Diecast copper rotors improve motor efficiency.

Electric motors consume so much energy that a 1% increase in motor efficiency would save 20 billion kWh/yr. (more than $1 billion in electricity) with accompanying decreases in air contamination and contribution to global warming. High electrical conductivity of copper in the rotor structure of an induction motor can achieve an 11-19% reduction in overall energy losses by increasing energy efficiency, however, diecasting a copper-base rotor is challenging.

High pressure diecasting is the most economical process to form the squirrel cage of the induction motor rotor. However, copper-base alloys (despite their fluidity which makes them ideal for diecasting) have a high melting temperature that leads to a shorter diecasting die (tool) life and increased costs. As a result, diecast aluminum rotors have been used in most small and medium-sized integral horsepower motors even though copper is the logical material of choice.

In pursuit of a new diecast copper alternative, the Copper Development Assn., New York, worked with various casting and motor producers to develop a manufacturing process for a diecast copper rotor to take advantage of the higher conductivity and improve motor performance. The project addressed tool life through the use of heated nickel-base alloy diecasting dies.

To diecast a rotor, a stack of magnetic steel laminations is placed on a mandrel and the assembly is loaded into the diecasting machine. The laminations have punched-out slots around the diameter, creating a path for the molten copper. The diecasting process pushes molten metal into the tooling under pressures of at least 8,000 psi. The rotor cavity has multiple gates for the metal to flow through at an end ring. Molten copper must be forced through the gates to fill 44 small cross-section conductor bars and the far end ring.

Copper wire rod (C10100) is melted on a shot-by-shot basis for pressure injection into the die and around the steel. Melting one shot at a time at the speed of the diecasting machine controls oxygen and hydrogen pickup during melting and helps to achieve high cast density. The finished casting in Fig. 1 consists of an 8-lb (3.63-kg) copper squirrel cage in and around a 4.5-in. (11.43-cm)-long steel lamination shaft, measuring 4.53 in. (11.51 cm) in diameter and weighing 29 lb. (13.15 kg).

Testing of motors fitted with copper rotors has demonstrated energy loss reduction of 11- 19% (depending on rotor design), which is a minimum 1.5% efficiency improvement compared to aluminum rotors. The copper rotor is more attractive to manufacturers because it also can allow a reduction in manufacturing cost by 4-7% or weight by 3-6% because less stator iron, copper winding and rotor iron are required. In addition, motors with copper rotors run cooler than their aluminum counterparts, and the reduction in operating temperature translates to increased motor life expectancy.

The ability to successfully diecast these copper rotors could mean increased business from motor and pump manufacturers.

Following is a brief description of the results from copper rotor motors tested in application. These results were reported by the Copper Development Assn., New York.

Agricultural Water Pumping

A project testing the suitability of the copper rotor technology upgrade for motors used for water pumping in agriculture was carried out by a cluster of motor and pump manufacturers. Copper rotors were diecast for all the tests. Rotor laminations designed for aluminum were used in this direct substitution evaluation.

Motors were built and tested by six motor manufacturers. Field tests of motors fitted to pumps pumping water for agricultural use and one test of a motor driving a doffing machine in a textile plant then were conducted. Results for two of the two-pole motors and two of the 4-pole motors are shown in Tables 1-4. All of these motors are 415 V, 50 Hz, 3 phase. As expected with a higher conductivity rotor material, the speed is increased slightly, the slip is reduced and the efficiency is increased.

The two-pole 2-Hp (1.5 kW) and 5- Hp (3.7 kW) motors (Tables 1 and 2) were applied to pumping water for agricultural use. The tests comparing the pumping performance of motors with copper and aluminum rotors were decisive in terms of pumping time to fill the tanks and energy consumed in pumping a liter of water.

The 2-Hp (1.5 kW) motor-pump combination was tested filling a 2000L tank. The tank was brought near to the top in 823 sec. with the copper rotor motor, which was 170 sec. faster than with the aluminum rotor motor. But, more importantly, less total energy was consumed (even at the higher pumping rate by the copper motor) and the volume of water pumped per kWh was 8.9% higher.

The larger motor was tested filling a 5,000-L tank. The copper motor reduced filling time by 82 sec. and increased the volume of water pumped per unit of energy by 10.1%. The four-pole 5-Hp (3.7 kW) motor (Table 4) was tested in the doffing operation in a textile plant. The hourly rate of energy consumption decreased from 1.95 kWh with the aluminum rotor to 1.68 kWh with the copper. This translates to an annual energy savings of 2365 kWh.

Power costs were $0.109 per kwh at the location of this textile plant so annual electricity cost would be reduced by $265.00. The initial cost of the copper rotor version of the motor was $167.08, which was 10.35% higher than the aluminum rotor motor.

