---
title: "True/False: air gap length and cost"
subject: "Electric Machine Design"
chapter: 5
key: "semester_5/electric_machine_design:tf-air-gap-cost"
source: https://www.bctengineer.com/q/semester_5/electric_machine_design/tf-air-gap-cost
site: BCT Engineering
---

# True/False: air gap length and cost

Electric Machine Design · Chapter 5: DC Machine Design

- Asked in: 2069 Asar, 1+3 marks
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machine_design/tf-air-gap-cost
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-5

## Question

State and justify whether the following statement is TRUE or FALSE: Keeping large value of length of air gap increases cost of dc machine.

## Short answer

TRUE. A larger air gap requires more field ampere-turns, leading to more field copper, bigger poles and frame, and higher cost.

## Answer

**TRUE.**

The air gap needs most of the field ampere-turns ($AT_g = 800000\,B_g K_g l_g$). A longer gap therefore needs more field mmf, which means more field copper, larger field coils and poles, and a larger frame. Field copper loss also rises. All these increase the cost of the machine. (A longer gap does reduce armature reaction and noise, but it is kept just long enough to limit armature reaction because of this extra cost.)

**Example:** doubling $l_g$ from 3 mm to 6 mm roughly doubles the gap ampere-turns, which form the largest part of the field mmf per pole, so field copper and field loss rise almost in the same proportion. So the gap is not made longer than needed: in practice the gap ampere-turns are kept at about 0.5–0.7 times the armature ampere-turns per pole, just enough to control armature reaction.

## Check yourself

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

1. Keeping a large air gap increases cost. Which is correct?
   A. True: more field ampere-turns, field copper and frame size
   B. False: it reduces cost
   C. False: it reduces field mmf
   D. True: because of armature reaction increase
   Answer: A. The gap needs most of the field mmf. (Key point: True or false)

2. Which expression gives gap ampere-turns?
   A. AT = Bg lg
   B. AT = 800000 Bg Kg lg
   C. AT = 800 Bg / lg
   D. AT = 8000 Bg Kg / lg
   Answer: B. Proportional to lg. (Key point: Gap mmf)

3. What benefit of a longer gap is offset by cost?
   A. Reduced armature reaction and noise
   B. Lower iron loss
   C. Lower copper loss
   D. Better efficiency
   Answer: A. So the gap is just long enough to limit armature reaction. (Key point: Benefit)

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