- Conductivity
The
resistivity of copper to aluminum at constant temperature
= 0.021 × 10 –
6 / 0.034 × 10 – 6
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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