Copper Motor Rotor Research Update
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 fabrication by pressure die casting is a well established and economical method. Only small numbers of very large motors utilize copper in the rotors by mechanical 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.
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. As a result, the projected energy savings of the copper rotor motor is a significant national consideration. Efficiency increases as a result of the motor rotor program are projected to result in total energy savings in the year 2010 of 20.2 E+12 Btu/yr at only 10% market penetration of the targeted motor sizes. This grows to 143 E+12 Btu/yr at the expected market penetration of 50 to 70% (different sized motors have different expected market penetrations). These numbers are equivalent to the yearly output, respectively, of 0.5 to 3.5 600 MW generating plants operating at 75% of capacity.
The objective of the program is to identify high temperature, thermal shock resistant materials, and then to 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.
The consortium of partners for this program was assembled during 1995 by the Copper Development Association Inc. The lead industrial partner is ThermoTrex Corporation (TTC), a subsidiary of Thermo Electron Corporation. Other partners in the program are Baldor Motors, Air Conditioning and Refrigeration Institute (ARI), Buhler North America and THT Presses.
This consortium of partners has been formed by Copper Development Association Inc., which is managing the project and providing technical expertise on the processing/handling of copper. Copper industry funding is provided by the International Copper Association.
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
- 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%.
The problem encountered in attempting to die cast copper motor rotors is thermal shock and thermal fatigue of mold materials. That is, suddenly pouring very hot molten copper into a die greatly shortens the life of the die. Because copper has a higher melting point than aluminum the problem is more severe. To be economically feasible, mold life must be measured in thousands of casting cycles.
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 identified tungsten and molybdenum as good candidate materials for copper casting. They 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.
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 the nickel-based superalloys and the beryllium-nickel alloys. None of these materials has the low expansion of tungsten or molybdenum, 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.
The first copper die casting trial was very successful. The die cast copper exhibited some defects but none that significantly degraded the electrical conductivity. Five copper castings were metallurgically, chemically and physically analyzed. The electrical conductivity varied from 95% to 101% IACS (pure copper is 100% IACS).
Copper die cast motor rotors would result in attractive improvements in motor energy efficiency. Advances are being made toward the development of durable and cost effective mold materials, presently the last major hurdle preventing die casting of the copper rotors.
An expanded version of this article is available.
Also in this Issue:
- Adjusting to Dramatic Changes Occurring in the Interconnect Market - What’s Next for the World of Interconnects?
- The Metal of Civilization Standards B115 and B49
- Copper Motor Rotor Research Update
- Copper: Technology & Standards
- New Copper Alloys - Summer 1998