Indianapolis-Vertical, self-discharging, centrifugal separators must operate at high speeds for separation, then achieve high torque at low speeds for scraping. Classic designs are complicated, involving two motors connected to the load with chains, sprockets, pinion gears, and clutches. One motor spins the rotor at high-speed during the separation cycle, the other, typically a gearmotor, must produce enough torque to break the solids free from the bowl wall during the discharge cycle.

"Such complexity makes this class of centrifuges notoriously unreliable, and difficult to work on," explains Jeffery Beattey, president of centrifuge-manufacturer Midwest Engineered Products Corp. That's why he recently invented the CentraSep(TM), an all-new centrifuge with a patented bowl/ blade clutch.

An ac motor drive from ABB (New Berlin, WI) allows a single ac motor from Marathon Electric Mfg. Corp. (Wausau, WI) to achieve both high speeds and high torque. "As it eliminates a number of moving components, using a single-motor is key to improving the reliability and efficiency of this centrifuge," notes Beattey.

Since gearmotors essentially trade off speed for torque, it wouldn't make sense to use one to spin the rotor at high speed. So Beattey looked at several different ac motor control and drive scenarios before coming up with the ABB solution. "None of the drives we tried had the torque we needed at the low end, and they couldn't handle the load on deceleration without tripping out or breaking resistors," explains John Emmert, president of Carmel, IN-based Electrical Controls Design, a builder of industrial control panels for original equipment manufacturers. Emmert is the electrical designer charged with building the panel, and making the PLC, drive, and motor, all work together on the CentraSep project.

"No one has ever tried a single-motor design before. It was questionable whether the single motor would work right up until we were actually able to run fluids in it, collect solids, and then scrape them out of the bowl," explains Beattey. "In fact, we even designed a second hole into the motor mounting plate for a gearmotor driving a three-pulley system with an overrunning clutch, just in case we couldn't get it to work."

Beattey never had to implement the gearmotor back-up plan because an ACS 600, 10-hp unit solved the problem. However, it wasn't an out of the box solution. Working with engineering help from ABB, Emmert and Beattey co-developed the proprietary software needed to program and operate the drive at extended torque parameters.

A Siemens S7226 PLC and the ACS 600 drive work together to achieve high torque output at speeds less than 100 rpm. In fact, says Emmert, "we can command and receive 200% of the rated torque of the motor for two seconds."

"We couldn't have done it without the ABB drive," says Beattey. The secret to the ACS 600 drive's success, according to Kaylan Gokhale, R&D manager for drives at ABB, is its open-loop Direct Torque Control (DTC), which hit the market in the mid-1990s.

DTC is what Gokhale calls a "native" encoder-less torque control. It controls torque without actually measuring the rotor position, unlike other vector control methods such as SVC (sensorless vector control) or FOC (field oriented control) that were first conceived as encoder-based, full field-oriented control, then modified using sensor-less torque control algorithms to eliminate the encoder.

"ABB drives calculate the motor torque and flux 40,000 times per sec," says Gokhale, "achieving more precise torque control so that the drives can adapt to and handle changes in load, over-voltages, and even short circuits as a result."

For more information on drives from ABB: Enter 501 For more information on motors from Marathon Electric Mfg. Corp.:Enter 502 For more information on PLCs from Siemens: Enter 503

Additional Details

Contact Jeff Beattey, Midwest Engineered Products Corp., 5060 East 62^nd St., Indianapolis, IN 46220; Tel: (317) 257-2714; Fax: (317) 255-7399: www.centrasep.com, or Enter 504