Kent, OH--For decades, vacuum cleaners and other floor-care appliances have used the venerable brush-type motor. And while brush-type motors provide good performance at low cost, they do have one drawback: relatively short life. In a vacuum cleaner, a brush-type motor typically lasts from 500 to 1,500 hours. Unless you happen to be the Oscar Madison type who doesn't vacuum at all, that's not a particularly long time.

Now, however, engineers from the Lamb Electric Div. of Ametek believe they've found a cost-effective alternative to the brush-type motor. Using switched-reluctance technology, they've boosted the life of the vacuum cleaner motor to at least 5,000 hours. And they've accomplished that while maintaining the cost of the motor at $150 or less. That price tag, they believe, will put the switched-reluctance technology well within the cost constraints of commercial vacuums, central vacuums, and commercial scrubbers.

Although virtually all appliance manufacturers use brush-type motors because of their relatively low cost, several vacuum-cleaner manufacturers are testing the Infin-A-Tek, as Ametek calls the new switched-reluctance motor. It's yet another development in the appliance world that aims to leverage technology to help us keep our spaces clean. Previously, Cybermotion Inc. developed a mobile cleaning robot called "Dottie" that reportedly tidies up office buildings in four to six shifts of continuous work. Sporting a 2/3 hp brush-type dc motor, a vacuum with 150-cfm air flow and a remote PC, among other technology, "Dottie" cleans 8,000 to 18,000 sq ft of floor space per hour.

Although available for many years, conventional switched-reluctance motors did not offer the combination of long life and low cost that Ametek's Infin-A-Tek motor reportedly offers. To achieve both goals, Ametek engineers applied a unique twist to conventional switched-reluctance technology. Their concept: a two-phase, four-pole design.

The two-phase, four-pole design enabled them to significantly cut cost because it eliminated a substantial number of switching transistors, capacitors, power field effect transistors, and diodes from the control board. Three-phase designs, for example, typically employ as many as six big switching transistors. The new motor uses just two. And three-phase designs typically employ at least five big capacitors, while the new motor again uses just two.

An on-board microprocessor controls current switching in the stator by sensing the position of the rotor to soft-start the motor and control its speed and torque. Most slow starts in other brushless motors require positioning magnets on the rotor. Possible add-on features include performance shaping for peak power and optimum performance; external speed control; and remote on/off switching and control.

Ametek engineers say they could have used two-phase switched- reluctance technology long ago, but would have faced other barriers to success. Two-phase designs, they say, would have been plagued by mechanical and electrical noise, as well as efficiency issues.

To solve those problems--while still maintaining low cost--they made two key design changes to conventional technology. First, they employed an asymmetrical stator configuration. "By switching the motor's pole off the true 90-degree center line, we were able to make the material in the laminations thicker, and therefore make the part more robust," notes Gene Bennington, engineering manager for Ametek's Lamb Electrical Div. Result: The stator vibrates less and operates more quietly than it otherwise would.

To maximize efficiency and minimize electrical noise, Ametek engineers used a unique rotor configuration that was designed to work in conjunction with the stator. Result: less harmonic noise, greater efficiency.

The new design therefore dramatically lowers the cost of switched reluctance technology, while dealing with the technical issues that have previously prevented it from competing in the floor-care market. That's critical, because lower cost enables it to leapfrog past brushless designs in such applications. Up to now, three-phase brushless technology has offered much longer life than brush-type motors, but at costs as high as $350 per motor.

With its combination of cost and performance, the Infin-A-Tek is expected to have a niche in floor- care applications. "Under normal operating conditions in residential vacuums, brush-type motors perform fine," Bennington says. "But if you put them in an industrial or commercial application, such as a hotel, brush-type motors don't offer long enough life. That's why we developed this switched-reluctance design. It meets the needs of those applications without breaking the bank."

Switched reluctance: rugged and fast

The switched-reluctance motor is gaining popularity in recent years, mainly because it offers an unusual combination of ruggedness, reliability, and high-speed operation.

The motor, which derives its name from the magnetic circuit characteristic that produces flux, is deceptively simple-looking. It uses no brushes, windings, bars, or permanent magnets. Instead, its rotor is essentially just a stack of laminations. "It's inherently simpler and less expensive than other rotors," notes Dave Gill, a senior project engineer for Ametek.

Appearances aside, however, the switched-reluctance motor is hardly simple. Because it has no windings, it doesn't use the time-honored technique of moving the rotor by creating a magnetic field. Instead, it produces flux lines between opposing poles of the stator. As the flux lines move, the rotor follows along, thus creating torque. "You have to keep stepping the rotor around--it's like keeping a carrot one step ahead of a mule," Gill says.

The real complexity of the motor lies in the shaping of the current responsible for the flux lines. To accomplish that, the motor uses microprocessor control and well-tailored software. Together, the two reconfigure the current in response to changes in speed.

Such complexity makes customization more difficult than it would be with, say, a series universal motor. Still, the motor offers features that users can't get elsewhere. "It's a highly engineered motor," Gill says. "But the advantages make the engineering worthwhile."

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