The Efficiency Advantage of Using Die-Cast Copper Rotors

Introduction

Die-cast copper rotors offer significant efficiency advantages for synchronous electric motors, making them highly attractive to both industrialized and developing countries. This is particularly relevant in developing regions where electric power is often scarce and expensive. For instance, India, the world’s second-largest emerging energy market, faces a chronic 10% energy shortage, which can spike to 20% during peak periods. Energy use in India is growing at a rate of 9-10% per year, with 63% of rural households lacking electricity.

To address this issue, the International Copper Promotion Council India, supported by the Small Scale Industries Development Bank of India and funded by the USAID Eco Project, tested copper rotors in motors used for water pumping—a major agricultural electricity use in India.

Testing and Results

Rotors were cast by Kitra Industries (India) by substituting copper for aluminum as the conductor-bar material, without altering the design or armature laminations. Tests were conducted by six Indian manufacturers and several pump fabricators in Coimbatore, Tamil Nadu. SITARC, an engineering laboratory, also conducted additional testing.

The tests included the following motor types, all with both copper and aluminum rotors:

– Two-pole, 2-hp (1.5-kW), 415-V, 3-phase, 50-Hz

– Two-pole, 5-hp (3.7-kW), 415-V, 3-phase, 50-Hz

– Four-pole, 3-hp (2.2-kW), 415-V, 3-phase, 50-Hz

– Four-pole, 5-hp, (3.7-kW), 415-V, 3-phase, 50-Hz

Encouraging Test Results

The test results demonstrated that using higher conductivity copper rotor material increased motor speed slightly (reducing slip) and significantly improved efficiency by 1.3 to nearly 4 percentage points, despite the motors not being optimized for copper.

Rotor Material

Load (%)

Input 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%

Rotor Material

Load (%)

Input 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%

Rotor Material

Load (%)

Input Power (W)

Speed (RPM)

Efficiency (%)

Copper

100 4344

1473

85.97%

Aluminum

100 4544

1429

83.01%

Copper

75 3280

1473

85.54%

Aluminum

75 3400

1433

82.56%

However, because copper’s higher conductivity was not taken into account in the motor’s slot design, there was a reduction in locked rotor torque.

HP (KW)

Poles

Rotor Material

Locked rotor Torque, % of Rated Torque

Temp. Rise °C (°F)

2 (1.5)

2 Cu

406.2 39.6 (103.3)

2 (1.5)

2 Al

442.2 36.8 (98.2)

5 (3.7)

2 Cu

174.0 66.7 (152.1)

5 (3.7)

2 Al

260.9 80.1 (176.2)

3 (2.2)

4 Cu

242.4 57.7 (135.9)

3 (2.2)

4 Al

268.4 68.8 (155.8)

5 (3.7)

4 Cu

168.2 61.8 (143.24)

5 (3.7)

4 Al

205.2 68.9 (156.0)

Copper rotors reduce operating temperatures due to their lower I²R losses. This is true in all examples shown here with the exception of the 2-hp (1.5-kW) motor, which contained no cooling fins. The corresponding aluminum rotor did have fins. Even without fins, however, the motor with the copper rotor ran only about 5.4°F (3°C) warmer than the cooled aluminum rotor motor. The temperature rise data in above table obtained by the winding resistance method. Separate tests that tracked temperature rise by direct measurement of the core temperature showed the same trends, but the temperatures reported were from 36°F (20°C) to as much as 72°F (40°C) lower.

Elevated temperatures accelerate degradation of the insulation on a motor’s windings, eventually leading to failure. The general rule of thumb in the motor industry is that for every 10 degrees Celsius hotter a motor runs, life expectancy can be cut in half. Conversely, cooler motors last longer. While copper rotor technology is still too young to provide long-term endurance data, it would be reasonable to expect that motors fitted with such rotors will provide longer service.

Breakdown torque values listed for the 4-pole motors described in Tables 3 and 4 are given in Table 6. In these motors, copper rotors apparently raised torque values by more than 50 percentage points over those for similar motors with aluminum rotors.

