February 2009 Electrical Apparatus

February 2009 Electrical Apparatus

This is a summary of the Electrical Apparatus February 2009 featured technical article,  by Richard L. Nailen, P.E.    

In a centrifuge–sometimes called a separator or clarifier–a container rotating at high speed, either horizontally or vertically, causes centrifugal force to separate components of differing mass from a liquid suspension. This is useful in many industrial operations such as sugar refining, and in medical laboratory work. Two characteristics define acceleration capability of the driving motor: low load counter-torque; and extremely high inertia because centrifuge rotational speed typically far exceeds standard motor RPM (belt drives are common).

High system inertia would ordinarily cause severe motor overheating. Temperature rises in squirrel-cage rotor bars and end rings are normally calculated on the basis of all acceleration heating being stored in those components. Acceleration time is too short to allow dissipation of much of the heat to the core laminations or surrounding air.

However, because centrifuge load torque is low, reduced-voltage motor starting will greatly prolong acceleration time. Star-delta starting is a common practice. When acceleration then takes from three to six minutes, much of the generated heat will flow into the rotor and stator cores, reducing the peak temperature rises.

Thermal calculations take several forms, depending upon motor size and manufacturer experience. When rotor bar depth exceeds 20-25 mm, for example, the designer may take account of the deep-bar effect by considering that within successive time intervals during acceleration the fraction of each bar throughout which current flows–and therefore within which heat is generated–will predictably increase. To allow for heat dissipation from the bars, the adjacent lamination teeth may be added to the bar mass early in the acceleration, and the remainder of the core laminations may be added later. Both influences will be greatest in a cast rotor cage.

As further aids to cooling in a fabricated rotor cage, bars may be extended further than usual from the ends of the core. That reduces bending stress where the bars emerge from the slots, caused by thermal growth of the end rings. Also, extra fan blades will act both as heat sinks and as additional air movers.

To order a back issue with the full article, “When long acceleration is desirable: the centrifuge drive,” call 312-321-9440 or visit our online webstore.

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