Motion System Design

Reeling in efficiency with cast rotors

An ac induction rotor starts as a stack of steel plates stamped with slots. Either molten aluminum is injected into the slots and flowed over the ends, or (in very large motors) bars of aluminum or copper are pushed through them to form conductive poles. But both cast aluminum and bar rotor types have inherent inefficiencies: In the former, balance can be temperature dependent (because aluminum expands when heated) and conductivity is limited. In the latter, current must fight through imperfections at bar/end-ring solder joints. These inefficiencies not only make for electrical energy losses (for loss of power) but also generate heat — which in turn causes bearings, laminations, and other sensitive motor parts to fail faster.

Die-cast copper rotors are one alternative that eliminates these inefficiencies. Copper is more conductive than aluminum, and casts don't have the imperfections of bar construction. But casting copper is so difficult that until now, only limited quantities of custom motors were made with cast-copper rotors. For starters, copper's melting point is very high — 1985° F compared to aluminum's 1221° F — so it takes more energy to melt. And, copper-casting tools take a bigger beating: thermal cycling usually destroys aluminum molds after 15 to 20,000 shots, but molds for copper rarely survive half that long. Another challenge: Even when it's fully melted, copper is very viscous. In large molds especially, by the time slow-flowing copper gets to one end, areas on extremities can begin to freeze — and create voids and imperfections in the cast.

Now Siemens Energy and Automation, Inc., Alpharetta, Ga., has addressed these casting issues to make cast-copper rotors for larger integral (1 to 20 hp) standard-production motors. Because denser copper is inherently more conductive (reducing electrical-energy losses by 20% over aluminum rotors) less heat is generated, so cooling fins are eliminated — and this in turn eliminates wind losses. Too, because the casting results in less porosity, balancing is easier and doesn't require sprues.

The end result is a shorter, torque-denser motor that exceeds NEMA-premium efficiency ratings.

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