Rotating Machinery Services

Magnetism in Machinery

Magnetism in 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.

Since the turn of the century, manufacturers of electrical equipment have recognized and protected against the effects of electrical shaft currents. Bearing insulation has customarily been utilized for such purposes.

Only since the mid-1970's has the need for protective measures on totally mechanical systems been fully realized. 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. Propitious placement of the coils in slots and other design features result in the conversion of mechanical energy to electrical energy. This produces electrical voltage and power in the windings that is then delivered to the electrical load or power system.

A turbine, compressor, or any other rotating machine that is magnetized behaves much the same way. The magnetic steel parts provide a magnetic circuit, and are also electrically conducting so that voltages are generated, producing localized eddy currents and circulating currents. These currents will be either alternating or direct, and can spark or discharge across gaps and interfaces, producing sparking with frosting, spark tracks, and, in the extreme, welding. They can cause increased temperatures and inflict or initiate severe damage.

The generator action occurs as a result of relative movement between the magnet and the "conductors." Hence, either the frame of a machine or the rotor can be magnetized, and the same action exists when relative motion occurs between the rotating and stationary parts.

The magnetic field density in the air gap of assembled and operating motors and generators is designed to be in the order of 7,000 to 9,000 gauss. These fields are capable of generating from watts to megawatts of electrical power, depending upon the speed and size of the generator.

The field levels due to residual magnetism in turbomachinery 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 from 2 gauss to thousands of 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 of damaging currents.