Such a field is generated in known manner by an electromagnetic inductor which, in the case of stirring a molten metal, may be referred to as a "stirrer". The inductor includes the following components:
a magnetic circuit constituted by a yoke for looping a magnetic flux and including wide teeth which form a sequence of projecting pole pieces;
prefabricated excitation windings, each constituted by an insulated conductor wound about a winding axis, the windings being coaxially mounted around respective pole pieces of the magnetic circuit; and
electric power supply means for supplying the windings with excitation electrical currents, in particular alternating currents having phases forming a sequence in time that is associated with the sequence of poles so as to generate a moving magnetic field.
The magnetic circuit of a stirrer is conventionally formed by a stack of plane magnetic laminations which are clamped together by means of tie bars. The magnetic circuit may be manufactured using two different known manufacturing methods. In the first one of these methods, the magnetic circuit is obtained by mechanically assembling:
a yoke made of a stack of plane magnetic laminations; and
pole pieces each of which is made of a stack of plane magnetic laminations and is then mechanically fixed to the yoke; in this case, by construction, the pole pieces are detachable and are referred to as "attached" pieces.
In the second method of manufacturing a magnetic circuit, the circuit is made of a stack of plane laminations which have been cut out so as to have a solid portion and wide teeth; in this case, by construction, the yoke and the pole pieces are integral.
In order to make such a stirrer, the large alternating electromagnetic forces to which the conductors in the winding are subjected when in use make it necessary to use shims, bindings and an impregnation resin.
Cooling the windings by causing water to circulate is also known.
Cooling may be internal. In which case, the water flows through the inside channel of a tubular conductor making up the winding. This type of cooling requires distilled water to be used and leads to problems arising from the need to make relatively complex and bulky hydraulic connections inside the limited volume available for the inductor and from the need to use sufficiently powerful pumps.
These hydraulics problems are simplified in inductors using external cooling. In this case, the windings are disposed inside a housing in which cooling water is caused to circulate. The windings are therefore immersed in the water and they are cooled via the walls of the winding block they form together with the resin with which they are impregnated. Sealing arrangements are provided so as to prevent any water reaching the copper of the conductors.
In such a cooling system, the use of distilled water is no longer necessary, but the life-span of such immersed windings is essentially linked to the continued effectiveness of the sealing arrangements against the surrounding water--both for the conductors and for the impregnation. That is why numerous precautions are taken in selecting the insulating materials, the manufacturing and impregnation methods, and the cooling water--in particular, efforts are made to obtain water having a low solids content, especially of ferromagnetic particles, and to limit the size of these particles. In spite of all these precautions, the life-span of such windings is limited in the long-term and generally (for all windings) by the water-absorption capacities of the various materials currently available. These absorption capacities relate to microscopic pores through which water leaks develop towards the masses of metal constituted essentially by the magnetic circuit of the inductor. The power dissipated by the leaks transforms the microscopic pores into macroscopic pores which leads to complete breaks in the insulation in the medium term. Since the copper is then no longer protected, it is attacked by electrolysis and this generally leads to the winding conductor breaking.
The life-span may also be reduced drastically and locally (e.g. on one winding) by accidental phenomena (e.g. a foreign body in the volume containing the windings, a malfunction in the water circuit, etc.).
Therefore, there are two problems. The first is to increase the overall life of the inductor. The second is to facilitate replacing a faulty winding.
An object of the present invention is to solve these two problems more simply and better than before.