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Machine Design

Motor and drive put new spin on centrifuge

Manufacturing firms that rely on deionized water, especially those using electrical-discharge machining, face the problem of superfine particles getting into the water and increasing its conductivity. To keep the water free from superfines and any other particles measuring between the submicron level and one-half inch, engineers at Midwest Engineering, Indianapolis, designed a centrifuge that uses a single ac motor and drive and cleanses water at a rate of 135 gpm.

The CentraStep's design synchronizes the bowl and blade assembly. The assembly has two blades for cleaning the bowl and two stilling vanes for centrifugal processing. A positive-locking clutch couples the bowl's main spindle and blades so both rotate at precisely the same speed when processing fluids. The motor links to the main spindle via a single chevron-style, nonslip timing belt. Synchronization ensures heavier particles are thrown outward against the bowl. It also prevents blade oscillation, thus minimizing bearing wear while maximizing separation efficiency. A thick, stainless-steel bowl further reduces vibration and oscillation.

To quickly accelerate and turn the bowl against the scraper blades, the designers used a motor with high breakaway torque and precise motor control the ACS 600, 10-hp drive with built in open-loop Direct Torque Control from ABB Automation Inc., Milwaukee. The combination calculates the proper motor torque and flux 40,000 times/sec. It lets the drive handle changes in load, overvoltages, and short circuits almost immediately. This level of responsiveness makes the motor practically tripless, says ABB. And the lack of an encoder for feedback from motor to drive reduces costs for the drive by as much as 25%. MTBF for the drive is at least 150,000 hr, and the company has recorded no drive failures in any production units.

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