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-> Research on Copper Motor Rotors
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| Research on Copper Motor Rotors: Application of High
Temperature Mold Materials to Die Cast the Copper Motor Rotor. |
By Dr. John G. Cowie, CDA-USA
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It is well known that incorporation of copper for the conductor bars
and end rings of the induction motor in place of aluminum would result
in attractive improvements in motor energy efficiency.
Die cast motor rotors are universally produced in aluminum today because
of inadequate die life for the process with copper. Only small numbers
of very large motors utilize copper in the rotors by fabrication.
Such fabrication involves intensive hand labor and therefore is expensive.
Die casting, when it can be performed, is widely recognized as a low
cost manufacturing process. For these reasons, die casting has become
the fabrication method of choice and aluminum the conductor of choice
in all but the largest frame motors. Tool steel molds as used for
the aluminum die casting process have proved to be entirely inadequate
when casting higher melting point metals including copper. Lack of
a durable and cost effective mold material has been the technical
barrier preventing manufacture of the copper cast rotor (CCR).
An important study sponsored by the Department of Energy found that
motors above 1/6 Hp used about 60% of the electricity generated in
the United States.(1) Medium horsepower motors, 1-125 Hp, use about
60% of the electricity supplied to all motors. Because of the proliferation
of electric motors in this horsepower range, the target of this project,
the projected energy savings of the copper rotor motor is a significant
national consideration. Efficiency increases (a function of motor
size) are projected to result in total energy savings in the year
2010 of 2.02 E+12 Btu/yr at only 10% market penetration and 14.3 E+12
Btu/yr at the expected market penetration of 50 to 70% (dependent
on motor size). These numbers are equivalent to the yearly output,
respectively, of 0.5 to 3.5 600 MW generating plants operating at
75% of capacity.
Problem Statement
Utilizing high temperature, thermal shock resistant materials, design,
fabricate and demonstrate molds designed to withstand the copper motor
rotor die casting environment for an economically acceptable life,
i.e., thousands of casting cycles.
Program Summary
Several candidate die materials have been identified; beryllium-nickel,
a molybdenum/tungsten composite, and one or more compositions in the
tungsten-based composite family produced by a high speed chemical
vapor deposition (CVD) technique by the ThermoTrex Corporation. This
project will in its first phase fabricate and test simple end ring
molds of the several materials on an 800 ton horizontal shot controlled
pressure die casting machine to be located at a new Casting Development
Center being established by Buhler North America in Denver, Colorado.
In the second phase of this project, the most promising mold material
from Phase I will be fabricated into motor rotor molds and run for
an extended number of shots at this same facility. For these runs,
motor company partners will supply iron lamination stacks for appropriate
motors designed to use copper rotor conductors. The motor partners
will test the performance of the copper cast rotors.
Background
Recent analysis by two U.S. motor manufacturers shows that the economics
of motor operation and manufacture favor the use of copper in all
classes of motors if the die life in the pressure die casting process
can be extended to the order of 20,000 shots.
Die Cast Copper Rotors (CCR's) can provide advantages to motor manufacture
or performance in three ways:
• improvement in motor energy efficiency in operation
• reduction in overall manufacturing cost
• reduction in motor weight
The motor manufacturer can accentuate one of the advantages at the
expense of the other two. For example, in the case of a premium 10
Hp motor recently analyzed, the motor efficiency is 91.0%. Three design
scenarios using CCR have been analyzed: (1) seeking maximum efficiency
improvement; (2) seeking maximum manufacturing cost reduction; and
(3) seeking motor weight reduction.
Motors losses result from primary (stator winding) IČR (usually 34%
to 39%), secondary (motor) IČR (usually 16% to 29%), iron (core),
friction and windage, and stray load.(2) In addition to direct reduction
in rotor loss with CCR's, designs achieve additional reductions from
overall motor re-optimization of iron, strays, etc. CCR-based designs
show overall loss reduction from 15% to 20%.
