- 2074 Asoj · 5 marks
Conducting versus insulating material characteristics
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.
Compare the characteristics of conducting and insulating materials used in DC machine.
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 | (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.
✓ Answer checked by Claude Sonnet 5.5
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