Machine Design

Copper rotors: cast for the part

Dr. John G. Cowie
Copper Development Association
New York, N.Y.

Six different sizes and types of rotors were die cast using pure, highconductivity copper. Tests showed a 1.2 to 1.7 % improvement in efficiency.

When it comes to energy efficiency, every little bit helps. Motor manufacturers are always looking to cut losses and increase efficiency. One innovative way may be to use die-cast copper rotors instead of the conventional aluminum rotors found in most motors.

Die-cast copper rotors can provide several advantages. The first and most significant is better motor energy efficiency. Production runs show an improvement of 1.2 to 1.7% in efficiency, although actual numbers depend on the rotor design. They also reduce electrical energy losses by 15 to 23% compared with motors using aluminum rotors.

Another benefit is that motors run cooler. This is a big deal because a lower operating temperature means longer motor life. Tests show that copper rotors decrease operating temperature by as much as 5°C. As a general rule, for every 10° increase in the motor operating temperature, insulation life of the motor is cut in half.

Today, die-cast motor rotors are universally produced in aluminum because processing copper yields inadequate die life. Thermal shock and fatigue of mold materials are the biggest problems encountered in casting copper rotors. Thermal cycling of the mold surface limits the mold life even in aluminum die casting. However, cyclic thermal stresses are more severe in copper die casting.

Thus far, lack of a durable and cost-effective mold material has been the technical barrier preventing mass production of cast-copper rotors. Several materials could solve the problem. Among them are beryllium-nickel, molybdenum/tungsten, and tungstenbased composites produced by a high-speed chemical-vapor-deposition (CVD) technique.

Several major motor manufacturers undertook the effort in conjunction with a CDA-managed research and development program begun three years ago. The project was funded in part by the U.S. Department of Energy, the Air Conditioning and Refrigeration Technical Institute, and the International Copper Association.

Further improvements in process and rotor design, such as optimization of the steel laminations, should extend copper's lead in efficiency over that of aluminum.

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