---
title: "Factors for air gap length in induction motor"
subject: "Electric Machine Design"
chapter: 4
key: "semester_5/electric_machine_design:air-gap-length-factors"
source: https://www.bctengineer.com/q/semester_5/electric_machine_design/air-gap-length-factors
site: BCT Engineering
---

# Factors for air gap length in induction motor

Electric Machine Design · Chapter 4: Three phase induction motor design

- Asked in: 2081 Bhadra, 6 marks; 2080 Bhadra, 6 marks; 2078 Bhadra, 8 marks; 2082 Chaitra (new course), 6 marks (asked 4 times)
- Video: [3.9 Factors affecting the choice of air gap length](https://www.youtube.com/watch?v=qPWo2ab5LJE) — Nisha Prakash
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machine_design/air-gap-length-factors
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-4

## Question

What are the factors that must be considered while selecting the length of air gap in induction motor?

## Short answer

Air gap is kept small for high power factor and low magnetising current, but increased to improve overload capacity, reduce pulsation loss, noise, unbalanced pull, and aid cooling. Empirical: lg = 0.2 + 2√(DL) mm.

## Answer

The **air gap** of an induction motor is kept as small as mechanically possible, because the gap needs most of the magnetising mmf. Its final length is a compromise between the following factors.

#### 1. Power factor (magnetising current)
The gap needs the largest share of magnetising ampere-turns. A larger gap increases the magnetising current and lowers the power factor. This is the main reason to keep the gap small.

#### 2. Overload capacity
A larger gap reduces the zigzag and other leakage reactance. Lower leakage reactance gives a larger maximum (breakdown) torque, so the overload capacity improves with a larger gap.

#### 3. Pulsation (tooth) losses and noise
Slot openings cause flux pulsations in the tooth tips. With a larger gap these pulsations are smoothed, so pulsation losses, magnetic noise and vibration are reduced.

#### 4. Cooling
A larger gap allows more air to pass and gives better cooling of the rotor and stator surfaces.

#### 5. Unbalanced magnetic pull
Any eccentricity of the rotor produces a one-sided pull. Its effect is relatively smaller with a larger gap because the percentage change in gap is smaller.

#### 6. Mechanical considerations
The gap must allow for shaft deflection, bearing wear, and manufacturing tolerances of stator and rotor. Machines with long cores or large diameter need larger gaps.

#### 7. Harmonics
A larger gap reduces the effect of slot harmonics, so harmful harmonic torques (cogging, crawling) are reduced.

#### Empirical formula
For small and medium machines (Sawhney):
$$
l_g = 0.2 + 2\sqrt{DL}\ \text{mm}
$$
with $D$ and $L$ in metres. Typical values are 0.25 mm to 2 mm; for example, a motor with $D = 0.16$ m and $L = 0.085$ m has $l_g = 0.2 + 2\sqrt{0.0136} = 0.43$ mm.

| Small gap | Large gap |
|---|---|
| Low magnetising current | High magnetising current |
| High power factor | Low power factor |
| Higher leakage reactance | Lower leakage reactance |
| Lower overload capacity | Higher overload capacity |
| More noise, pulsation loss | Less noise, pulsation loss |
| Poorer cooling | Better cooling |

## Check yourself

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

1. Why is the air gap kept as small as possible?
   A. A larger gap raises magnetising current and lowers power factor
   B. A larger gap raises the power factor
   C. A larger gap reduces the torque to zero
   D. A larger gap lowers iron losses to zero
   Answer: A. The gap takes most of the magnetising mmf, so a larger gap means more magnetising current and a poorer power factor. (Key point: Power factor)

2. What does a larger air gap do to overload capacity, and why?
   A. Lowers it, because leakage reactance increases
   B. Improves it, because leakage reactance falls and maximum torque rises
   C. No effect on it
   D. Lowers it, because the cooling gets worse
   Answer: B. A larger gap reduces zigzag and other leakage reactance, so the breakdown torque and overload capacity increase. (Key point: Overload capacity)

3. What is the effect of a larger gap on pulsation losses and noise?
   A. They increase because the flux path is longer
   B. They are unchanged
   C. They become zero for every machine
   D. They decrease because slot-opening flux pulsations are smoothed
   Answer: D. A larger gap smooths flux pulsations from slot openings, reducing pulsation loss, noise and vibration. (Key point: Pulsation losses and noise)

## Related in Electric Machine Design

- [Effect of too wide air gap in induction machine](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Awide-air-gap-effects) — Electric Machine Design, chapter 4
- [Air gap and rotor bars, 10 kW motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Acage-rotor-10kw-design-data) — Electric Machine Design, chapter 4
- [True/False: air gap length and cost](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Atf-air-gap-cost) — Electric Machine Design, chapter 5
- [600 kW generator: dimensions, poles, air gap](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Adc-generator-600kw-constraints) — Electric Machine Design, chapter 5
- [1000 kW generator: dimensions, poles, air gap](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Adc-generator-1000kw-air-gap) — Electric Machine Design, chapter 5
- [Stator bore, length and conductors of 70 hp motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Astator-bore-70hp-motor) — Electric Machine Design, chapter 4

## Related in other subjects

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- [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
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- [Factors for selecting construction materials](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Afactors-selection-construction-material) — Civil Engineering Materials, chapter 1, asked 9×