Degaussing

Does anyone know a mal695:ethod of removing the magnetic field?

Dear al695.

Last year we have successfully to degaussing of GE Turbine 180 MW and Rotor Generator , this scope is a part of the total scope for rewinding of rotor generator 180 MW, to removing the magnetism in the parts ( shaft, gear, bearings, casing, rotor, blades, etc) you requred a auto degaussing instrument such as MPS, pls check out the following article and my experiences to remove of magnetizing in the Turbine parts and Rotor generator.

Before degaussing the magnetism are 20 to 52 gauss, we degauss up to 0 - max 6 gauss on the rotor generator (slot portions) and 0 - 2 gauss at shaft, bearings, turbine rotor, couplings, etc and 0-6 gauss on the turbine casing.

Any questions and required technical support you can contact me at :

sis_cahya@yahoo.com

fig1. gauss measurement on the LP Turbine by digital gauss meter

Fig.2. Gauss Measurement on the generator coupling

Fig.3__ gauss measurment on the LP Turbine Shaft

Fig.4__Degaussing on the Rotor Generator Shaft

Fig.5__Degaussing on the bearings

Fig.6__ Degaussing on the bearing supports

Fig.7__ MPS Auto Degaussing

Fig.8__ Digital Gauss Meter

Fig.9__Gaus Meter Analog

Fig 10__Degaussing on the rotor generator when rotor still in place

Demagnetizing

Magnetism in Machinery, account for many machinery failures, in particular the deterioration of bearings, seals, geras, couplings and journal has been attributed to electrical currents in machinery. Often on machinery groupings contain no components with electrical windings or intended magnetism, ie. No motors, generators. Manufacturers of electrical equipment have recognized and protected against the effects of electrical shaft currents, bearing insulation has been utilized for such purposes.

There are 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 that can creating residual magnetism is the passing of electrical current through the parts, electrical system faults nearby heavy electrical currents such as rectified supplies and lightning, electrostatic discharges, which are credited with causing bearing and seal pitting, the use of electrical welding and heaters on pipes and other parts is common and if not used properly can induce residual magnetism.

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.

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 be insidious sources in magnetization 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 following the test.

Components that have come in contact with magnetic chucks and magnetic bases often display multiple adjacent poles of a residual field.

Field Measurement and Demagnetizing

Magnetic fields are measured with devices called gaussmeters. As the name implies, these meters measure magnetic flux density in units of gauss. Most meters utilize a probe that works on the "Hall Effect." This type of probe utilizes high frequency currents in its semiconductor chip to produce a characteristic proportional to the magnetic field. Usually, only the very tip of the probe is field sensitive.

Fields that pass perpendicular to the face of the Hall Effect semiconductor are measured. The Hall Effect probe performs expertly in measuring direct (DC) magnetic fields. Its accuracy, and the interpretation of just what is being read if it is employed for measuring alternating (AC) fields, is uncertain and questionable.

Reliable AC gauss or milligauss measurements are usually obtained employing separate circuitry in the gaussmeter, and a different probe encompassing an AC fields sensor coil. It can be used on operating machines or in environments containing alternating fields.

Magnetic fields are directional, being "North" at one end and "South" at the other. Hall Effect probes are directionally sensitive. If the probe is flipped over, a reverse reading is indicated, consistent with sensing either a North or South pole. With digital metering, the sign of the field is automatically indicated (+ or -). Analog meters with a zero center scale will read to the left or right of zero. Keeping one side of the probe (such as the side with the calibration number) always facing away from the object being measured and calling it a North pole when the meter reads positive, establishes a convention for locating North and South poles, and the orientation and path of the magnetism that is creating the poles.

This arbitrary choice for a North pole may be opposite from the established convention; however, it is fully adequate in locating magnetic circuits and their magnetizing forces for purposes of degaussing. Should agreement with established convention on polarity identification be important, the North pole side of an identified reference magnet may be utilized for determining which outward-facing side of the Hall Effect probe produces a positive magnetism reading, which should be labeled North.

A good gaussmeter has a calibration means to verify proper probe calibration, as well as a zeroing adjustment. The zeroing procedure requires that the probe be temporarily inserted in a "zero gauss chamber." These small chambers shield out stray fields, including the Earth's field, so that a true zero field is realized. A typical Gaussmeter

Maximum Allowable Residual Magnetic Field Levels (As Measured in Open Air)
2 gauss:	Bearing components, including pads and retainers, journals, thrust disc, seals, gears and coupling teeth.
4 gauss:	Bearing housings.
6 gauss:	Mid-shaft and wheel areas, diaphragms, etc.
10 gauss:	Components remote from minimum clearance areas, such as casings, pipings, etc.

Reference :

Principles of Magnetism and Stray Currents in Rotating Machinery

By Paul I. Nippes, P.E., President Magnetic Products and Services, Inc.

Good Luck

Rgds

Siswanto