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Demagnetizing Effect of External Magnetic Field

The operation of a permanent magnet in a circuit is expressed by the B-H characteristics shown in the second quadrant and is usually analyzed using an equivalent network. Although, for design purpose, it is possible to analyze the operation of a static magnetic circuit by simply analyzing its second quadrant.
We must also be able to analyze the magnetizing force(4πI=B-H) when the circuit is to be used as a revolving-field type (for example, generators and/or motors) in which demagnetizing fields are induced by the armature.
The coercive force in the second quadrant (BHC) is the value of magnetic field which causes the flux density B to become O and the intrinsic coercive force (iHC) is the value of magnetic field which causes the magnetization (4πI) to become O.
Therefore, a magnetic circuit operating in an external field like the reaction of an armature is analyzed using the B-H curves including those in the third quadrant (viz. 4πI-H curve) instead of a B-H curve drawn only in the second quadrant.

Graphic Analysis of Demagnetizing Action

See Fig. 3. A 4πI-H curve is developed from the B-H curve.
The operating point obtained form the magnetic circuit related to the operating line Pc is at point A on the B-H curve and the corresponding point on the 4πI-H curve is B.
Suppose that a demagnetizing field △H is applied to a magnet; the operating point will then move to point D on the B-H curve.
This point is determined by drawing the operating line Pi′ parallel to the operating line Pi to obtain an intersection of the Pi′ line with the 4πI-H curve (viz. point C) and then drawing a perpendicular line from point C to intersect the B-H curve.
After the demagnetizing field has been removed, the operating point will move to point A′.
This tells us that the flux density at the initial operating point has been reduced from Bd1 to Bd2 by the application of a demagnetizing field.
This means that a non-reversible demagnetization equivalent to Bd1 minus Bd2 has occurred in the magnet.    