Conducting Material (Cu vs Al)
•Electrically Conducting Material :- This materials carry current in an electrical machine.
•Highly Conducting Material :- Copper, Aluminum, Gold, Silver.
•Highly resistive material:- Alloys (heating elements)
Now, R=Ro(1+αΔT)
Where α= temperature coefficient of resistance
ΔT= Change in temperature of per degree Celsius
ΔT= Change in temperature of per degree Celsius
R & Ro = Initial and final value of resistance
Properties of Highly Conducting material
- Highest Possible Conductivity
- Less temperature Coefficient of Resistance.
- Adequate Mechanical Strength and absence of brittleness.
- Rollability and Draw ability.
- Good weldability and solder ability to ensure the low resistance of joints.
- Adequate resistance to corrosion.
COPPER
- Machine Windings
Ϭau > Ϭag > Ϭcu > Ϭal
(Gold) > (Silver) > (Copper) > (Aluminum)
Ϭ= Conductivity of the material
- It is used to construct winding of an electrical machine because the cost is not very high and it is highly conducting.
- Copper is resistance to oxidation and corrosion.
- Hard drawn copper wires are used in electrical machine because their mechanical strength is high.
αcu =0.00393 per degree Celsius (high)
ALUMINIUM
- Since copper is getting depleted aluminum is the next best choice for conducting material it is abundantly available.
- Aluminum cannot be drawn into thin wires but it can be used to form thin sheets.
Comparison of Aluminum(Al) and Copper(Cu)
Why cross sectional area of Aluminum(Al) is higher than the cross sectional area of the copper(Cu) ?
- As we know that Ϭcu > Ϭal
- So, R = ∫ * l/A = l/Ϭ * A
- Since, Rcu = Ral
- Now, l/Ϭcu * Acu = l/Ϭal * Aal
- Therefore Aal = Ϭcu * Acu /Ϭal > Acu
Where ∫ = resistivity of the material and Ϭ = conductivity of the material and l=length of the material and A= cross sectional area of the material.
Hence cross sectional area of Aluminum is higher then the cross sectional area of Copper.
- For Al wire the size of slot required is higher as compared to Copper.
- For Induction motor above 100KW output Aluminum can be used for cage rotor.
- Aluminum can be used to form foil type low voltage winding in a transformer.
- It can be used to construct tank of a transformer to reduce stray losses.
- It easily gets oxidized to form AI2O3 which prevent further oxidation.