---
title: "Effect of too wide air gap in induction machine"
subject: "Electric Machine Design"
chapter: 4
key: "semester_5/electric_machine_design:wide-air-gap-effects"
source: https://www.bctengineer.com/q/semester_5/electric_machine_design/wide-air-gap-effects
site: BCT Engineering
---

# Effect of too wide air gap in induction machine

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

- Asked in: 2071 Chaitra, 8 marks
- Video: [Length of Air Gap in Induction Motors | Design of 3-Phase Induction Motors #airgap #design #machine](https://www.youtube.com/watch?v=8XiABWWvxk8) — Prasant Decodes
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machine_design/wide-air-gap-effects
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-4

## Question

What will be the effect if air gap length is too wide in induction machine? Explain different factors to be considered when selecting suitable air gap length.

## Short answer

A too-wide air gap increases magnetising current and lowers power factor, raising copper loss and reducing light-load efficiency, though it lowers noise, leakage reactance, and unbalanced pull. Selection factors include power factor, overload capacity, pulsation loss, cooling, unbalanced magnetic pull, mechanical constraints, and harmonics.

## Answer

The **air gap** is the main reluctance in the magnetic circuit of an induction motor, so its length strongly affects performance.

#### Effects of a too-wide air gap
- **High magnetising current**: the gap mmf $= 800000\,B_g K_g l_g$ rises in proportion to $l_g$, so the no-load current rises sharply.
- **Low power factor**: the larger magnetising (reactive) current lowers the power factor; this is the most serious effect.
- **Higher copper loss and lower efficiency** at light loads because of the large no-load current.
- On the good side: lower leakage reactance (higher overload capacity), less noise, better cooling and less unbalanced magnetic pull.

So a wide gap is used only when overload capacity or mechanical reasons are more important than power factor.

#### Factors in selecting the air gap length
1. **Power factor**: small gap for low magnetising current and high power factor.
2. **Overload capacity**: larger gap lowers zigzag leakage reactance and raises the maximum torque.
3. **Pulsation loss and noise**: larger gap smooths the tooth-ripple flux, reducing pulsation losses, noise and vibration.
4. **Cooling**: larger gap allows more cooling air.
5. **Unbalanced magnetic pull**: larger gap reduces the effect of rotor eccentricity.
6. **Mechanical factors**: shaft deflection, bearing wear, length of core and manufacturing tolerances set a minimum gap.
7. **Harmonics**: larger gap reduces slot harmonic fields and their parasitic torques (cogging, crawling).

#### Empirical formula
$$
l_g = 0.2 + 2\sqrt{DL}\ \text{mm} \quad (D, L \text{ in m})
$$
Example: $D = 0.25$ m, $L = 0.16$ m gives $l_g = 0.2 + 2\sqrt{0.04} = 0.6$ mm.

| Factor | Small gap | Large gap |
|---|---|---|
| Magnetising current | Low | High |
| Power factor | High | Low |
| Overload capacity | Lower | Higher |
| Noise, pulsation loss | More | Less |
| Cooling | Poorer | Better |
| Unbalanced pull | More | Less |

## Check yourself

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

1. What is the most serious effect of too wide an air gap?
   A. Low power factor because magnetising current rises
   B. Higher starting torque
   C. Higher leakage reactance
   D. Greater unbalanced magnetic pull
   Answer: A. Gap mmf is proportional to gap length, so magnetising current increases and the power factor falls. (Key point: Main effect of wide gap)

2. How does a larger air gap affect overload capacity?
   A. It lowers maximum torque
   B. It lowers leakage reactance and raises maximum torque
   C. It has no effect
   D. It raises leakage reactance
   Answer: B. Zigzag leakage falls with a larger gap, which increases the overload capacity. (Key point: Overload capacity)

3. Which of these is a benefit of a larger gap?
   A. Higher power factor
   B. Lower no-load current
   C. Less noise, less pulsation loss and less unbalanced pull
   D. Lower copper loss at light load
   Answer: C. A larger gap smooths tooth ripple and reduces the effect of eccentricity. (Key point: Other benefits)

4. What is the empirical formula for gap length used in the answer?
   A. lg = 2 + 0.2 sqrt(DL) mm
   B. lg = 0.2 sqrt(DL) m
   C. lg = 0.2 + 2DL mm
   D. lg = 0.2 + 2 sqrt(DL) mm
   Answer: D. With D = 0.25 and L = 0.16, lg = 0.2 + 2 x 0.2 = 0.6 mm. (Key point: Empirical gap length)

## Related in Electric Machine Design

- [Factors for air gap length in induction motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Aair-gap-length-factors) — Electric Machine Design, chapter 4, asked 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
- [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
- [Choice of specific electric loading in induction machine](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainduction-specific-electric-loading) — Electric Machine Design, chapter 4, asked 3×
- [Choice of magnetic loading in induction machine](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainduction-magnetic-loading) — Electric Machine Design, chapter 4
- [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

## 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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- [True length: tape found 2.5 cm too long](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atape-825-plus-750-m) — Surveying I, chapter 2