High conducting materials: requirements and classification

Conducting materials carry current in windings, busbars, commutators and leads. A material of high conductivity (low resistivity) is needed so that $I^2R$ loss and voltage drop are small.

Fundamental requirements of a high conducting material

1. High electrical conductivity (low resistivity), for low copper loss and voltage drop.
2. Low temperature coefficient of resistance, so that resistance and loss do not rise much when hot.
3. High tensile strength to withstand winding tension, electromagnetic forces and centrifugal forces.
4. High ductility and malleability, so it can be drawn into thin wires and strips and bent into coils without cracking.
5. Good solderability and weldability, giving low-resistance joints.
6. Resistance to corrosion and oxidation.
7. Good thermal conductivity, so heat is carried away from hot spots.
8. Low cost, light weight and easy availability.

Classification of electrical conducting materials

Conducting materials are classified by resistivity into two groups.

1. High conductivity (low resistivity) materials carry current with minimum loss.

• Copper ($\rho \approx 1.72\times10^{-8}\ \Omega$m at 20 °C). Annealed (soft) copper is used for windings. Hard-drawn copper is used for overhead lines, commutator bars and busbars.
• Aluminium ($\rho \approx 2.8\times10^{-8}\ \Omega$m). It is light and cheap, but needs about 1.6 times the copper area. Used in squirrel-cage rotors (die-cast), transformer foil windings, busbars and cables.
• Silver and gold: silver has the best conductivity of all metals, and gold does not oxidise. Both are costly, so they are used only for relay and switch contacts.
• Copper alloys: brass for terminals; bronze, cadmium-copper and beryllium-copper for springs, contact wires and slip rings.
• Steel: used only where strength matters, for example ACSR cores and rail conductors.

2. High resistivity materials are used where a resistance is wanted.

• Manganin, constantan (eureka): precision resistors, shunts and standards (very low temperature coefficient).
• Nichrome, kanthal: heating elements (resist oxidation at high temperature).
• Tungsten: lamp filaments. Carbon: brushes and resistors.
• Cast iron and alloy steel grids: starter and rheostat resistances.