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Thread: Refurbishing a pair of 2440

  1. #46
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    You cannot make the magnet stronger than its specification. However if the charging pulse is too weak or too strong (overcharging) the magnet will not reach the specified magnetic flux.

    Because of this one should measure the gap flux after charging. This is why the cone (or diaphragm) should be removed during charging. You cannot measure with the cone still mounted.

    If the guy doing the charging is experienced, he will probably put just the right amount of charge into the magnet with the cone still mounted.

    I have read somewhere that low frequencies will discharge alnico magents. So cone speakers might need recharging. Compression drivers will be less affected. I had two tweeters, four 1" drivers and a 2220 checked. Only the 2220 needed recharge.

  2. #47
    Senior Member herki the cat's Avatar
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    Quote Originally Posted by grumpy View Post
    ....http://cnx.org/content/m28345/latest/ ....I'm not sure I agree that overcharging is really an issue....
    grumpy,You are absolutely correct. No one has yet presented proof...."that overcharging is really an issue".... Very likely the problem has been "Cock Pit Errors, Operator Care-Lessens, or Charger Hardware Failure if not a "Defective Work Piece Assembly."

    Three Magnet Charging items were always observed in the RCA Camden factory:
    #1) Always begin with a completely demagnetized Magnet Structure.
    #2) Ramping up the charging current very slowly, & especially ramping down the current to zero. A failure mode causing "Instant switching of the charge current to zero with out instant disconnection from the current source" will result in a demagnetizing event due to the severe current transient in the charging winding L/C components.
    #3)
    Always provide an air gap, like a 2 inch thick wooden block, between the Magnet Work Piece and the vertical Charger Component snugging the work piece in place, other wise, removing the charged magnet structure from intimate clamping between the top & bottom poles of the charger would severely de-magnetize the work piece.

    At page bottom, See figure 1.15 "Magnetic circuit including both a permanent magnet and an excitation winding."

    grumpy, thank you for presenting the link:
    http://cnx.org/content/m28345/latest/ of The Textbook [# 1 Electric Machinery - A.E. Fitzgerald, Charles Kingsley, Jr., Stephen D. Umans- 6th edition- Mc Graw Hill series in Electrical Engineering. Power and Energy] which describes with "Maxwell's Equations" a leveling off of flux value due to saturation of the magnetic iron."

    Assuming the hard magnetic material is initially unmagnetized, corresponding to point (a) of the figure 1.16, consider what happens as current is applied to the excitation winding. With a core of infinite permeability, the horizontal axis can be considered a measure of the applied current & a measure of H, the magnetic field intensity, in the magnetic material.

    As the current i is increased to its maximum value, the B-H trajectory rises from point (a) toward its maximum value at point (b). To fully magnetize the material, assume the current has been increased to a value/max sufficiently large that the material has been driven well into saturation at point (b). When the current is then decreased to zero, the B-H characteristic will begin to form a hysteresis loop arriving at point (c) at zero current. At point (c), notice that H in the material is zero, but B, the magnetic flux density is at its remnant value Br.

    As the current then goes negative, the B-H characteristic continues to trace out a hysteresis loop.
    In Fig. 1.16, this is seen as the trajectory between points (c) and (d). If the current is then maintained at the value H to the (d) power, the operating point of the magnet will be that of point (d). Note, this same operating point would be reached if the material were to start at point (c) and with the excitation held at zero, with an air gap inserted in the core.

    Should the current then be made more negative, the trajectory would continue tracing out the hysteresis loop toward point (e).

    However, if instead the current is returned to zero, the trajectory does not in general retrace the hysteresis loop toward point (c).

    It then traces a minor hysteresis loop between point (d) & point (f) at zero current. If negative polarity current is then varied between zero Br point and point (d), the B-H characteristic will trace out the minor loop as shown.

    This happens when the voice coil current is driven into the negative polarity region from "zero drive" Br point toward point (d), creating the minor loop/(d to f) recoil line in Alnico alloys.
    herki[Quote/]

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    Last edited by herki the cat; 01-18-2012 at 08:24 PM. Reason: DE NATHA

  3. #48
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    Well. I was only quoting the guy who did my remagnetization. He has been working with magnetization for at least 20 years. It could be something with their machine that made the result less optimal if "overcharging".

    Anyways, you should be able to measure BL after recharging, so there is no need to remove the cone for measurements.

  4. #49
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    Would it be possible to see if the optimum charge has been applied to the magnet by a proxy such as measuring the impedance curve of the driver?

  5. #50
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    For a cone driver you can measure BL, for a compression driver I donīt know.

  6. #51
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    Forum member Charles0322 asked if I could send the drawings for the throats. Unfortunately I had lost the original drawings, so I had to make a new improved version. This drawing has an exponential flare, mine are straight.

    I have used a cheap caliper measuring both the original throat and the one I made previously. This drawing should be 0.05 mm accurate.

    My throat has more material compared to the original. It should fit snugly in the casing and magnet, so it can be used to center the magnet when remagnetizing.

    I have also calculated fc of the throat, it is 186,5 Hz
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