---
title: "Conducting versus insulating material characteristics"
subject: "Electric Machine Design"
chapter: 1
key: "semester_5/electric_machine_design:conducting-vs-insulating"
source: https://www.bctengineer.com/q/semester_5/electric_machine_design/conducting-vs-insulating
site: BCT Engineering
---

# Conducting versus insulating material characteristics

Electric Machine Design · Chapter 1: Materials used in electrical equipment

- Asked in: 2074 Asoj, 5 marks
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machine_design/conducting-vs-insulating
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-1

## Question

Compare the characteristics of conducting and insulating materials used in DC machine.

## Short answer

In DC machines copper and carbon conductors have very low resistivity and good thermal conductivity; resistance rises with temperature. Insulating materials such as mica, enamels and varnish have high resistivity but limited thermal endurance; their resistance falls with moisture and heat. Copper is used for windings and commutator, mica between segments, and carbon/graphite for brushes.

## Answer

In a DC machine, **conducting materials** carry the armature and field currents and the commutator current. **Insulating materials** keep these currents in their intended paths: between turns, between coils and core, and between commutator segments.

| Characteristic | Conducting material | Insulating material |
|---|---|---|
| Function | Carry current with minimum loss | Prevent current flow between parts |
| Resistivity | Very low (Cu 1.72×10⁻⁸ Ω·m) | Very high (10⁹–10²⁰ Ω·m) |
| Key electrical property | High conductivity, low temperature coefficient | High dielectric strength, low dielectric loss |
| Thermal property | High thermal conductivity; works well at high temperature | Poor conductor of heat; temperature limited by its class (A, B, F, H) |
| Effect of temperature | Resistance rises with temperature | Resistance falls and ageing speeds up with temperature |
| Mechanical need | Tensile strength, ductility for winding | Flexibility, strength, resistance to vibration |
| Moisture effect | Little (corrosion only) | Large: absorbed moisture lowers resistance |
| Usual materials in DC machine | Copper (armature, field, interpoles, commutator bars), hard-drawn copper or cadmium copper (commutator), carbon/graphite brushes, aluminium | Enamel, cotton, glass covering on wire; mica/micanite between segments; pressboard, Nomex slot liners; varnish; fibre wedges |
| Cause of losses | $I^2R$ (copper) loss | Dielectric loss (small) |

#### Special notes for DC machines
- **Commutator bars** use hard-drawn copper (or silver-bearing copper) for strength and wear resistance. **Mica** is used between bars because it withstands heat and sparking and wears at a similar rate to copper (it is undercut to avoid high mica).
- **Brushes** are carbon or graphite. Their fairly high contact resistance helps commutation.
- **Field coils** use enamelled or cotton-covered copper, and are insulated from poles by pressboard or mica.

## Check yourself

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

1. How does temperature affect the resistance of conductors versus insulators?
   A. Conductor resistance rises; insulator resistance falls
   B. Both rise
   C. Both fall
   D. Conductor falls; insulator rises
   Answer: A. Conductors have positive coefficient; insulation resistance falls and ageing speeds up. (Key point: Temperature effect)

2. Why is mica used between commutator bars?
   A. Withstands heat and sparking and wears similarly to copper
   B. It is a conductor
   C. It is cheap
   D. It is hygroscopic
   Answer: A. Mica is also undercut to avoid high mica. (Key point: Commutator mica)

3. Why are carbon/graphite brushes used?
   A. Fairly high contact resistance helps commutation
   B. Zero resistance
   C. Insulating
   D. Very low cost only
   Answer: A. Their contact resistance aids commutation. (Key point: Brushes)

## Related in Electric Machine Design

- [Good insulating material requirements and applications](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainsulating-requirements-application) — Electric Machine Design, chapter 1, asked 2×
- [Insulating materials for different machines](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainsulator-materials-machines) — Electric Machine Design, chapter 1
- [High conducting materials: requirements and classification](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ahigh-conducting-materials) — Electric Machine Design, chapter 1, asked 3×
- [Insulating material requirements and silicon steel](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainsulating-requirements-silicon) — Electric Machine Design, chapter 1, asked 2×
- [Insulating materials and their classification](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainsulating-material-classification) — Electric Machine Design, chapter 1
- [Insulating materials: classes, requirements and properties](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainsulating-material-properties) — Electric Machine Design, chapter 1

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