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Demag effects include:

• Once the rotor magnet is first removed from the magneto (first exposed to the maximum air gap), the rotor loses up to 25% of it's energy. The magneto will never be as strong after you remove the coil or magnet from the magneto.

• Once you throw the magneto rotor on the workbench and let other metal articles stick to it (spurious contacts) the rotor looses up to 25% of it's energy. The magneto will never be as strong.

• After the first exposure the rotor stabilizes and repeated exposure to the same air gap or the same spurious contacts does not further reduce the magnet's strength

Working Gap Variation

Starting with a demagnetized rotor magnet, as an external magnetizing force (H) increases, the magnet becomes increasingly magnetic (B). Eventually the magnetizing force has a decreasing effect, so that at point Bs the magnet is fully saturated and is incapable of further increases in magnet strength.

 

What is a Hysteresis Loop? A plot of magnetization versus applied magnetic field When a ferromagnetic material is magnetized in one direction, it will not relax back to zero magnetization when the imposed magnetizing field is removed. It must be driven back to zero by a field in the opposite direction. If an alternating magnetic field is applied to the material, its magnetization will trace out a loop called a hysteresis loop. The lack of retraceability of the magnetization curve is the property called hysteresis.

Demagnetization is accomplished by pulling a component out and away from a coil with AC passing through it. The same can also be accomplished using an electromagnetic yoke with AC selected.

 Once the external magnetic field is turned-off the magnet drops down the demagnetizing curve from point Bs to point Br. This is the highest point of residual magnetism that the magnet can retain. Notice that point Br is at the zero axis of the demagnetizing force axis (H). Point Br only occurs when the magnet's force field is contained within the magnet. This can only occur in a fully closed ring (no air gap between the north and south pole). Any leakage of magnetic force lines imposes a demagnetizing force on the magnet. This demagnetizing force (-H) moves the magnet down the hysteresis curve to some level below Br.

 If the magnet poles are exposed to an air gap the magnetic lines of force can not so easily travel from pole to pole and the magnetic circuit is opened. The air gap causes a demagnetizing force (-H) that drops the magnet down the demagnetizing curve to point A. As the air gap increases there is a reduction in the forces holding the magnetic domains in alignment thus the effect of an air gap is that some of the domains shift out of alignment and demagnetization occurs. A new hysteresis loop is formed at point A with a lower level of magnetic induction (strength). This loop is the dashed inner loop in Figure 2. Notice that magnetic induction is reduced from Br to Cr. When the magneto for the first time a new hysteresis loop is formed at point A.

 Keepers are used to minimize leakage of the magnetic field. If we place a piece of soft ferromagnetic material between the magnet north and south pole then most of the field travels from pole to pole and very little escapes from the magnet. Keepers, by shorting the magnetic poles, minimize the demagnetizing force.

 Once the keeper is removed from the rotor magnet or once the rotor is removed from the magneto and exposure to an air gap reduces magnet strength by approximately 25%. The lost magnet strength does not return when the rotor is reinstalled into the magneto rotor magnets.

Spurious Contacts

Contact demagnetization occurs when any ferrous material comes in contact with a magnet at a location other than the pole faces. Such spurious contacts redirects the magnetic field away from the other pole causing localized domain realignment and often occurs when magneto rotors are placed on the work bench where screws, washers, and other parts attach themselves to the magnets.

Contact demagnetization is progressive in that the more contacts that occur the lower the magnet falls on the demagnetization curve until approximately 40 contacts where the magnet stabilizes. The avoidance of air gaps and spurious contact demagnetization is avoided by the use of keepers and proper handling.

 Contact demagnetization does not occur in some magnet materials such as the rare earth or ferrite magnets. Such magnets have  a high coercive force.  

 Steps in obtaining maximum rotor magnet induction:

 1. Demagnetize the magnets to establish a new hysteresis curve.

 2. Magnetize the magnets while keepered minimizes air gap demagnetization. Special blocks called Mu blocks are used for this.

3. Prevent exposure to spurious contacts or fields to minimize demagnetization.

 4. Install rotor into magneto coil in place minimizes air gap demagnetization.

 5. Allow magneto minimizes air gap demagnetization.

6. Bendix rotors have a N mark on the rotor. Magnetize these with the north magnetic field at the N. Otherwise use the same field direction. We have seen some Bendix rotors magnetized with the field direction reversed. I believe that many  magneto magnets are from Alnico (an alloy of aluminum, nickel, iron, cobalt) 5 or 8 which are anisotropic grades and have a preferred direction of magnetic orientation. 

 Conclusion

 The energy output of the magnetos operate at reduced energy levels because of improper rotor magnet care. This is not surprising since to my knowledge there has been nothing previously published concerning this subject. 

Easy Comparative Method of Measuring Magnet Strength

   If you own a pocket compass you can accurately comparatively measure magnet strength. The basic idea is that the earth pulls on the magnetic compass with a strength of approximately .45 Gauss (horizontal component).  Bring a  magnet  towards the compass at 90 degrees to the NS needle until the magnet pulls the NS needle with the same strength as the earth -- 45 degree deflection.

At this distance the rotor is pulling on the compass with a strength of .45 Gauss. Now mark this distance. Place another magnet at the same spot. If the compass needle points less than 45 degrees then this magnet is weaker than the first magnet. You can compute the difference in strength by taking the tangent of the degree difference. 

For example, if the compass moves 6 degrees toward N then the magnet is 10.5% weaker

For my set-up using magneto magnet rotors, I mount the compass on a long cardboard box at one end and put a hole in the other end so I can set the rotor shaft into the hole. This allows me to rotate the rotor to get the point of strongest attraction. Then I move the compass on the box until I get the 45 degree reading.  I mark the spot on the box where a fully magnetized rotor pulls the compass at 45 degrees. Now I can compare any rotor quickly and easily.

Shipping Magneto Rotor's

According to the United States Department of Transportation and the Office of Hazardous Materials Safety, it is against regulations to ship a magnet by air that maintains a field of more than 0.00525 gauss measured at 4.5 meters (15 feet) from any surface of the package. This is to prevent the magnet from interfering with the operation of the aircraft's navigational compass. At the time of this writing there are no federal regulations that restrict the shipping of magnetized materials by ground transportation.

Notes & Definitions

note 1

If a magnet has not been stabilized, thermal cycling provides external energy to allow weak magnetic domains to reverse and bringing it down the demagnetization curve. Thermal cycling within the magnetic circuit (reduced air-gap) does not have as large an affect.

Before the Curie temperature is reached where the metal may undergo metallurgical changes that adversely affect the amount of magnetization.

Airgap: A gap in the magnetic circuit i.e. open circuit. Can be air or a non magnetic material.  An airgap exists when a magnetized magnet is by itself with no external flux path of high permeability material.

Coercive force: The demagnetizing field required to reduce the magnetic induction to 0 in a fully magnetized magnet

Keeper: Soft magnetic material placed between pole of a magnet to reduce the airgap

Anisotropic magnet: A magnet with a predetermined direction of magnetization, in which the properties are superior

Curie temperature: The critical temperature when a magnetic material loses its magnetic properties.

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