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Principle: A movable piece of a magnetic core is pulled into the gap by the reluctance force
The reluctance force or Maxwell's force arises due to the change in magnetic resistance , which is also referred to as reluctance . The reluctance force always works in such a way that the magnetic resistance decreases and the inductance increases and is attributable to the magnetostatics .
the face of the magnetic circuit at the air gap through which the field lines of the magnetic field pass
the sum of the size of both air gaps.
The (idealized) area available for the magnetic circuit is given by
The direction of the deflection is irrelevant, hence the amount bars . The size denotes the depth.
Inserting supplies
so that on the moving part of the deflected core a force
acts that pulls him towards the center. This is independent of the size of the deflection, except if the above derivation is no longer valid. This is the case when it gets too big.
Variable air gap
Tensile force in the air gap
The same applies as above
.
The following applies approximately to the inductance
Inserting it into the formula for gives the result:
.
Since the inductance increases when the air gap is reduced, the reluctance force acts in this direction. The force decreases with the width of the air gap. The maximum of the reluctance force is reached when the air gap approaches zero. However, if the air gap is very small, the approximate formula for the inductance no longer applies, since the magnetic resistance of the core can then no longer be neglected.
literature
Hans-Dieter Stölting, Eberhard Kallenbach (ed.): Manual electrical small drives . 3. Edition. Hanser, ISBN 3-446-40019-2 , pp.460 .