Electrical Apparatus magazine, February 2003 Which rotor design, copper or aluminum?

Because of cost, structural integrity, and design flexibility, standard squirrel-cage motors up to several hundred kilowatts in rating are universally manufactured with die-cast aluminum rotor conductors. In larger sizes where that is impractical, fabricated cages (made up of individual bars and end rings) use either aluminum or copper and its alloys. Some manufacturers have standardized on one or the other, but most will supply whichever one best suits the application.

Comparisons between copper and aluminum have emphasized differences in melting point, specific heat, conductivity, and physical strength. However, most such comparisons are seriously flawed. The differences between the two materials normally render impossible any direct replacement of one by the other in the same large motor.

To achieve the same accelerating torque, locked-rotor current, efficiency, and safe stall time, copper and aluminum rotor bars must have quite different shapes and areas. Even the depth of current penetration at locked rotor (the "deep bar effect") differs between the two materials (regardless of bar shape or size). Those variations affect full-load speed, power factor, and stray load loss as well as temperature rise.

Among the many conflicting and misleading statements made in comparing copper and aluminum rotors are these:

1. "Copper is 300% stronger than aluminum" (untrue; the difference ranges from 0% to 40% depending upon the type of applied stress).  

2. "Yield strength of...aluminum...[is] somewhat higher than copper" (the reverse is true for the commonly used aluminum alloy).

Other claims are valid but insignificant. For example, during heating and cooling, an aluminum bar does expand lengthwise in the slot more than copper. But the difference is slight; the problem has always existed for copper, and is controlled by various manufacturing techniques equally usable for aluminum.

As in other electrical apparatus, such as switchgear, aluminum can function as well as copper in many motor applications. Depending upon the specific performance requirements, either version can provide long service life.

Industry news...

Featured Technical Article
February 2003

On this page is a summary of the Electrical Apparatus February 2003 featured technical article, by Richard L. Nailen, P.E. , "Which rotor design, copper or aluminum?" Welche Rotor-Ausführung - Kupfer oder Aluminium? ... La conception du rotor doit-ell utiliser le cuivre ou l'aluminium? ¿Cuál diseño de rotor es mjor, cobre o aluminio? ...

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