Permanent Magnets

Permanent Magnets

There are different type of magnets available

1. Alnoco
2. Ferrite (Ceramic)
3. Samarium Cobalt
4. 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