motor**.

## Check yourself

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

1. What speed does the stator field rotate at?
   A. N_s = 120f/P
   B. N_s = N(1-s)
   C. N_s = 60f/P
   D. N_s = fP/120
   Answer: A. The field rotates at synchronous speed 120f/P. (Key point: Rotating field)

2. How is torque produced?
   A. Permanent magnets
   B. Induced rotor currents interact with the stator field to give force F = BIl
   C. Resonance
   D. Direct supply to the rotor
   Answer: B. Faraday induction yields rotor currents that experience forces in the field. (Key point: Torque production)

3. Why does the rotor turn in the direction of the field?
   A. Because of the shaft
   B. Fleming's right-hand rule
   C. Lenz's law: it opposes the relative motion
   D. Because of friction
   Answer: C. The rotor tries to reduce relative speed. (Key point: Direction)

4. What happens if the rotor reaches N_s?
   A. Torque is maximum
   B. Slip becomes 1
   C. Current doubles
   D. Relative speed is zero so emf, current and torque are zero
   Answer: D. No flux is cut at synchronous speed, so no torque. (Key point: Why N < N_s)

## Related in Electrical Machine

- [Operating principle of 3-phase induction motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-motor-operating-principle) — Electrical Machines I, chapter 5
- [IM sync speed, slip, rotor frequency](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Aim-208v-60hz-4pole-1755rpm) — Electrical Machine, chapter 5
- [Synchronous speed and why rotor lags stator field](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Asynchronous-speed-rotor-direction-slip) — Electrical Machines I, chapter 5
- [Synchronous speed; rotor follows rotating field](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Asynchronous-speed-rotor-direction) — Electrical Machines, chapter 5
- [Delta induction motor: slip, rotor emf, turns ratio](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aim-delta-440v-1440rpm-rotor-emf) — Electrical Machines, chapter 5, asked 2×
- [No-load and blocked rotor tests of induction motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aim-noload-blocked-rotor-test-explain) — Electrical Machines I, chapter 5

## Related in other subjects

- [Output equation of three phase induction motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainduction-motor-output-equation) — Electric Machine Design, chapter 4, asked 6×
- [When terminal voltage is less or more than emf](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aterminal-voltage-less-or-more) — Electric Machines II, chapter 1
- [Single phase IM principle, torque-slip and starting](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Asingle-phase-principle-torque-slip) — Electric Machines II, chapter 3
- [Rotor rheostat speed control of slip ring motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Arotor-rheostat-speed-control) — Electrical Machines, chapter 5