Liquid stirrer with magnetic coupling

Details Liquid stirrer with magnetic coupling Liquid stirrer with magnetic coupling

2001-04-05

2003-02-11

Drodge, Joseph W. (Department: 1723)

Agitating

Stirrer within stationary mixing chamber

Magnetic stirrer

C366S274000

Reexamination Certificate

active

06517231

ABSTRACT:

This application is a 35 U.S.C. sec. 371 of PCT Application Ser. No. PCT/FR99/02385, filed on Oct. 6, 1999.
The present invention relates in general to liquid stirrers, and more particularly to stirrers with magnetic coupling.
DISCUSSION OF THE RELATED ART
Stirrers with magnetic coupling generally transmit rotary motion without contact from a driving magnetic portion to a driven magnetic portion. The driving and driven portions are disposed respectively outside and at the bottom of a receptacle of liquid to be stirred. This transmission of motion can be described as “coupling of the axial type without guidance for the driven portion”.
Compared with less-recent stirrers having mechanical coupling, transmitting rotary motion without contact makes it possible to avoid having passages through the walls via mechanical rotary gaskets which present a risk of leakage.
FIGS. 1A and 1B
show two known stirrers with magnetic coupling.
The stirrer of
FIG. 1A
comprises a driving magnetic portion
1
which can be rotated about an axis by a motor
2
, in particular via transmission means
3
, and a driven magnetic portion
4
. The driven portion
4
is placed on the bottom of a receptacle
5
which is supported by means of a support
6
over the driving portion
1
. The receptacle
5
and the support means
6
are made of non-magnetic material. The driving and driven portions
1
and
4
, the transmission means
3
, and the motor
2
are centered on a vertical axis of symmetry
8
. The driven portion
4
is typically a permanent magnet in the form of a bar having a pair of north and south magnetic poles. The driving portion
1
is constituted by a U-shaped permanent magnet whose magnetic poles face the magnetic poles of the bar
4
when the stirrer is at rest. The magnets
1
and
4
are generally made of materials such as ferrites or alnicos (aluminum, nickel, cobalt).
When the motor
2
is in operation, the driven portion
4
is rotated about the axis of symmetry
8
by magnetic coupling with the driving portion
1
via an air gap
7
. More precisely, when the driving portion
1
rotates about the axis
8
under drive from the motor
2
, torque is transmitted to the driven portion
4
, thereby causing it to turn about the same axis.
The receptacle
5
contains chemical reagents in liquid form. Rotation of the bar magnet
4
about the axis enables the liquids contained in the receptacle
5
to be stirred and, for example, favors the production of precipitates which are evacuated via an evacuation outlet
50
on a side face of the receptacle
5
. With such a precipitation reaction, the stirrer shown in
FIG. 1A
can be referred to as a“precipitator”.
FIG. 1B
shows another known stirrer. In
FIG. 1B
, those elements which are identical to elements of
FIG. 1A
are designated using the same references. The stirrer of
FIG. 1B
differs from the stirrer shown in
FIG. 1A
in that the driving portion
1
, the motor
2
, and the transmission means
3
are replaced by a static drive
9
powered by an alternating (square wave) voltage source
10
. The static drive
9
has vertically disposed electromagnets (one of which is shown diagrammatically in
FIG. 2B
) which are powered by a voltage delivered by the source
10
and are switched in alternation. The static drive
9
produces the same effect as the elements
1
,
2
, and
3
in
FIG. 1A
, i.e. it produces a rotating magnetic field which causes the bar magnet
4
to rotate by magnetic coupling about a vertical axis of symmetry
8
′.
The stirrer with static drive shown in
FIG. 1B
presents several advantages over the stirrer with rotary motor of FIG.
1
A. In particular, it does not require mechanical moving parts to be used and it is more compact. In addition, with the stirrer with static drive it is possible to vary the torque transmitted to the driven portion merely by modifying the amplitude of the current delivered to the coils of the electromagnets. In the case of the stirrer with rotary motor, the transmitted torque can be adjusted only by physically varying the size of the air gap by means of a mechanical device.
A major drawback of prior art stirrers as shown in
FIGS. 1A and 1B
lies in the fact that the amount of torque that can be transmitted to the driven portion is limited. Increasing this torque increases the attraction force pulling the bar
4
against the bottom of the receptacle and increases wear by friction both of the bar and of the bottom of the receptacle.
FIGS. 2A and 2B
are diagrammatic front views of the relative positions of the driving and driven portions when the driving portion is constituted by a permanent magnet
1
(
FIG. 2A
) and when it is constituted by electromagnets
9
(FIG.
2
B).
FIG. 2C
is a diagrammatic plan view showing the arrangement of FIG.
2
A. As shown in
FIG. 2C
, when the stirrer is in operation, the driven portion
4
continuously lags behind the rotary field produced by the driving portion by an angle &agr;. The field lines, represented by arrows
11
(
FIGS. 2A and 2C
) and
12
(
FIG. 2B
) between the poles of the driving and driven portions have a horizontal component (
FIG. 2C
) which contributes to the torque transmitted to the driven portion, and a vertical axial component (
FIGS. 2A and 2B
) parallel to the axis of rotation
8
,
8
′. The axial force due to the axial component constitutes a very large fraction of the interaction energy between the driving and driven portions. Any increase of the torque transmitted to the driven magnet
4
automatically increases the axial attraction between the driving and driven portions and thus increases wear of the driven magnet
4
and of the bottom of the receptacle
5
because of the presence of a significant axial component in the field lines.
SUMMARY OF THE INVENTION
The present invention seeks to provide a liquid stirrer which, compared with prior stirrers, makes it possible to reduce the wear on the driven portion and the bottom of the receptacle for given torque transmitted to the driven portion.
To this end, the invention provides a liquid stirrer with magnetic coupling comprising a driving portion and a driven portion, the driven portion being for placing on the bottom of a receptacle containing a liquid to be stirred, and control means for controlling the driving portion so as to drive the driven portion in rotation about a predetermined axis of rotation by means of magnetic coupling with the driving portion, the stirrer being characterized in that the driving and driven portions are configured so as to encourage the field lines that result from the magnetic coupling to extend substantially perpendicularly to said axis of rotation in the vicinity of the driven portion.
In practice, the driving and driven portions are preferably configured so as to encourage the field lines resulting from the magnetic coupling to extend substantially parallel to a longitudinal axis of the driven portion in the vicinity of the driven portion.
The predetermined axis of rotation is typically, but not necessarily, a vertical (virtual) axis of symmetry of the driven portion and/or of the driving portion. When the stirrer is in operation, the driven portion is merely placed on the bottom of the receptacle and is therefore subject only to its own weight, to friction forces with the bottom of the receptacle, and to the electromagnetic forces generated by the driving portion through the receptacle. The receptacle, or at least a portion thereof close to the driving portion, is made of a non-magnetic material, so as to allow the field lines to pass through.
Thus, contrary to prior stirrers, in the stirrer of the invention, a large part of the field lines in the vicinity of the driven portion have an axial component (parallel to the axis of rotation) which is small compared to their horizontal component. The axial attraction force which is an undesirable force because of the wear phenomena to which it gives rise both on the driven portion and on the bottom of the receptacle, is therefore smaller for identical torque. Consequently, a larger torque c

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