Residual Magnetism in High Speed Rotating Machinery.

Residual magnetism in high-speed machinery accounts for many previously unexplained machinery failures. In particular, the deterioration of bearings, seals, gears, couplings and journals has been attributed to electrical currents in machinery. Often, such trains or machinery groupings contain no components with electrical windings or intended magnetism, i.e., no motors or generators. The evolution of turbine and compressor systems towards high speeds and massive frames is acknowledged as the cause for a new source of trouble from magnetic fields.

An electrical generator converts mechanical power to electrical power through magnetic fields. A conventional generator rotor is essentially a magnet that is rotated in such a manner that its magnetic field flux passes through coils of windings. This produces an electrical voltage and power in the windings.

A turbine, compressor, or any other rotating machine that is magnetised behaves much the same way.

The magnetic steel parts provide a magnetic circuit, and are also electrically conducting so that voltages are generated, producing localised eddy currents and circulating currents. These currents will be either alternating or direct, and can spark or discharge across gaps and interfaces, resulting in erosion of component material in the form of frosting, spark tracks, and, in the extreme, melting and welding. They can cause increased temperatures and initiate severe bearing damage.

Fixed pad thrust bearing showing moderate amount of frosting due to spark erosion. Unit was equipped with OEM carbon brushes. (source: sohreturbo.com)

Typical frosting on bearing and seal area. This case is unusual in that frosting extended around only 1/2 of the circumference.(source: sohreturbo.com)

The field levels due to residual magnetism in turbo-machinery occur not from design but from manufacturing, testing, and environmental causes. They have been measured at the surface and in gaps of disassembled parts of a machine at levels ranging from a few gauss to thousands of gauss (1 Tesla = 10,000 Gauss). These increase significantly in the assembled machine where the magnetic material provides a good closed path for the magnetism and the air gaps between parts are reduced considerably. This combination can set up conditions for generation of notable stray voltages and the circulation or discharge of damaging currents.

There are a number of ways in which steel machinery parts can become magnetized. Placing a part in a strong magnetic field can leave substantial residual magnetism. Mechanical shock and high stressing of some materials can also initiate a residual field. Another method of creating residual magnetism is the passing of electrical current through the parts. In increasing order of their effect, following are the known examples: Electric system faults; nearby heavy electrical currents, such as rectified supplies and chemical processes; and lightning.

Electrostatic discharges, which are credited with causing bearing and seal pitting, can also play a role in magnetisation of shafts. The use of electrical welders and heaters on pipes and other parts is common and, if not used properly, will induce residual magnetism. Items that have been subjected to magnetic particle inspection often retain residual magnetism because of insufficient or improper demagnetizing techniques following the test. Components that have come in contact with magnetic chucks and magnetic bases often display multiple adjacent poles of a residual field

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