Pioneering the way ahead for copper rotor technology

20/05/2015 20
By Nuno Fernando

Research underway in Germany could further accelerate market acceptance of copper rotor technology for asynchronous motors in a variety of applications

 

The general consensus is that a permanent magnet synchronous motor is more efficient than using a copper rotor in an asynchronous motor. But is such a statement based on a level playing field? Is it a fair comparison to make?

Peter Szilagyi, Sales Director and András Bárdos, Chief Technical Officer at die-casting company Breuckmann GmbH, would say a resounding “no” to both questions. They point out that for the synchronous motor using a permanent magnet, major investments have been made in recent years to develop new and better motor lamination designs. However, for asynchronous motor technology, equivalent investments have not been made. “This means that we are still using the old motor lamination geometrics that have been around for the last two or three decades,” says Szilagyi. “Moreover, these are designed for aluminium rotors and have not been optimized for copper rotors.”

Does this old lamination technology account – if only partially – for the efficiency lag? What if a copper rotor’s lamination design was totally changed to specifically fit asynchronous motor technology? What efficiency levels would then consistently be within reach?

Finding the answers to these intriguing questions is the goal of a research project that Breuckmann is currently conducting with the University of Applied Sciences in Dresden, Germany. This major two-year project aims to examine the consequences of totally redesigning the lamination slots in a copper rotor.

Initial results are extremely encouraging. “They support our hypothesis completely. With a new rotor lamination design specifically created for a copper rotor, we are already seeing significantly higher efficiency levels,” explains Bárdos. “We are excited about the potential of further improvements to the design, as well as seeing what effect superior electrical steel windings might have.”

 

Obstacles and challenges

They do admit however, that even should these preliminary results be verified, other obstacles still need to be addressed before copper rotor asynchronous motor technology becomes more widely accepted.

Cost is one challenge. The argument is frequently heard that a copper rotor is more expensive than an aluminium one of the same rating. Yet here too, the question arises of whether the playing field is level. “It’s important not just to compare rotors, but look at the whole system cost,” says Szilagyi.

He explains that a copper rotor is generally more expensive than an aluminium rotor of the same size, but points out that rotor costs are frequently compared without including the cost of other components, such as an inverter: “The inverter necessary for asynchronous motor technology is significantly less expensive than an inverter for a synchronous motor, which of course reduces the overall system cost.”

Such a claim is backed up by research. Comparisons of system costs between the two technologies indicate that an asynchronous motor with a copper rotor is less expensive than synchronous technology offering the same performance. This is because to obtain the same output with a copper motor as with an aluminium motor, the copper motor will be smaller. So the housing is smaller, the laminations necessary are less, and the amount of wire in the stator is less.

Research has also been conducted to compare a permanent magnet wheel motor with an asynchronous copper rotor wheel motor. It concluded that when total lifetime costs of the complete system were calculated, the most cost-effective solution was the copper rotor asynchronous motor solution.

Another issue is the current insecurity of supply faced by motor manufacturers in Europe, because there are only three companies capable of casting high-quality copper rotors in Europe. This will only be addressed when copper rotor manufacturing becomes more competitive. Against this argument though is the possibility to produce the highest efficiencies at a reasonable price, without using rare earth materials for permanent magnets.

 

New applications

More and more companies are identifying copper rotor asynchronous motors as the way forward.  KESSLER, based in Bad Buchau, Germany, and with affiliates in the USA, China, Taiwan and Russia is a major manufacturer of components for the machine tool industry. Products include electric motors, spindles, directly driven swiveling spindle heads and tool axes. They recently switched from aluminium to copper rotors for some of their high-speed milling machines. The existing aluminium rotor was not able to pass the stress test at 16,000 rpm. The copper rotor runs at a lower temperature, the x-deviation is smaller, and the accuracy of the milling machine is much higher.

Another German company, ATE from Leutkirch im Allgäu, designs and builds powerful electric motors and drives for high-speed applications. They too have begun to use more copper rotor technology in their drilling and milling machines. And many large pump manufacturers are moving towards copper rotor technology too.

Meanwhile in the automotive industry a lot of developments are focusing on asynchronous motors built around copper rotor technology. “For all types of electric vehicles, asynchronous motors using copper rotors look the most capable of delivering the highest efficiencies necessary,” says Bárdos. “We hope that our research into new lamination designs will help to accelerate this evolution, and look forward to asynchronous motors using copper rotors consistently achieving IE4 or even IE5 efficiency levels.”