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Copper Versus Aluminum Rotor Construction

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.  

Copper ... 

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, ETI's copper rotor construction gives you greater reliability, efficiency, and versatility with which aluminum cannot compare.

• 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.  ETI's copper rotor design ensures higher reliability compared to aluminum bars.
• Tensile Strength - 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.
• Higher Melting Point - Copper can better withstand thermal cycling over the life of the motor.
• Conservative Temperature Rise - Copper features conservative temperature rise limitations which prevent excessive deterioration in mechanical properties.
• Improved Joint Design - ETI'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.

Aluminum ... 

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.  ETI uses only copper on large AC machines because our experience leads us to believe that aluminum rotors are more likely to fail.

• 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.
• Fatigue Sensitivity - Many tests indicate that aluminum is more susceptible to fatigue than copper or copper alloy.
• High Temperature Rise - 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 heat-tested aluminum will decrease even at temperatures as low as 150º C. Temperature rises on bars much higher than this are not unusual during starting and acceleration.
• Poor Joint Design - Welded Aluminum joints, with attendant stress concentration, residual thermal stresses, and reduction in physical properties, can result in failure.
• Laminations Become Loose - Through repetitive thermal cycling, aluminum laminations can become loose, resulting in early failure.

Our experience leads us to believe that aluminum rotor designs are more likely to fail and are more difficult to repair compared with copper.  The primary causes for most rotor failures are loose rotor bars and poor bar to end ring welds.  By now you already understand how copper's inherent advantages -lower coefficients of expansion, higher tensile strength, and improved joint design -make it a superior material.  ETI's practice of swaging, brazing, and thru-bolt construction further ensure long- term reliability.  In the unlikely event that a copper rotor does fail, it is significantly easier to repair than aluminum. Copper's usual rectangular design makes it easy for repair shops to rework bars and laminations.  Aluminum bars, on the other hand, are built in a wider variety of shapes with correspondingly designed slots in the rotor laminations.  We believe that the resulting lamination slot design often makes aluminum rotors either difficult or impossible to repair.  Don't be misled by aluminum's lower initial selling price. Because of the risk of failure, aluminum may cost you a lot more in the long run.

         

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Your Electric Motor Specialists !