---
title: "Choice of specific electric loading in induction machine"
subject: "Electric Machine Design"
chapter: 4
key: "semester_5/electric_machine_design:induction-specific-electric-loading"
source: https://www.bctengineer.com/q/semester_5/electric_machine_design/induction-specific-electric-loading
site: BCT Engineering
---

# Choice of specific electric loading in induction machine

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

- Asked in: 2079 Bhadra, 4 marks; 2075 Asoj, 5 marks; 2072 Chaitra, 4 marks (asked 3 times)
- Video: [5.3. Choice of specific electric loading in electrical machines](https://www.youtube.com/watch?v=MBKqCx7TXqw) — Nisha Prakash
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machine_design/induction-specific-electric-loading
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-4

## Question

What are the factors to be considered while determining the ampere conductor per meter (specific electric loading) of an induction machine?

## Short answer

Factors include copper loss and temperature rise, operating voltage, size and cost, overload capacity, machine diameter, and slot space factor. A higher ac reduces size and cost but increases copper loss, temperature rise, and leakage reactance, thereby lowering overload capacity.

## Answer

**Specific electric loading** ($ac$) is the total stator ampere-conductors per metre of gap periphery:
$$
ac = \frac{6 I_{ph} T_{ph}}{\pi D}
$$
Typical values for induction motors are 5000–45000 A/m. Its choice depends on:

1. **Copper loss and temperature rise**: a higher $ac$ means more copper per unit surface, so more $I^2R$ loss and a higher temperature rise. Good ventilation allows a higher $ac$.
2. **Operating voltage**: high-voltage machines need thicker insulation, so less slot space is left for copper and a lower $ac$ must be used.
3. **Size and cost**: from $Q = C_0D^2Ln_s$, a higher $ac$ gives a smaller and cheaper machine.
4. **Overload capacity**: a higher $ac$ means more turns and higher leakage reactance, which lowers the maximum torque and overload capacity.
5. **Diameter of the machine**: larger machines (larger $D$) have more space for copper and can use a higher $ac$.
6. **Space factor and slot design**: a high $ac$ needs more copper in the slots. Deeper slots raise slot leakage, and wider slots weaken the teeth.

| Higher $ac$ gives | Effect |
|---|---|
| Size and cost | Smaller, cheaper |
| Copper loss | Higher |
| Temperature rise | Higher |
| Leakage reactance | Higher |
| Overload capacity | Lower |
| Suitability for high voltage | Poorer |

Example: a motor with 1800 conductors carrying 11.16 A on a 0.276 m bore has $ac = 1800 \times 11.16/(\pi \times 0.276) \approx 23000$ A/m, a normal value for a medium motor.

In short, a high value saves material but raises temperature rise and leakage reactance. The designer chooses a value consistent with the required overload capacity and cooling.

## Check yourself

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

1. What does a higher specific electric loading do?
   A. Lowers copper loss and temperature rise
   B. Raises copper loss and temperature rise, though it gives a smaller machine
   C. Raises only the iron loss
   D. Has no effect on temperature
   Answer: B. More ampere-conductors per metre mean more I^2R loss and heating, but less material for a given kVA. (Key point: Copper loss and temperature rise)

2. Why is a lower ac used for high-voltage machines?
   A. High voltage needs thicker insulation, leaving less slot space for copper
   B. High voltage needs thinner conductors
   C. High voltage reduces the temperature rise
   D. High voltage raises the power factor
   Answer: A. Thicker insulation at high voltage reduces the space available for copper, so ac must be lower. (Key point: Voltage rating)

3. How does a high ac affect overload capacity?
   A. Raises it by reducing the turns
   B. No effect
   C. Lowers it because more turns give higher leakage reactance and lower maximum torque
   D. Raises it by cooling the machine
   Answer: C. Higher ac means more turns and leakage reactance, reducing the maximum torque and overload capacity. (Key point: Overload capacity)

## Related in Electric Machine Design

- [Choice of ampere conductors per meter in DC machine](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Adc-ampere-conductors-selection) — Electric Machine Design, chapter 5, asked 4×
- [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
- [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 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×
- [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×

## Related in other subjects

- [Conductor sizing factors and armoured cable](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aconductor-size-factors-armoured) — Industrial Power Distribution and Illumination, chapter 3