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Hysteresis Loop

A lag between cause and effect.

When the switch is closed, as seen in Drawing 3, the current (Drawing 2) is increased and flux density (Drawing 1) increases until saturation point b is reached. This part of the wave form (from point a to b ) is exactly the same as the B-H curve.

The current continues on beyond saturation point b; however, the flux density cannot increase beyond saturation.

At point c the magnetizing force (current) is zero and B (flux density) falls to a value c that is the positive magnetic flux remaining after the removal of the magnetizing force H.

This particular value of flux density is called remanence or retentivity.

The current or magnetizing force now reverses. The amount of current in the reverse direction that causes flux density to be brought down from c to zero d after the core has been saturated is called the coercive force. The current, and therefore magnetizing force, continues on toward a maximum negative until saturation in the opposite magnetic polarity occurs at point e.

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