Induction furnaces having a crucible for receiving metallic material to be melted, hereinafter called a "charge", are well known. Such a crucible is generally called a "cold crucible" because it is not substantially heated by the induction device, and it is permanently cooled by a cooling device. In such furnaces, it is also well known to cause levitation of the charge by using electromagnetic confinement phenomenon, resulting in a separation of the charge to be melted from the inner crucible walls.
It is common practice to make the crucible from a plurality of metal segments, electrically isolated from each other to reduce electromagnetic losses in the segments. Collectively, the segments form the metallic wall of the crucible.
It is also well known to form such a crucible in a generally cylindrical shape. The bottom part of this shape is hemispheric or conical and includes a central hole for the release of the melted charge.
A major preoccupation in the design and operation of such furnaces is avoidance of excessive heating of the crucible wall. The division of the crucible wall into a plurality of segments, electrically insulated from each other results in a reduction of the induction effects on the wall and limits heating of the wall. However, it is not possible to completely suppress such heating. Furthermore, the charge melting in the crucible transmits a certain amount of heat to the crucible wall. Therefore, it is necessary to substantially cool the segments forming the crucible wall. Generally, this cooling is carried out by forming the segments of the crucible wall of a metal, such as copper, which conducts heat, and by providing holes in the segments. These holes accommodate pipes extending parallel to the longitudinal axis of the crucible, and containing a cooling fluid such as water.
Generally, the segments forming the crucible are relatively thick because they have to be large enough to contain the diameter of the cooling pipes provided inside the segments. As the segments necessarily have a thickness greater than the penetration depth of the magnetic field generated by the induction system, it has not been possible to substantially avoid magnetic induction effects in these segments. Thus, in order to limit induction effects, the lateral dimensions of the crucible, i.e., its diameter, have been very limited. The width of the segments could not be made less than a determined value because they must include at least one longitudinal hole for a cooling pipe. Thus, design flexibility has been seriously curtailed.
For these reasons, known furnaces must use magnetic induction means operated at intermediate frequencies, i.e. greater than about 400 Hz. Such furnaces have other drawbacks such as high manufacturing costs, which increase with the number of segments used. Moreover, input and output pipes for the refrigerant must be provided for each segment. It is also necessary to provide the fluid connection of these pipes, as well as the electrical insulation separating the segments from each other. Such a crucible is therefore difficult to produce and its cost is relatively high.
Another drawback is that magnetic induction coils operating at intermediate frequency allow only relatively low currents. Therefore, the voltage across these coil terminals is relatively high, greater than 80 volts, for example. It is therefore necessary to take precautions with such systems to avoid risk of electrocution and to limit electric arcing.
Yet an additional drawback is that in order to supply an intermediate frequency electric current to such a magnetic induction system, it is necessary to provide an intermediate frequency electric supply transformer. Such a supply system is relatively expensive, particularly if it delivers high levels of electric energy. It has not been possible heretofore to build an induction furnace having a cooled wall, with a capacity to contain a volume of charge greater than about 5000 cm.sup.3. Generally, the inner diameter of such a crucible is never greater than 150 mm.
In such prior art furnaces, a problem arises concerning the mixing of metal melted in the crucible. This mixing is often not satisfactory and this has heretofore limited furnace capacity.