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Frequently Asked Questions About Rules of Thumb and Quick Calculations

  1. How are energy product and magnet strength related?
  2. How do I determine the magnetic flux density at a particular distance away from a magnet?
  3. What is the "working l/d ratio" for various magnet materials?
  4. What is the holding force of a magnet?
  1. How are energy product and magnet strength related?
    Magnet strength needs to be defined. Magnet strength actually is a measure of total flux in the area of interest. If greater holding strength is desired, then more Br is needed, and since holding force is proportional to B^2, the improvement will be proportional to the ratio of Br's squared. In a spectrometer application, charged particle deflection will be directly proportional to the Br increase. If resistance to demagnetization is the reason for wanting "greater strength", then a magnet with a higher Hci value is needed. Even if the high Hci magnet shows lower open circuit flux density, it could provide more under operating conditions.

  2. How do I determine the magnetic flux density at a particular distance away from a magnet?
    To calculate simple field values, you can use the Calculators available in the Resource Center of our website.

  3. What is the "working l/d ratio" for various magnet materials?
    "Working l/d ratios" are those which produce a load line through the Bd/Hd values associated with the maximum energy product. Typically you would want to use a l/d ratio somewhat higher (in the final circuit) to protect against unplanned demagnetization effects.
      
    As a rule of thumb, the "equivalent" Length/Diameter ratio for an isolated magnet should be (0.5 * Br) / Hc, or more. That would give an Alnico 5 magnet length/diameter ratio of 10, while ceramic or rare earth would operate with a 0.5 ratio. A high permeance coefficient is especially important for Alnico 5 because of its low coercivity and the pronounced "knee" in its second quadrant demag curve. Its low resistance to demagnetization makes Alnico susceptible to demagnetization due to shock, heat or magnetic fields from adjacent magnets.
      
    It should be emphasized that the working permeance coefficient is the prime concern; the value achieved when the magnet is placed in the final circuit. Open circuit permeance coefficients are only important if the magnet operates by itself, or the value is so low that irreversible flux losses are likely before the magnet is placed into the magnetic circuit. In the latter case, magnetization after assembly would be advised.

  4. What is the holding force of a magnet?
    Force is calculated by
      
    F = 0.577 * B^2 * A
      
    where B is the flux density in the gap in kG, and A is the magnet area in square inches.
      
    For round or rectangles shapes, use current sheet calculations to estimate B in the gap, or at the end of the magnet. These equations are available in uner the Tools section of the navigation bar on our Web site. They can also be found in ourReference and Design Manual, availabel for download under our Engineering Support section.
      
    If the magnet is in contact with steel, double the magnet length since the steel will mirror the magnet's properties. For round or rectangles some distance off a steel keeper, use current sheet equations to calculate flux at half the separation distance. This is a ballpark calculation and empirical values are usually higher.