Permanent Magnets
There are different type of magnets available
- Alnoco
- Ferrite (Ceramic)
- Samarium Cobalt
- Neodium Iron Boron
Selection of magnet plays a major role in performance and
cost. Ferrite magnets are most popular and is inexpensive.
Samarium cobalt and Neodium magnet are high performance.
Neodium is popular in high performance application and cheaper than samarium cobalt.
Magnets are made in two types bonded and sintered
Bonded magnets are made by suspending powdered magnet
material in non conductive non magnetic resin. Magnet formed in this way are
not suitable for high performance application since substantial fraction of
magnet is made by non magnetic material
Sintered magnets are made by sintering process allows magnet
to be formed without bonding agent
Permanent magnets are magnetic material with large
hysteresis loop
The hysteresis loop is formed by applying large possible
field intensity to an unmagnetized material and shutting it off. This allows material
to recoil and is called demagnetization curve
If two ends of magnets are shirted by infinite permeable
material and material recoils and attain a final point at H=0. The flux density
leaving the magnet at this point is equal to remanence or residual flux density
Br
Remanence is the maximum flux density that the magnet can
produce by itself
If permeability surrounding the magnet is zero, no flux will
flow-out of magnet, final point attained is B=0 at this point magnetic fled
intensity across the magnet is equal to negative of coercivity or coercive force
Hc
For permeance value between zero and infinitive operating
point of magnet lies in second quadrant
Between remanence and coerecivity.
The magnitude of slope in second quadrant is known as
permeance coefficient Pc
When Pc =0, magnet is operating at coerecivity B=0 , H = -Hc
Pc = infinity B=Br and H=0
Samarium cobalt and Neodium magnet have straight demagnetising
curve throughout the second quadrant at room temperature
At higher temperature demagnetizing curve shrinks towards origin.
As demagnetizing curve shrinks towards origin, flux
available from orgin drops however this effect is reversible
The maximum energy product BHmax is the maximum product of
flux density and field intensity
BHmax is usually mentioned in Million of Gauss-Oer-steds
MGOe
