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.
For over a century, Westinghouse motors have been synonymous with quality and reliability. Over time, product enhancements have been continually refined, result.ing in dependable, long-term performance year after year.
Today, as always, all large Westinghouse Motor Company Induction motors greater than 250 HP feature copper or copper alloy rotor bars and end rings.
Some of our competitors will try to persuade you to save a few dollars and settle for aluminum rotors on their large machines. We believe you cannot afford to operate without copper rotor construction. Please read on and decide for yourself.
The real test for rotor material is during starting because this is when the motor is subjected to extreme operating conditions. During this period, large AC motors can be rotor limited . This means that the rotor cage will reach its thermal limit before the stator winding.
In this demanding period of operation, copper offers a significantly higher range of resistivity and a much higher thermal margin.
Copper also provides a lower coefficient of expansion, higher tensile strength, and ideal conductivity.
Without a doubt, Westinghouse Motor Company's copper rotor construction gives you greater reliability, efficiency, and versatility with which aluminum cannot compare.
Cast aluminum rotors appeared years ago on small motors and are now accepted as the industry standard for those machines.
Today, many motor manufacturers have switched to fabricated aluminum rotors In their large motors, presumably because of the lower cost of aluminum.
In fact, most of our competitors have standardized on aluminum rotors.
Aluminum rotors have many limitations compared with copper. For example, an aluminum die cast rotor cannot be repaired. And aluminum fabricated rotors only can be repaired with great difficulty.
The Westinghouse Motor Company uses only copper on large AC machines because our experience leads us to believe that aluminum rotors are more likely to fail.
These damaged aluminum rotor bars were removed from competitors' failed motors. Note the severe deformity and degradation.
Copper Advantages :
- Lower Coefficient of Expansion
Aluminum will creep and move approximately 33% more than copper. This large movement will eventually lead to fatigue failure in rotor material due to thermal expansion and contraction.
Copper is 300% stronger than aluminum and thus able to withstand high centrifugal force and repeated hammering of the current-induced forces during each start.
Copper can better withstand thermal cycling over the I ife of the motor.
- conseivative Temperature Rise
Copper features conservative temperature rise limitations which prevent excessive deterioration in mechanical properties.
Westinghouse Motor Company's improved bar to end ring joint design minimizes stresses and stress concentrations.
Swaged rotor bars ensure long life by minimizing the movement and vibration that can cause bar fatigue and failure.
- High Frequency Induction Brazing
End rings are joined to the bars by high frequency Induction brazing on most rotors. This reduces stresses and hot spots in the joint.
These ensure increased rotor integrity by preventing cage migration from end to end.
Aluminum Disadvantages :
- Limited Range of Resistivity A
far greater range of resistivity is available on copper alloys than with aluminum and aluminum alloys. As a result, performance may have to be compromised if aluminum is used.
Many tests indicate that aluminum is more susceptible to fatigue than copper or copper alloy.
Aluminum has a 35% higher temperature rise than copper for the same KW loss.
- Adverse Effect of Temperature on Mechanical Properties
The physical properties of heattested aluminum will decrease even at temperatures as low as 150° C. Temperature rises on bars much higher than this are not unusual during starting and acceleration.
Welded Aluminum Joints, with attendant stress concentration, residual thermal stresses, and reduction in physical properties, can result in failure.
Through repetitive thermal cycling, aluminum laminations can become loose, resulting in early failure.