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Answer by Hugh Falkner
The lower resistivity of copper compared to aluminium means that using it for the motor rotor bars will give a lower resistance (higher conductivity of 57 MS/m vs. 37 MS/m) and hence reduced energy losses. This benefit can be taken in two ways:
- Higher efficiency in the same size.
- More compact motor with the same efficiency. The cost savings in reduced laminations and stator wire approximately compensate for the higher rotor cost.
Depending on the details of the motor design, copper rotors can offer a mixture of the following additional benefits:
- Less heat generated, and so less need for fan cooling, with attendant savings in energy and noise.
- Less heat generated, and so longer motor lifetime.
- More frequent stop starts due to more rapid conduction of heat from the rotor bars to the end rings.
- Shorter stack, enabling installation in tighter spaces.
- High quality of casting with low risk of porosity.
- Low corrosion in critical applications.
- Lower total weight of motor.
For the motor designer, a new rotor lamination design will be necessary to maximise the benefit of using lower loss copper, and changes to the die-casting process will be needed to achieve high quality casting. But apart from the design changes above, in use the user should not be aware of the change to a copper rotor motor.