Diecast Copper Rotors & Modified Design

One manufacturer of gear motors tested a range of high efficiency motors up to 50 Hp (37 kW). The higher efficiency had been obtained by employing electrical grade copper in the rotor as well as the use of stator lamination and modified winding designs. These modifications succeeded in raising efficiency over the entire load spectrum while at the same time maintaining torque at critical points on the torque-load curve including starting torque.

Table 5 presents efficiency data for 1.1 kW and 5.5 kW aluminum and copper rotor motors. Comparison of these motors is especially interesting because two different design concepts have been employed for the 1.1 kW and the 5.5 kW copper motors.

The 1.1 kW motors essentially have the same layout of stator and rotor laminations. Aluminum rotor bars have simply been replaced by diecast copper but the lamination material is of a higher grade. In contrast the high efficiency 5.5 kW has a completely new lamination and winding design. The data in Table 5 shows that the copper rotor leads to a significant increase in efficiency while maintaining the outer motor dimensions standard for aluminum regardless of design.

RELATED ARTICLEL: Good enough for the military.

Copper has been essential to power generation since 1831, when British scientist Michael Faraday's experiments with copper wire, motion and magnetic force led to the development of the first modern electric motor, generator and transformer. Today's innovations involving copper hold great promise for making the electric products used in our homes and industries more energy-efficient.

The development of a diecasting process for copper rotors makes it commercially viable for companies to manufacture electric motors using all-copper rotors, which have been found to reduce heat loss and increase motor efficiency by 1.2-1.7% over traditional aluminum rotors.

That may not sound like much, but considering that electric motors account for 23% of America's energy use (nearly 70% of which is consumed by manufacturing), a 1% increase in motor efficiency would save $1.1 billion in energy costs annually, according to the U.S. Department of Energy. Other benefits are longer motor life, more lightweight motors and a reduction of carbon dioxide and other harmful emissions.

Recognizing the potential of the copper rotor and the new Copper-Based Casting Technology (C-BCT) used to produce it, the U.S. government appropriated $1 million in fiscal 2004 for an industry consortium (led by the Copper Development Association, New York) that will develop more efficient, durable and lightweight motors for the military's defense systems.

In addition to military and industrial applications, some European companies already are working on using the new copper rotors in motors designed to make appliances like washing machines and vacuum cleaners more energy-efficient.

--Information courtesy the Copper Development Association, New York

For More Information

"Recent Advances in Development of the Die-cast Copper Rotor Motor," E.E Brush, Jr., BBF Assoc., D.T. Peters and J.G. Cowie, Copper Development Association, Inc., and M. Doppelbauer and R. Kimmich, SEW Eurodrive Gmbh & Co KG., 2004 International Conference on Electrical Machines.

"Advances in Pressure Die Casting of Electrical Grade Copper," E. E Brush, Jr., BBF Assoc., D.T. Peters and J.G. Cowie, Copper Development Association, Inc., and S. P. Midson, 2002 AFS Transactions (02-002).

Table 1. Test Results for 2-Hp (1.5 kW), 4154, 2-pole,
3-phase, 50-Hz Motor, Copper Rotors Compared to Aluminum

                               Input
Rotor Material   Load (%)    Power (W)    Speed (rpm)   Efficiency (%)

Copper             100         1824          2949           82.54
Aluminum           100         1856          2926           81.14
Copper              75         1440          2955           79.19
Aluminum            75         1456          2940           77.80

Table 2. Test Results for 5-Hp (3.7 kW), 4154, 2-pole,
3-phase, 50-Hz Motor, Copper Rotors Compared to Aluminum

                               Input
Rotor Material   Load (%)    Power (W)    Speed (rpm)   Efficiency (%)

Copper             100         4256          2947           87.09
Aluminum           100         4496          2925           83.99
Copper              75         3232          2960           85.99
Aluminum            75         3408          2935           82.19

Table 3. Test Results for 3-Hp (2.2 kW), 415-V, 4-pole,
3-phase, 50-Hz-Motor, Copper Rotors Compared to Aluminum

                              Input
Rotor Material   Load (%)   Power (W)   Speed (rpm)   Efficiency (%)

Copper             100        2600         1451           85.88
Aluminum           100        2660         1411           83.55
Copper              75        1960         1465           84.15
Aluminum            75        2040         1433           82.82

Table 4. Test Results for 5-Hp (3.7 kW), 4154, 4-pole,
3-phase, 50-Hz Motor, Copper Rotor Compared to Aluminum

                              Input
Rotor Material   Load (%)   Power (W)   Speed (rpm)   Efficiency (%)

Copper             100        4344         1469           85.97
Aluminum           100        4544         1429           83.01
Copper              75        3280         1473           85.54
Aluminum            75        3400         1443           82.56

Table 5. Full-Load Efficiency Comparison
for Copper and Aluminum Rotors

Copper Rotor Motors     50 Hz   60Hz

       1.1 kW           82.8%   84.1%
       5.5 kW           88.1%   89.7%

Aluminum Rotor Motors

       1.1 kW           75.7%   77.4%
       5.5 kW           84.8%   86.6%