Electromagnetic bearing for high temperature environment

Electromagnetic bearing within a high temperature environment. The bearing comprises electromagnets, each one constituted by at least one coil made of conductors, and one armature in ferromagnetic material. Each coil is produced by weaving and comprises a warp constituted from at least one insulated conducting wire and a weft constituted from a strip of insulating material, the whole coil assembly being thereafter molded in a high temperature cement.

The present invention relates to an electromagnetic bearing designed to be 
placed in a high temperature environment and comprising electromagnets, 
each one constituted of at least one coil and one armature in 
ferromagnetic material. 
The structure of a magnetic bearing is now well-known and the applications 
of such bearings continue to extend. For a number of special conditions of 
operation, the conventionally known structure of magnetic bearings has not 
proved completely satisfactory and has been found rather fragile. 
This is the case for example, when a magnetic bearing is designed to be 
used in a machine inside which very high temperatures prevail. For 
example, inside a high-temperature reactor comprising a blast-engine 
working with hot helium, the end of a shaft may be brought to temperatures 
of about 1000.degree. C. and the bearings to temperatures of around 
400.degree. and 500.degree. C. With such working conditions, it has been 
proposed already to mold the coils in a high temperature cement in order 
to produce a massive block which is more able to withstand the action of 
heat. Under repeated thermal shocks, the block of cement tends however to 
crack and short-circuits may occur inside the coil, the wires of which are 
only very slightly insulated. 
It is the object of the present invention to overcome the aforesaid 
disadvantages and to enable the production of magnetic bearings working 
reliably even when they are fitted inside an environment of high 
temperature. 
This object is reached due to the fact that according to the invention, 
each coil is produced by weaving and comprises a warp constituted from at 
least one insulated conducting wire and a weft constituted from a strip of 
insulating material, the whole coil assembly being thereafter molded in a 
high temperature cement. 
A woven coil, in which the different turns of one particular conducting 
wire are held one with respect to the other in specific relative 
positions, and without contacting, permits to eliminate definitely all 
possibility of short-circuits, even if under the action of thermal or 
mechanical shocks, the cement block cracks or if the insulating wire layer 
of the coil is damaged. 
Thus the warp wire and the weft tape of the coil are woven loosely, 
providing interstices which help the penetration of the molding cement. 
Advantageously, the insulated conducting wire is constituted by a 
nickel-plated copper wire insulated by a layer of ceramics, wich can in 
turn be coated with a layer of varnish which is designed to prevent the 
ceramics layer from becoming detached during the winding operation. 
The tape of insulating material can be a tubing of glass-fiber. 
The coil according to the invention is easy to manufacture from an assembly 
composed of grouped wires connected in parallel, presented for example as 
a twine, which is woven with the insulating tape. A blotter-like ceramic, 
or any other heatwithstanding material such as glass-wool can also be 
placed on the side portions of the coil between the ends of the insulating 
tape (not shown in the drawings).

FIG. 1 shows how, in principle, a woven coil is produced according to the 
invention. An insulated conducting wire 1 forms a warp, whereas an 
insulating tape 2 constitutes the weft of the weave formed by interweaving 
the wire 1 and tape 2. The weaving is relatively loose and a space e is 
provided between two adjacent turns of the conducting wire 1 to produce 
interstices facilitating the penetration of a coating cement which is 
designed to produce a massive block around the weave. However, even in the 
event of cracks appearing in the massive block, the formed weave keeps 
relatively fixed distances e between two adjacent turns of the conducting 
wire 1 so that said turns are never contacting with one another. In 
practice, the different adjacent turns of a layer are produced 
successively from a single wire or from an assembly of wires and every 
time, the tape 2 is inserted between adjacent portions of wire. 
The insulating tape 2 which is used as a weft designed to hold the turns of 
the conducting wire in well-defined respective positions can be 
constituted by a tubing in braided glass-fiber. The thickness of tape 2 is 
about a few tenth of a millimeter, for example 0.5 mm, and its width is a 
few millimeters for example 2 mm (see FIG. 3). 
The conducting wire 1 (FIG. 4) is advantageously constituted by a copper 
core 10 coated with a layer 11 of nickel, which layer is itself coated 
with an insulating layer 12 of ceramics. A varnish can also be coated over 
the insulating layer 12 to prevent same from becoming detached during the 
winding operation. The diameter of the core 10, 11 in nickel-plated 
copper, of the wire 1 is about a few tenths of a millimeter and remains 
preferably under one millimeter. In those cases where strong energies are 
required, a plurality of wires 1 are grouped in parallel and form a unit 
1' (see FIG. 7). It is thus possible to easily avoid using conducting 
wires of too large a diameter which are always difficult to wind, and to 
reduce the radius of curvature of the coil 100. 
FIG. 5 shows a coil 100 with several layers of insulated conducting wire 1 
interwoven with a flat insulating tape 2 which keeps a pre-set distance 
between two adjacent turns produced from the insulated conducting wire 1. 
The coil 100 which is produced around a yoke 6 composed of a stack of 
ferromagnetic plates 61, is molded in a high temperature cement 4, which, 
due to the interstices provided between the different wires 1 kept apart 
one from the other by the tape 2, can be distributed evenly throughout the 
coil. 
In case of repeated thermal shocks, cracks appear in the molded-over part 
4, but the weave nonetheless provides a good mechanical strength to the 
coil and preserves the insulation even though the ceramic layer 12 on the 
wire 1 may be partly damaged. The weaving indeed ensures permanent 
cohesion of the massive block 4, even if the latter is cracked. Thus the 
present invention permits to use ordinary insulated conducting wires, even 
relatively fragile wires capable of having their ceramic insulation 
damaged under the effect of thermal shocks, without this affecting the 
good operation of the assembly due to the distances which are permanently 
kept between the turns of the wire 1 of the coil 100. 
In the side portions of the winding, and in particular close to the side 
walls 5 of the covering 4 (FIG. 5), a ceramic blotter can be inserted to 
increase the quality of the insulation. Any other material withstanding 
heat, such as glass-wool can also be used. 
As already indicated, it is possible when effecting the weaving to use a 
single conducting wire 1 or a single group 1' of wires 1 connected in 
parallel, to form not only adjacent portions of wires in one single layer, 
but also to form successively several superimposed layers of the coil. The 
turns of each layer can then be offset with respect to the corresponding 
turns of the adjacent layers. 
FIG. 6 shows, in the radial plane of the device, a shaft 20 supporting an 
annular armature 22 constituted of laminated sheet-metal 21, a stator of 
providing an electromagnetic bearing comprising a yoke 6 composed of 
laminated sheet-metal 61 and having four pairs (Ex, Ex', Ey, Ey') of poles 
around which are wound the coils 100 according to the invention. Each one 
of the coils 100 may be composed of a plurality of layers of wire 1 woven 
with a flat tape 2, as shown in FIG. 5. The electromagnetic bearing 
assembly is thus adapted to withstand very high temperatures.