At the Global Electric Motor Systems Energy Efficiency Forum (Eemods) conference, Richard de Fay, Sustainable Energy Program Manager at the Copper Development Association, presented a report on the test results of copper rotor motors and permanent magnet motors.
At the conference, DeFi pointed out, "The industry once believed that permanent magnet motors were more efficient than induction motors, but this is not true." The Copper Development Association plans to conduct a relatively fair comparison of permanent magnet (PM) motors and copper rotor motors (CRM) using drives specified by the PM motor manufacturers to determine the validity of marketing claims.
The first phase of testing compared 7.5HP (5.5kW) copper rotor and permanent magnet motors from three different manufacturers. The tests, conducted at the Advanced Energy Motor Testing Laboratory in North Carolina, lasted several months. The motor tests were divided into three phases, all using the same frequency converter, and efficiency comparisons were made based on motor load and speed.
The study found the following data comparing the energy efficiency of permanent magnet motors and copper rotor motors at different speeds (1800, 1600, 1400, 1200, 1000 RPM): (Blue represents copper rotor motors, red represents permanent magnet motors).
Data results show that at different frequency levels (90Hz for permanent magnet motors and 60Hz for copper rotor motors), the system energy efficiency of copper rotor motors is higher than that of permanent magnet motors.
The research from the second testing phase shows that at different frequency levels (150Hz for permanent magnet and 60Hz for copper rotor), the copper rotor motor system has higher energy efficiency than the permanent magnet motor when the torque percentage is low. However, when the torque percentage reaches 75%, the system efficiency of the copper rotor begins to fall below that of the permanent magnet.
The third phase of testing showed that, under the same frequency conditions, the copper rotor motor is more efficient than the permanent magnet PM motor.
“This comparative analysis is quite interesting and indicates that further research on the motors at different horsepower speeds is necessary,” Mr. DeFi said. “We now have the opportunity to conduct further, larger-scale evaluations to more accurately assess copper rotor motor technology.” Future tests may include motors with rated power up to 20 horsepower (14.9 kW).
As the examples above demonstrate, asynchronous motors using copper rotors not only have significantly higher energy efficiency than aluminum rotor motors, but also offer irreplaceable advantages over permanent magnet motors. For instance, permanent magnet materials are prone to demagnetization in environments such as short circuits, high temperatures, and strong vibrations; they lack demagnetization protection during faults; and under abnormal conditions like overspeed, the permanent magnet poles are easily affected. These inherent defects of permanent magnet synchronous generators make it difficult to meet the high reliability and maintenance-free performance requirements of systems, while copper rotor motors excel in these areas. Motor manufacturer Reliance Electric believes that copper rotor motors are the only motors capable of meeting the stringent requirements of the military.
