---
title: "Choice of ampere conductors per meter in DC machine"
subject: "Electric Machine Design"
chapter: 5
key: "semester_5/electric_machine_design:dc-ampere-conductors-selection"
source: https://www.bctengineer.com/q/semester_5/electric_machine_design/dc-ampere-conductors-selection
site: BCT Engineering
---

# Choice of ampere conductors per meter in DC machine

Electric Machine Design · Chapter 5: DC Machine Design

- Asked in: 2082 Baishakh, 8 marks; 2078 Kartik, 8 marks; 2072 Kartik, 8 marks; 2082 Chaitra (new course), 5 marks (asked 4 times)
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machine_design/dc-ampere-conductors-selection
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-5

## Question

Explain the factors to be considered while selecting the ampere conductor per meter (AC) in a dc machine.

## Short answer

Specific electric loading (ac) choice depends on temperature rise (higher ac raises copper loss and heat), speed (better ventilation allows higher ac), voltage (thicker insulation limits space so lower ac), machine size, armature reaction, commutation, copper loss, and efficiency. High ac reduces size and cost but worsens heating and commutation.

## Answer

**Specific electric loading** ($ac$) of a dc machine is the total armature ampere-conductors per metre of armature periphery:
$$
ac = \frac{I_z Z}{\pi D}
$$
where $I_z$ is the current in each conductor ($I_a/a$) and $Z$ the total conductors. Usual values are 15000–50000 A/m. Its choice depends on:

#### 1. Temperature rise
Higher $ac$ means more copper loss per unit armature surface. Since heat is removed through that surface, temperature rise increases. Machines with good cooling (forced ventilation) can use a higher $ac$.

#### 2. Speed of machine
A high-speed armature is better ventilated (fan action), so a higher $ac$ can be used.

#### 3. Voltage
High-voltage machines need thicker insulation, which leaves less slot space for copper. Hence a lower $ac$ is used for high voltage.

#### 4. Size of machine
Larger machines have deeper slots and more room for copper, so they can use a higher $ac$.

#### 5. Armature reaction
Armature mmf per pole $= ac\,\pi D/(2P)$. A high $ac$ means strong armature reaction, so more field mmf (more field copper, larger poles) is needed to avoid excessive distortion and loss of flux.

#### 6. Commutation
Reactance voltage is proportional to the ampere-conductors per slot and $ac$. A high $ac$ makes commutation difficult and may need interpoles.

#### 7. Copper loss and efficiency
A higher $ac$ increases copper loss and lowers efficiency.

#### 8. Size and cost
From $P_a = C_0D^2Ln$ with $C_0 = \pi^2B_{av}\,ac\times10^{-3}$, a higher $ac$ gives a smaller, cheaper machine.

| Higher $ac$ | Effect |
|---|---|
| Size, cost | Reduced |
| Copper loss, temperature rise | Increased |
| Armature reaction | Increased |
| Commutation | Worse |
| Efficiency | Lower |

## Check yourself

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

1. What is the specific electric loading ac of a dc machine?
   A. Armature flux per metre of periphery
   B. Total armature current per pole
   C. Ampere-conductors per slot
   D. Armature ampere-conductors per metre of armature periphery, Iz Z / (pi D)
   Answer: D. ac = Iz Z/(pi D), usually 15000-50000 A/m. (Key point: Definition of ac)

2. Why does a higher ac raise temperature rise?
   A. Iron loss rises
   B. Friction rises
   C. More copper loss per unit armature surface that must carry away the heat
   D. Less copper is used
   Answer: C. Copper loss per unit cooling surface rises, so temperature rises. (Key point: Temperature rise)

3. Why is a lower ac used in high-voltage machines?
   A. High voltage needs more current
   B. Armature speed is lower
   C. Commutator is longer
   D. Thicker insulation leaves less slot space for copper
   Answer: D. Insulation takes more slot space at high voltage, restricting copper. (Key point: Voltage effect)

4. What is the effect of a higher ac on the machine size?
   A. Larger, since losses rise
   B. No effect on size
   C. Smaller and cheaper, since C0 = pi^2 Bav ac x 10^-3 rises
   D. Larger, since armature reaction rises
   Answer: C. Higher C0 gives smaller D^2L, though copper loss, armature reaction and commutation worsen. (Key point: Effect on size)

## Related in Electric Machine Design

- [Choice of number of poles in DC machine](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Adc-number-of-poles) — Electric Machine Design, chapter 5, asked 8×
- [Choice of specific loadings in DC machine](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Adc-specific-loadings-selection) — Electric Machine Design, chapter 5, asked 2×
- [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×
- [True/False: ampere conductors and temperature rise](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Atf-ampere-conductor-temperature) — Electric Machine Design, chapter 5
- [True/False: armature reaction and ampere conductors](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Atf-armature-reaction-ampere-conductor) — Electric Machine Design, chapter 5
- [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

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