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Tests cast doubt on higher efficiencies of PM motors

17 September, 2015

A series of tests, funded by the Copper Development Association (CDA), comparing the efficiencies of a copper-rotor motor (CRM) with three permanent magnet (PM) motors, have failed to show that one type is clearly more efficient than the other. In one test, the efficiency of a copper-rotor induction motor was found to be higher than that of a commercially available PM motor.

The results of the tests were revealed in a paper presented by Richard deFay, project manager of the CDA’s sustainable energy programme, at the 2015 Energy Efficiency in Motor Driven Systems (Eemods) conference, held this week in Finland.

“The efficiency of the permanent magnet motor has been widely regarded as superior to that of the induction motor,” deFay told the meeting. “What CDA set out to do was conduct a relevantly equal ‘apples to apples’ system comparison between a PM motor and a CRM, using a drive specified by the PM motor manufacturer to determine the validity of marketing claims.”

The first round of testing used a 7.5hp (5.6kW) copper-rotor motor and PM motors from three different manufacturers. The tests were conducted at the motor-testing laboratory of Advanced Energy in North Carolina over a period of several months.

The motors were tested in three phases over a variety of speeds and torque values on the same variable-frequency drive to compare their efficiencies. In two of the phases of the study, the efficiency of the copper-rotor motor was found to be higher than that of the commercially available PM motor. In the third phase, there was no clear difference in efficiency between the motors.

deFay: further studies are needed

“The findings of this comparative analysis are quite interesting, and indicate that further motor studies must be undertaken at different horsepower speeds,” says deFay. “We now have an opportunity to conduct further evaluations on a larger scale to more accurately evaluate copper-rotor motor technology.”

Future tests could include motors rated up to 20hp (14.9kW).

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