Better manufacturing techniques may let rotors cast from copper become a mainstay of superefficient motor technology
The stringent efficiency levels now spelled out in NEMA Premium tables have fostered interest in motor architectures that can deliver high efficiency. One of those architectures uses a squirrel-cage rotor cast from copper rather than the traditional aluminum. Copper cuts down on the I2R losses by up to 40% and overall motor loss by 20%, though it increases the weight of the rotor a bit. The motivation for a copper-rotor design stems not only from efficiency considerations but also from a desire to minimize the size of the motor.
But until recently, copper rotors were tough to cast because of copper’s higher melting point (1,083°C compared to 660°C for aluminum), and a tendency to oxidize. Now, breakthroughs in copper die casting have made it more economical to mass produce copper rotors. Moreover, the economics could be favorable enough to let copper-rotor designs show up in hybrid and electric vehicles.
A group of companies led by the International Copper Association (ICA), New York City, launched the Copper Rotor Motor Project in 1994 with the aim of devising methods, materials and processes for mass producing copper rotors. By 2006, Yunnan Copper Co. Ltd. and Nanyang Explosion Protection Group Co. Ltd. in China had established a joint-venture called Yunnan Copper Die-Casting Co. Ltd. (YCD), which aimed to commercialize copper rotor motors in that country. The collaboration has produced results, and recent advances have made possible the efficient industrial production of die-cast copper-rotor motors.
The copper die-casting process is in many ways similar to that of aluminum. However, unlike aluminum, once molten, copper reacts with oxygen in the air to form cuprous oxide, which reduces its electrical conductivity and, consequently, motor efficiency. To keep the oxygen content within a range of 300 to 600 ppm, the ICA and YCD developed a gas-blanket protection method employed while the copper melts.
Another problem arises because the molten copper must be introduced into the die cavity quickly. Both the gas entrained in the molten copper and the shrinkage during solidification can make the casting porous. Historically, porosity has also been a problem for aluminum die casting, though manufacturers have developed casting procedures to manage it. For copper die casting, the YCD/ICA team developed a software model that simulated the heat and fluid flows during casting, filling, and solidification. The model predicted where porosity defects would arise and suggested ways to minimize them. The result of these simulations and extensive experimental testing is a reliable casting process and a way to design the die and die-runner system.
Copper’s high melting point also required copper rotors be cast at 1,200°C or higher. Use of conventional materials and die designs would have resulted in a copperrotor die having a useful life of only 600 cycles compared to 50,000 cycles for a comparable aluminum die. To boost die life, the Chinese team developed a special heating system that preheated the die by 500 to 600°C, near the temperature of molten copper. This preheating helps alleviate the thermal shock and subsequent cracks that normally reduce die life.
To make die temperature more uniform, a software model simulates the die-temperature field during the preheating and casting stages. The software helps optimize the design of die-heating and cooling systems, thereby extending die life. Collectively, these new horizontal diecasting technologies and processes have let the Chinese move into commercial mass production of copper rotors and improve yield rates to between 92 to 98%.
It is easy to see why copper rotors make for more-efficient motors. Copper has the second highest electrical conductivity of all metals. It is 70% more conductive than aluminum, and only 6% less conductive than silver. So replacing aluminum with copper lets the rotor conduct heat and electrical current more efficiently. The resulting lower temperatures let the motor run cooler and extend its life — motor life is generally halved for every 10°C rise in operating temperature.
Some copper-rotor induction motors generate so little heat that they don’t need cooling fans that typically sit at the end of conventional induction motors. And a copperrotor motor uses less conductive material to maintain the same power and efficiency as its aluminum counterpart, so it can occupy up to 20% less volume and be more compact.
The copper-rotor motor is finding its place in China. In parallel with the rest of the world, China has been adopting the new IE3 and IE4 standards for high-efficiency motors used to drive commercial equipment. (IEC 60034- 30 specifies electrical efficiency classes for single-speed, three-phase, 50 and 60-Hz, cage-induction motors that have two, four, or six poles, have rated output between 0.75 and 375 kW, have a rated voltage up to 1 kV, and are rated on the basis of either continuous or intermittent duty with a rated cyclic duration factor of 80% or higher. IE3 is the efficiency class in Europe for 50 Hz. With some exceptions it is identical to NEMA Premium for 60 Hz. The IE4 Super Premium Efficiency class has not been formally standardized, though some manufacturers claim to meet its proposed efficiency levels.
Chinese motor makers are now producing IE3 singlephase motors with copper rotors that match the high efficiencies of three-phase aluminum motors. The copper-rotor construction also runs more quietly (because it needs no fan), provides higher torque multiples, and can handle bigger overload capacities. In 2011, Chinese manufacturers produced the country’s first IE4 motor with a die-cast copper rotor for use in heavy-duty industrial pumps and compressors. At 30 kW and with six poles, it boasts an exceptional efficiency of 94.3%, with reliability and price performance exceeding that of currently used permanentmagnet motors.
Copper-rotor motors are also presenting an attractive alternative to the automotive industry for electric vehicles, where they are candidates to replace permanent-magnet motors. Permanent-magnet motors typically use rareearth magnets that are subject to volatile pricing and political pressures. Rare-earth metal prices rose by a factor of four from 2010 to 2012 because of booming demand and supply problems. Those higher costs are significant, adding around $520 to the parts cost of a hybrid-electric vehicle, and potentially double that amount to the consumer price.
Copper-rotor motors also offer a potentially longer lifetime than permanent-magnet motors: magnets lose strength when operated at high temperatures over lengthy periods. Experts say their efficiency is likely to decline after three to five years of use. In contrast, copper-rotor motors retain their efficiency for at least 10 years.
All in all, it looks as though mass adoption of copperrotor induction motors is not far off.