1. If motor re-design efforts were devoted solely to improving efficiency,
it is estimated that the new design with CCR could achieve 92.5% efficiency.
This CCR example creates a "super" premium efficiency motor with an
efficiency level (i.e., 92.5%) higher than currently available premium
efficiency motors.
2. If motor re-design efforts were devoted solely to reducing manufacturing
costs for the current 91.0% efficient premium motor, it is estimated
that the new design using CCR could be manufactured at a $36 reduction
in overall manufacturing cost (15% of current $240 estimated manufacturing
cost), maintaining exactly 91.0% efficiency.
3. If motor re-design efforts were devoted solely to reducing motor
weight, it is estimated that the new design could reduce weight by
5% to as much as 10%.
CCR's can be used in specific motors to achieve a multiplicity of
intermediate combinations of these design advantages. For example,
where a smaller efficiency increase is required, the CCR could be
used to achieve some reduction in manufacturing cost (stator winding,
core, etc.) than would otherwise have been the case with traditional
aluminum die cast rotor technology. The problem encountered in attempting
to die cast copper motor rotors is thermal shock and thermal fatigue
of mold materials. Thermal cycling of the mold surface limits the
mold life even in aluminum die casting. However, cyclic thermal stresses
are so severe in copper die casting that in at least one recent instance,
a mold-gate-plate made of high strength steel (H-13, a die casting
industry standard) being tested at a die machinery manufacturer's
facility, fractured in just five casting shots. To be economically
feasible, mold life must be measured in thousands of casting cycles.
A problem with common mold materials is that they lose strength at
high temperature thus requiring low mean operating (and pre-shot surface)
temperatures. A low initial temperature increases the temperature
differential at the surface of the die, and thus the stress in the
die, on each shot.. The high melting temperature, high heat of fusion,
substantial latent heat and high thermal conductivity of copper all
combine to maximize the thermal shock. The solution to the thermal
shock problem lies in the use of high temperature materials having
thermal and thermoelastic properties and thermal properties conducive
to minimizing thermally induced strain. Studies conducted by the International
Copper Research Association (INCRA) in the 1970's confirm these expectations.
Innovation
Tungsten and molybdenum identified in the INCRA studies as good candidate
materials for copper casting have not found use in the industry largely
because of fabrication costs. ThermoTrex Corporation has developed
a cost effective near-net shape material forming process, named Chemical
Vapor Composites (CVC), a high growth rate variant of the well-known
chemical vapor deposition method. TTC had been funded by ARPA through
Fall 1996 to scale this process for a tungsten composite mold manufacture
expressly for die casting the copper motor rotor. TTC is currently
fabricating the critical mold-gate-plate for a THT machine.
These tungsten CVC materials hold high promise for long life. However,
they may be still more expensive than necessary for all components
of the mold system. Several new materials not available in the 1970's
have been developed for high temperature applications. Two promising
candidates for parts of the die caster are a molybdenum/tungsten composite
and the beryllium-nickel alloys. None of these materials has the high
thermal conductivity and low expansion of tungsten, but they do retain
exceptional strength at high temperatures. Hence, even if it is found
that they cannot survive in the hottest sections of the die casting
equipment, they may serve well in other areas.
Motor Company Participation
Participation of motor manufacturers in this program is welcome. The
logical point for active participation is to contribute an electric
motor and accompanying rotor design for production of the rotor mold
and cast copper rotors beginning in the sixth quarter in the activity
time line outlined above. The program as funded presently contemplates
production of cast copper rotors of only one design. Funds for mold
fabrication and rotor casting of other designs would have to be provided
by the individual companies. These costs will have to be worked out.
We invite motor manufacturer representatives to participate in all
meetings and to stay in touch with this project through project update
mailings.
Contacts
Dr. John G. Cowie
Manager Technical Programs
Copper Development Association Inc.
jcowie@cda.copper.org |
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