Copper Rotor Motor Undergoes Efficiency Testing

21/01/2016 21
By Nuno Fernando

Efficiency comparison between a 7.5 HP copper rotor motor and three permanent magnet motors suggests some interesting results


Setting the scene

For some time, permanent magnet motor manufacturers have been promoting their products with claims of superior efficiency beyond what is possible with a NEMA Premium induction squirrel cage motor. But is this comparison a fair “apples-to-apples” one to make? Or is it more like comparing apples with pears?

The reason behind these questions is that frequently the comparison is made between a complete system and a single motor. In one corner of the ring is the permanent magnet motor (PMM), which requires a Variable Frequency Drive (VFD) in order to operate. In the other corner is the copper rotor motor, which does not necessarily need a VFD. What would the comparison look like if both motors operated off a drive recommended by the PMM manufacturer?


Putting the test in place

The Copper Development Association (CDA) decided to conduct a fairer comparative evaluation. They set up a series of tests between a 7.5 HP copper rotor motor with above NEMA Premium nameplate efficiency versus three permanent magnet motors from different manufacturers. The testing was conducted at the motor testing laboratory of Advanced Energy, Inc. of Raleigh, North Carolina under the supervision of Emmanuel Agamloh, Ph.D., P.E., and his staff over a period of several months. Motors were tested over a range of speeds and torques on the same variable frequency drive to compare efficiency.


The results

Richard E. deFay, project manager for CDA’s sustainable energy program, presented the findings at the 9th International Conference on Energy Efficiency in Motor Driven Systems (EEMODS) held in Helsinki, Finland, September 15-17, 2015. EEMODS is the leading electric motors conference in the world and is held every other year.

His presentation, entitled A Comparative Efficiency Analysis of a 7.5 HP Copper Rotor Motor and Three Permanent Magnet Motors, gave specific details of the study’s recent testing procedures and research data results. The results presented were very illuminating.

“I explained to my audience of 75 industry professionals that the findings of our analysis essentially debunked current wisdom promoted by manufacturers regarding efficiency of permanent magnet motors,” explains deFay. “In tests 1 and 3 of the study, the efficiency of the copper rotor motor was found to be higher than that of the commercially available permanent magnet motor at various loads and speeds. During test 2, it was found that neither motor’s system efficiency was clearly higher than the other.”

It should be noted that none of the motors achieved their nameplate efficiency during testing.


Positive feedback

The presentation was very well received and generated both interest and enthusiasm for the project on a global scale. “To see some of the most respected international experts supporting our work was heartening,” adds deFay. “I received several questions regarding the type of drive used for the PMM and CRM. There was also interest in securing data on our follow-up research.”


Follow-up research and other considerations

These tests certainly demonstrated a need for further testing to determine if this was a one-time phenomenon. More extensive testing is therefore being undertaken at different horsepower speeds to determine the efficacy of the original testing. This could include looking at 5, 10 and 20 horsepower motors. Results of these tests should be ready for release later in 2016.

Of course, efficiency is not the only consideration. What about the cost of the motors? The table below presents the costs of the four motors used. It’s clear that not only is a copper rotor motor significantly less expensive than the equivalent permanent magnet motor, but the lack of a drive is another huge cost saving.

Overview of costs of the four motors 7.5 HP used in the tests:

  • Permanent Magnet Motor A: $ 960   ($0 Motor, $899 Drive, $60 Tax)
  • Permanent Magnet Motor B: $ 1773 ($1097 Motor, $563 Drive, $112 Tax)
  • Permanent Magnet Motor C: $ 2034 ($893 Motor, $1000 Drive, $146 Tax)
  • Copper Rotor Motor:             $ 604   ($566 Motor, $0 Drive, $38 Tax)


Another factor to consider is manufacturing lead time? DeFay has a strong opinion on both cost and lead time: “When factoring in the cost of the motors and the extensive lead time to have the PMM’s manufactured – one took 17 weeks, another 10! – it became clear that the copper rotor motor was in many aspects a superior option.”

In the big picture, tests like this, and the further large-scale evaluation of copper rotor technology, can only help end users make better informed purchase decisions.