17 February 2018

Compare Aluminum and Copper

  • Conductivity

The resistivity of copper to aluminum at constant temperature
                              = 0.021 × 10 – 6 / 0.034 × 10 – 6 
                              = 0.61
As the resistivity of copper is lower than the aluminum for same temperature, the conductivity of aluminum is 61% lower than the copper conductor.

  • Density

The density of aluminum is approximately 1 / 3.5 that of copper it means that it mass reduces 1/3.5 times than that of copper conductor. The aluminum is cheaper and light in weight than copper.

  • Losses

Copper loss in the conductor = I2R ….( 1 )
Current density δ = I / a 
Therefore I = δa………………….…..( 2 )
Resistance R = ρL / a ………………..( 3 )
Where ρ = Specific resistance
From equation ( 1 ) , ( 2 ) and ( 3 )
Copper loss in the conductor = ( δa )2 ( ρL / a )
                                            = δ2 ρL a
Copper loss per unit volume = δ2 ρL a / La
                                           = δ2 ρ
Copper loss per unit mass = δ2 ρ / g
Where g = mass density
Therefore we can say that copper loss per unit mass is directly proportional to resistivity and inversely proportional mass density.
The resistivity of Aluminum at 75 oC = 0.034 × 10 – 6 ohm meter
The resistivity of Copper at 75 oC = 0.021 × 10 – 6 ohm meter 
The ratio of resistivity of Aluminum to Copper at constant temperature = 1.62
Therefore the copper loss in the aluminum conductor is 1.62 times that of copper conductor for same volume of conducting material.

  • Rating

The rating of machine depends upon temperature rise. The current rating of aluminum conductor is 1 / √ 1.62 = 0.78 times that of copper conductor. Therefore the rating of machine is reduced by 100 – 78 = 22% if aluminum conductor is used.

  • Conductor space

If we consider equal length and equal loss in the aluminum and copper conductor, the area occupies by the aluminum conductor is 1.62 times that of copper conductor. Therefore the slot depth has to be increased by amount 60% for aluminum conductor we slot width is considered to be fixed.

  • Weight

The aluminum is used for making transformer tank because of light weight and it will reduce stray load loss.

  • Effect of atmosphere

Aluminum quickly reacts with atmosphere and oxidation start very rapidly. The oxide layer on aluminum increases its resistivity. The copper conductor offers high resistance to corrison.

  • Soldering

Aluminum wire is hard to sold. It is soldered by special ultrasonic soldering iron.

  • Mechanical strength

The mechanical strength of hard drawn copper is much higher than the aluminum conductor. The aluminum is soft material but when alloyed with iron or silicon, its mechanical strength increases which can be used for overhead transmission line.

  • Contact Resistance

The copper offers low contact resistance at joints as compared to aluminum.

  • Application

The aluminum is used for flexible wire, bus bar, moulded case rotor and in the overhead transmission line. Die cast aluminum winding is used for squirrel cage induction motor. The copper is used mainly in the winding of the transformer and electrical machines.
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