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This system group is today used principally in stacking systems because
it allows a high switching frequency at the same time as a low compensation
current; this suits the increasing stacking speed.
Functional principle
The principle of the design of these magnet systems is presented on the
right side. Two types are distinguished. The Type
1 system shows the permanent magnet, the coil and the flux conductance
parts. The permanent magnet drives a magnetic flux through the iron items,
with the result that the ends of the field lines are able to leave the
north pole which is generated and enter the south poles. The pattern of
the field lines is, in rough approximation, similar to that of a semicircle.
It is also approximately the case that the radius of the semicircle is
a dimension for the deep action of the system. Deep action should be understood
in this context to refer to the distance at which a 0.5 mm thick item
of sheet metal just jumps with the whole area of the magnet system being
taken up. The permanent magnet is restricted on one side by a U-section,
on the other, an iron plate forms the close. This iron plate has a double
function. Firstly, it collects the magnetic flux of the permanent magnet
and conducts it via the centre bridge to the north pole of the system;
secondly, the iron plate forms the bypass for the flux compensation in
the event of the current flowing through the coil; the bypass is described
in greater detail below. If there is no current flowing through the coil,
the adhesive power of the magnet system is generated solely by the permanent
magnet. In the case of the coil, it is now possible to make current flow
either in the supporting direction or in the compensating direction. The
supporting direction refers to the permanent magnet being supported by
the coil current and the adhesive power thus being still further increased.
If the current flows in the compensating direction (usual case), the adhesive
power of the permanent magnet is weakened. This weakening can be taken
so far that almost none of the field lines leaves the magnet system, resulting
in the adhesive power disappearing. In this case, the iron plate on the
magnet has the function of conducting the magnetic flux of the coil in
such a way that the magnetic flux of the permanent magnet is exactly compensated.
This logic implies that the magnet system is magnetically neutral when
a current is flowing, or, viewed the other way, that the max. adhesive
power occurs when no current is flowing. This corresponds to the normal
type of operation in sheet metal stacking. The exact opposite is the case
as regards electromagnets.
Type 1: Displacement system
with one permanent magnet
Special
displacement system for holding and transporting sheet metal
Type 2A: Displacement system with two permanent magnets
Type 2B: Enclosed displacement system with two permanent magnets
The system of Type 2A is equipped with 2 magnets which
have their poles facing each other. These permanent magnets lead the magnetic
flux through the centre bridge of the adhesion area so that the indicated
polarity is generated at the adhesion area. The advantages of this version
can be seen in the higher adhesive power (2 magnets), although the greater
system height may cause difficulties. In particular, this is a system
which is „open“ at the top, i.e. the fieldlines leave the
system „upward“ to the same extent also; it should therefore
be ensured that no ferromagnetic material may be used directly above the
hanging installation of the system, since this would have the effect of
a magnetic „short circuit“. This would greatly reduce the
adhesive power. Type 1 systems are enclosed, have a low
construction height and can be more easily compensated as a result of
the bypass.
This configuration is recommended when RES magnets have to be used as a result of a high adhesive power requirement, because these magnets are markedly more difficult to compensate in the Type 2A configuration, since there is no bypass. As a result, the magnet itself must be weakened by the coil current. Owing to the high coil current, the heating is greater, leading to the possible switch-on period being shorter. The preferred use for displacement systems of Type 2A can thus be seen in the case of slower systems which have lower switching frequencies, such as in the case of sheet loaders and load-elevating systems.
These restrictions mean that one version has established itself which has an enclosed construction and which may be regarded as a compromise between Type 1 and Type 2A. This version is presented as Type 2B in the following sketch.
The iron casing at the back of the system makes it possible to obtain the magnetically enclosed configuration. However, the distance between the north pole, which is open at the back, and the large iron area (south pole) is very critical. If too small a distance is chosen, a great amount of adhesive power is lost. On the other hand, this „bypass“ helps in terms of compensation. This distance thus allows the compromise of high adhesive power and low compensation current to be optimized.
The systems of Type 1 and Type 2A may of course be interconnected, resulting in multipole versions.
