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Summary:
"Which rotor design, copper or aluminum?"
Whichever
is used, the machine must perform within industry standards and user
specification
With
trilingual summary
By
Richard L. Nailen, EA Engineering Editor
Which
rotor design, copper or aluminum?
Because
of cost, structural integrity, and design flexibility, standard
squirrel-cage motors up to several hundred kilowatts in rating are
universally manufactured with die-cast aluminum rotor conductors. In
larger sizes where that is impractical, fabricated cages (made up of
individual bars and end rings) use either aluminum or copper and its
alloys. Some manufacturers have standardized on one or the other, but
most will supply whichever one best suits the application.
Comparisons
between copper and aluminum have emphasized differences in melting
point, specific heat, conductivity, and physical strength. However,
most such comparisons are seriously flawed. The differences between
the two materials normally render impossible any direct replacement
of one by the other in the same large motor.
To
achieve the same accelerating torque, locked-rotor current,
efficiency, and safe stall time, copper and aluminum rotor bars must
have quite different shapes and areas. Even the depth of current
penetration at locked rotor (the "deep bar effect") differs
between the two materials (regardless of bar shape or size). Those
variations affect full-load speed, power factor, and stray load loss
as well as temperature rise.
Among
the many conflicting and misleading statements made in comparing
copper and aluminum rotors are these:
1.
"Copper is 300% stronger than aluminum" (untrue; the
difference ranges from 0% to 40% depending upon the type of applied
stress).
2.
"Yield strength of...aluminum...[is] somewhat higher than
copper" (the reverse is true for the commonly used aluminum
alloy).
Other
claims are valid but insignificant. For example, during heating and
cooling, an aluminum bar does expand lengthwise in the slot more than
copper. But the difference is slight; the problem has always existed
for copper, and is controlled by various manufacturing techniques
equally usable for aluminum.
As
in other electrical apparatus, such as switchgear, aluminum can
function as well as copper in many motor applications. Depending upon
the specific performance requirements, either version can provide
long service life.
Copyright 2003, Barks Publications, Inc., Chicago.
Reproduction by any means prohibited.
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