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Creating a New Market: Testing and Promoting the Copper Motor Rotor
(continued) IEEE Motor Tests The motors were tested using the IEEE 112B efficiency test standard. Overall, aluminum lost 179 more watts than copper – a 14 percent difference. Stator resistance and iron core loss between the two metals were equal since the same iron laminations and stator were used for both the copper and the aluminum rotors. However, copper performed significantly better than aluminum in rotor resistance. Copper lost 157 watts where aluminum lost 261 watts – a 40 percent improvement with copper. 
Measurements taken for windage and friction showed copper to have a 43-watt, or 37 percent, advantage over aluminum. Copper lost 32 less watts, or 23 percent less than aluminum, in stray load loss. This disparity is accounted for by the fact that copper castings had far less porosity and near-perfect balance.
An unexpected result of the tests was a large decrease in the operating temperature of the motor using the copper rotors – a drop of 4.5°C. Heat affects the life expectancy of the motor. Conventional guidelines estimate that for every 10°C hotter a motor runs, typical motor life can be reduced by half from its design point. With a reduction of 5°C in operating temperature, copper motor rotors may increase life expectancy by 50 percent. The tests did find one negative impact on motor performance due to using the copper rotors. Starting torque is reduced 36 percent. Breakdown torque is reduced 17 percent; and locked torque is reduced 4 percent. Starting and breakdown torque levels are reduced from historically high values but are still within NEMA minimum requirements. The locked rotor torque, nonetheless, is still impressive. As the motor was not optimized for copper rotors, the torque problem can be corrected with modifications to the lamination slot design. |
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