Copper Motor Rotors: Application of High Temperature Mold Materials to Die Cast the Copper Motor
Copper Applications in Innovative Technology
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.
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.
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.
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) I²R (usually 34% to 39%), secondary (motor) I²R (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%.
- 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.
- 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.
- 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.
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 mailing.
Also in this Issue:
- Electric Vehicles
- New Research Program Focuses on Improvements for the 21st Century
- A Discussion of Future Trends and Their Influences on the Copper Strip, Sheet and Plate Industry Part 2 of 2
- A Discussion of Future Trends and Their Influences on the Copper Strip, Sheet and Plate Industry Part 1 of 2
- Copper Motor Rotors: Application of High Temperature Mold Materials to Die Cast the Copper Motor
- CDA Backs DX Geothermal Heat Pump R&D