---
title: "Construction and working of 3-phase induction motor"
subject: "Electrical Machines I"
chapter: 5
key: "semester_4/electrical_machines_i:induction-motor-construction-working"
source: https://www.bctengineer.com/q/semester_4/electrical_machines_i/induction-motor-construction-working
site: BCT Engineering
---

# Construction and working of 3-phase induction motor

Electrical Machines I · Chapter 5: Three-Phase Induction Machines

- Asked in: 2082 Kartik (new course), 6 marks
- Video: [3 Phase Induction Motor  (Construction & Working) Electrical Machines BEE (EEE) Engineering 1st year](https://www.youtube.com/watch?v=S9aRRIVS1ec) — Gautam Varde
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines_i/induction-motor-construction-working
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines_i/chapter-5

## Question

Explain the construction and working principle of 3-phase induction motor.

## Short answer

Stator has a 3-phase winding in a laminated core; rotor is squirrel-cage (shorted bars) or slip-ring (3-phase winding with slip rings). A 3-phase supply creates a rotating field at Ns=120f/P, inducing rotor currents that produce torque; the rotor runs at speed N < Ns with slip s.

## Answer

A **3-phase induction motor** is an AC motor in which the rotor current is produced by electromagnetic induction from the stator's rotating field; no electrical connection to the rotor is needed.

#### Construction

**1. Stator (stationary part)**
- Frame of cast iron or steel for support and protection.
- Core of thin (0.4–0.5 mm) silicon-steel laminations with slots on the inner surface, to reduce eddy-current and hysteresis loss.
- A 3-phase distributed winding in the slots, wound for a definite number of poles, connected in star or delta and fed from the 3-phase supply.

**2. Rotor (rotating part)** — laminated cylindrical core with slots, mounted on the shaft. Two types:
- **Squirrel-cage rotor:** copper or aluminium bars in the slots, short-circuited at both ends by end rings. Simple, robust, cheap. Bars are usually skewed to reduce noise and cogging.
- **Slip-ring (wound) rotor:** 3-phase winding (usually star) with the same number of poles as the stator. The three ends are brought to three slip rings on the shaft; brushes allow external resistance to be added for starting and speed control.

**3. Other parts:** a small air gap (0.4–2 mm) between stator and rotor, shaft, bearings, end covers and cooling fan.

```text
   +-------------------------+
   |  Stator core + 3-ph wdg |
   |   +-----------------+   |
   |   |  air gap        |   |
   |   |   +---------+   |   |
   |   |   | Rotor   |===|===|== shaft
   |   |   +---------+   |   |
   |   +-----------------+   |
   +-------------------------+
```

#### Working principle

1. **Rotating magnetic field:** a balanced 3-phase supply to the 3-phase stator winding produces a magnetic field of constant magnitude rotating at synchronous speed $N_s = 120f/P$.
2. **Induced emf:** the field cuts the stationary rotor conductors, inducing emf in them (Faraday's law).
3. **Rotor current:** the rotor circuit is closed (end rings or external resistors), so current flows.
4. **Torque:** the current-carrying rotor conductors in the magnetic field experience a force. By Lenz's law, the rotor turns in the **same direction** as the field to reduce the relative speed.
5. **Slip:** the rotor can never reach $N_s$; if it did, there would be no relative motion, no emf and no torque. It runs at speed $N$ slightly below $N_s$, with slip $s = (N_s - N)/N_s$, typically 2–5% at full load.

Example: a 4-pole, 50 Hz motor has $N_s = 1500$ rpm; at 4% slip it runs at 1440 rpm and the rotor frequency is $sf = 2$ Hz.

## Check yourself

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

1. How does a slip-ring rotor differ from a squirrel-cage rotor?
   A. It has a 3-phase winding brought out via slip rings for external resistance
   B. It has no windings
   C. It uses permanent magnets
   D. It has only two poles
   Answer: A. Cage rotors have shorted bars; wound rotors allow external resistance. (Key point: Rotor types)

2. What does a balanced 3-phase stator supply produce?
   A. Pulsating field
   B. Constant-magnitude field rotating at Ns = 120f/P
   C. A static field
   D. A field rotating at rotor speed
   Answer: B. This rotating field cuts rotor conductors and induces emf. (Key point: Rotating field)

3. Why does the rotor turn in the same direction as the field?
   A. Lenz's law: it reduces relative speed
   B. Friction
   C. The supply polarity
   D. Armature reaction
   Answer: A. Induced current opposes the cause, the relative motion. (Key point: Direction of rotation)

4. Why can the rotor never reach synchronous speed?
   A. Friction prevents it
   B. At Ns there would be no relative motion, no emf and no torque
   C. Air gap is too large
   D. Supply frequency drops
   Answer: B. Slip s = (Ns - N)/Ns must be nonzero for torque. (Key point: Slip)

## Related in Electrical Machines I

- [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
- [Induction motor working and torque equation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aim-working-torque-equation) — Electrical Machines I, chapter 5
- [DC motor working principle and torque equation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Adc-motor-principle-torque-equation) — Electrical Machines I, chapter 4, asked 4×
- [Induction motor as generator in grid-connected mode](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-generator-grid-connected) — Electrical Machines I, chapter 5, asked 5×
- [Induction motor torque and condition for maximum torque](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aim-torque-derive-max-condition) — Electrical Machines I, chapter 5, asked 2×
- [Rotor copper loss and efficiency of 35 kW induction motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aim-4pole-1440rpm-35kw-losses) — Electrical Machines I, chapter 5

## Related in other subjects

- [Single phase motor working; effect of air gap](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aspim-working-air-gap-effect) — Electrical Machines, chapter 7
- [Split phase induction motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Asplit-phase-im) — Electrical Machine, chapter 7
- [Schrage motor construction and working](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aschrage-motor-construction) — Electric Machines II, chapter 3
- [Hysteresis motor construction and principle](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Ahysteresis-motor) — Electric Machines II, chapter 3, asked 2×