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`Ultra efficient` motors have copper rotors

01 April, 2006

`Ultra efficient` motors have copper rotors

Siemens has launched three ranges of high-efficiency NEMA-frame motors with die-cast copper rotors in the North American market. The Ultra Efficient family includes models with efficiencies up to 10% higher than the NEMA Premium efficiency standard - which is itself higher than the Eff1 specification used in Europe.

The three product lines consist of general-purpose cast-iron and die-cast aluminium models, and severe-duty cast-iron machines. Initially, they are available in ratings up to 20hp (14.9kW), with extensions planned to 30hp (22.4kW) for the aluminium range, and 400hp (300kW) for the cast-iron models.

Tony Giambra, a senior applications engineer with Siemens` Power Conversion Division, says that the new motors cost about 11% more than high-efficiency motors that use aluminium rotors, but adds that, in some applications, the extra cost can be recouped within six months through reduced energy bills. He reports that initial reaction to the motors from North American customers has been enthusiastic: "we can`t get enough of them".

Siemens is producing the copper rotors in Germany and assembling the Ultra Efficient motors in Mexico. Although there are plans to produce similar motors in IEC frames for the European market, Siemens is not revealing when these will go on sale.

The motors` high efficiency results from the inherently low I2R resistive losses of the copper squirrel cages, combined with optimised rotor and stator designs. Copper`s conductivity is up to 60% higher than that of aluminium.

Other improvements in the Siemens machines (shown in a cutaway above) include: redesigned cooling systems; the use of anti-friction bearings and polyurea-based grease; dynamically balanced rotors; and precision-machined mating surfaces for reduced vibration. The motors have a special insulation designed for use with variable speed drives.

Siemens` copper-rotor rotors are based on several years of research work by the Copper Development Association and the International Copper Association. Dr John Cowie, manager of the CDA research project, says that "the use of die-cast copper rotors reduces energy requirements, allows motors to run cooler, extends motor life and reduces overall weight and/or size.

"Siemens has raised the bar on electrical motor efficiency," he adds, "and we look forward to more manufacturers adopting the technology".

The first large-scale motor manufacturer to produce machines with copper rotors was SEW Eurodrive which offers copper-rotor motors in ratings from 0.75-37kW in its DTE and DVE ranges. A key attraction, according to SEW, is that the copper-rotor motor motors have higher power densities than high-efficiency copper-rotor machines of a similar rating, so can be made smaller. Conventional high-efficiency motors are often larger than standard motors of the same rating, which can make them difficult to use in retrofits and with gearboxes.

SEW adds that the DTE/DVE motors have reduced slip characteristics compared to aluminium-rotor machines, resulting in improved performance. Their torque/speed curves are also "stiffer", resulting in improved speed stability under varying torque conditions, and improving their performance in inverter applications, compared to conventional motors.

Another player in the copper rotor market is the French company, Favi, which is producing die-cast rotors for use by motor manufacturers including ITT/Grundfos. Favi is also working with Electrolux and Whirlpool in Europe to develop high-efficiency copper-rotor motors for appliances, and with the Brasilian firm Embraco on compressor applications.

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