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.
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.
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.
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