August 2010 Electrical Apparatus

August 2010 Electrical Apparatus

This is a summary of the Electrical Apparatus August 2010 featured technical article,  by Richard L. Nailen, P.E.  

Magnetic bearings that support a motor’s rotating shaft by a magnetic field rather than by an oil film were widely publicized more than 20 years ago as suitable for large, high-speed electric motor applications, in which lubrication problems and bearing failures had caused costly downtime. Such bearings consist of a laminated toroidal stator containing a set of peripheral electromagnets, surrounding a laminated rotor pressed on the shaft. An electronic control circuit continuously monitors the rotor position and adjusts magnet strength to keep it centered.

The commercial promise was never realized. Early bearing designs were bulky and expensive, difficult to maintain, of uncertain longevity, and involved complex electrical wiring to external control systems.

They did, however, offer one great advantage over conventional oil-lubricated journal (sleeve) bearings: the capability of operation at 10,000 RPM or more without friction loss. Consequently, a number of magnetic bearings were successfully used in large, high-speed turbines and compressors. No pressurized oil flow system was needed to supply lubricant and dissipate the heat generated by friction within the oil itself.

More recently, design improvements have produced magnetic bearings that are much smaller, with self-contained control circuitry, adaptable to 3600 RPM motors down to ratings of a few hundred kilowatts. They offer enhanced stability of the shaft/rotor assembly and reduced vibration. Heat losses appear comparable with fluid-film bearings but with the probability that external cooling (as with sleeve bearing diameters of about 90 mm or more) would not be needed. A slight improvement in motor efficiency is also expected from the elimination of bearing friction.

Magnetic bearings require a “backup bearing” to support the shaft long enough for safe shutdown if the magnetic bearing itself failed. At 3,600 RPM, that backup or “catch” bearing is a sleeve type, containing semi-solid lubricant, that can safely withstand a number of failures of the magnetic bearing.

Magnetic and sleeve bearings are not interchangeable, not only dimensionally but also because the fluid-lubricated bearing requires an oil sump, oil ring guides, and other complexities. Whereas repair or replacement of the sleeve bearing without motor disassembly, that’s not true for the magnetic bearing, which is not to be repaired in the field.

To order a back issue with the full article, “Is there new life for magnetic bearings?” call 312-321-9440 or visit our online webstore.

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