HP (KW)

Rotor Material

Breakdown Torque % of Rated Value

3 (2.2)

Cu

408.8 3 (2.2)

AI

340.5 5 (3.7)

Cu

350.7 5 (3.7)

Al

294.2

The two-pole 2-hp (1.5-kW) and 5-hp (3.7-kW) motors listed in Tables 1 and 2 were field-tested by fitting them to agricultural water pumps. The 2-hp (1.5-kW) motor-pump combination was tested by using it to fill a 528-gal (2000-l) irrigation tank, while the larger motor was tested by filling a 1321-gal (5000-l) tank. Results are shown in below table.

Motor Rotor Material

Input Power (KW)

Voltage V

Discharge Rate, gal/s

Energy, kwh

Gallons per kwh

-

2 HP

-

Cu

2.413

462 0.535 (2.43)

0.55 959 (3.630)

Al

2.171

446 0.442 (2.01) 5 HP

0.599 880 (3.333)

Cu

3.86

389 2.84 (12.93)

0.824 3,200 (12,114)

Al

3.77

377 2.56 (11.71)

0.894 2,907 (11,003)

Voltages in India can vary substantially over time and, in this case, were both higher and lower than the nominal 415-V line potential. Nevertheless, results of tests comparing the pumping performance of motors with copper and aluminum rotors were conclusive, both in terms of pumping time to fill the attached irrigation tanks and the energy consumed in pumping a single liter of water. The 2-hp copper-rotor motor (which turns faster due to its lower slip) filled the 528-gal tank in 170 s less than it took the motor with an aluminum rotor to accomplish the same task. But C and this is significant C the copper motor consumed less total energy even at the higher pumping rate, and the volume of water pumped per kWh was 8.9% higher. For the 5-hp motor/pump combination, filling time was reduced by 82 s with the copper motor. In this case, the volume of water pumped per unit of energy was increased by 10.1%, thanks to using copper in the rotor. These results are encouraging, albeit somewhat surprising. For centrifugal loads such as pumps and fans, higher-speed copper-rotor motors can actually result in higher energy consumption because the power consumed increases with the cube of rotational speed. However, in the Indian tests, the increase in motor efficiency due to the copper rotors was large enough to offset the increase in power consumption because, in these cases, the increase in speed was relatively small. Here, the higher flow rate was actually a benefit. Had the increase in running speed been large, any increase in power consumption could have been overcome by adjusting the gear or drive belt ratio to slow the fan or impeller.

Textile Plant Experience

The four-pole 5-hp (3.7-kW) motor described in Table 4 was also tested in a doffing machine at a local textile plant. The line voltage at the time was approximately 345 V. During the period when the copper-rotor motor was in service, the plant’s hourly rate of energy consumption decreased from 1.95 kW to 1.68 kW, which, if sustained, translates to an annual energy savings of 2,365 kWh and, consequently, a reduction in annual greenhouse gas emissions equivalent to 3,074 lb (1,400 kg) of carbon. Power costs are generally high in India. The rate at the textile plant was $0.109/kWh, meaning that the copper-rotor motor would reduce the plant’s annual utility bill by $265.00. Had the test been an actual commercial motor replacement, the copper motor driving the doffing machine would have paid for its extra cost (about 10% higher than a standard aluminum model) in only 22 days. Can copper simply be substituted for aluminum in induction motors to produce a more efficient machine? In the cases reported by the Indian motor manufacturers, the answer is clearly yes. Experience by others5,6 suggests that this is often, although not always, the case. If, for example, significant stray losses arise as a result of using poor-grade lamination steel, those losses can be increased by introduction of higher efficiency copper conductor bars, and overall efficiency will decrease. For best results, motor design should be optimized in order to take full advantage of copper’s ability to increase efficiency.

The efficiency advantage using Die Cast Copper Rotors Compare to Fabricated Copper Rotors.