---
title: "True/False: magnetic loading and teeth flux density"
subject: "Electric Machine Design"
chapter: 5
key: "semester_5/electric_machine_design:tf-magnetic-loading-teeth"
source: https://www.bctengineer.com/q/semester_5/electric_machine_design/tf-magnetic-loading-teeth
site: BCT Engineering
---

# True/False: magnetic loading and teeth flux density

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-magnetic-loading-teeth
- 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: Higher specific magnetic loading lowers the flux density in teeth.

## Short answer

FALSE. Higher specific magnetic loading raises gap flux density, which increases tooth flux density, not lowers it.

## Answer

**FALSE.**

The teeth carry the gap flux through an iron area smaller than the gap area (tooth width is less than slot pitch). Tooth flux density is roughly
$$
B_t \approx B_g \times \frac{\text{slot pitch}}{\text{tooth width} \times k_i}
$$
So a higher specific magnetic loading (higher $B_{av}$, hence higher $B_g$) **raises** the flux density in the teeth. This is why $B_{av}$ is limited: tooth density should not exceed about 2.1–2.2 Wb/m², otherwise the teeth saturate and a large field mmf is needed.

**Example:** with $B_g = 0.8$ Wb/m², slot pitch 30 mm, tooth width 15 mm and $k_i = 0.9$, $B_t \approx 0.8 \times 30/(15 \times 0.9) = 1.78$ Wb/m²; raising $B_g$ to 1.0 Wb/m² gives $B_t \approx 2.22$ Wb/m², which is already at the saturation limit.

## Check yourself

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

1. Higher specific magnetic loading lowers tooth flux density. Which is correct?
   A. True: it lowers it
   B. False: it raises it
   C. True: only at saturation
   D. False: tooth flux is unaffected
   Answer: B. Bt = Bg x slot pitch/(tooth width x ki). (Key point: True or false)

2. Why is tooth density higher than gap density?
   A. Teeth carry extra leakage flux
   B. Teeth are longer
   C. Slots carry no flux
   D. Teeth iron area is less than the gap area, since tooth width is less than slot pitch
   Answer: D. Same flux through smaller area. (Key point: Why teeth are higher)

3. What limit applies to tooth density?
   A. About 0.5 Wb/m^2
   B. About 5 Wb/m^2
   C. About 1.0 Wb/m^2
   D. About 2.1-2.2 Wb/m^2
   Answer: D. Beyond this, teeth saturate and need large field mmf. (Key point: Limit)

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