Aluminum is good, but copper is better -- at least when you're talking about die-cast rotors in three-phase induction motors.
Siemens Energy & Automation, based in Alpharetta, Ga., offers a new line of AC motors designed to meet or exceed National Electrical Manufacturers Association (NEMA) standards. Called New NEMA Motors, they feature aluminum and cast-iron frame configurations, and have a unique die-cast copper rotor that is said to reduce power consumption and extend the life of the motor while remaining in the normal frame length and diameter of the regular standard efficient product.
"To penetrate the market, we needed to be the efficiency leader," says John Caroff, Siemens E&A's Norwood, Ohio-based marketing manager. "The best way to get a next-step function higher efficiency would be go to the die-cast copper rotor."
A normal rotor is die-cast aluminum, which has more air pockets inside that keep the rotor from conducting electricity properly, according to Caroff. Copper rotors tend to have far fewer voids, so the conductivity is better and it is much easier to balance, which gives the motor a longer life. It also doesn't require internal cooling fins on the end of it like a normal aluminum rotor. With less resistance from the fins, it runs cooler.
According to Siemens, the company took a huge risk in designing die-cast copper rotors. For one, the melting point of copper is significantly higher than aluminum. Also, die-cast copper rotor equipment did not exist. Siemens had to design its own machine tools to tackle the job.
"Here was a unique motor design, but we also had to design the manufacturing equipment to produce it," says Caroff. "They keep the equipment very secret, because that is the real edge."
