Industrial electric heating furnaces – Induction furnace device – For furnace regulating
Patent
1996-11-21
1999-08-17
Hoang, Tu Ba
Industrial electric heating furnaces
Induction furnace device
For furnace regulating
373139, 373148, H05B 606
Patent
active
059404272
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a crucible induction furnace with at least two coils, which are connected in parallel to a load-commutated tuned circuit converter, run round the wall of a vertical melting crucible, are disposed one behind the other along the axis of the crucible and are connected in series with a capacitive impedance.
2. Description of The Prior Art
It is a universally known practice, when melting metals in a crucible induction furnace of the type stated above, to use induction coils to generate eddy currents in the material to be melted and thereby heat the metal. The melting power transmitted by a coil increases with the magnitude and frequency of a voltage applied to the coil. In order to be able to operate with higher frequencies than the mains frequency, use is generally made of tuned circuit converters, to the secondary side of which the induction coils are connected. However, the level of the secondary voltage in the case of tuned circuit converters is limited due to the tolerance to voltage of the semiconductor components.
It is furthermore common practice to increase the voltage applied to the coils of a crucible induction furnace by connecting a capacitive impedance in series with mutually parallel induction coils to the secondary side of a tuned circuit converter.
The purpose of the upstream capacitive impedance is to bring the tuned circuit into the resonant oscillation range. To explain its action, an ideal series tuned circuit with a coil of inductance L, a capacitor of capacitance C and a vanishing resistance is considered below. In this case, the following equation applies to the resonant frequency f.sub.0 : ##EQU1## If a tuned circuit of this kind is operated with an operating frequency f and an operating voltage U.sub.1, the voltage U.sub.2 in accordance with the equation:
In the circuit described above belonging to a crucible induction furnace and corresponding to the prior art, the same voltage is applied to all the coils. Such a circuit arrangement makes sense when the power requirement is the same at all the coils.
BRIEF SUMMARY OF THE INVENTION
Often, however, it is advantageous to supply different areas of the crucible with different power densities. Thus, for example, it is the object of DE 563 710 to supply the material to be melted with a power density which decreases or increases in the axial direction in order to influence the movement of the melt bath.
Another example of a differing power requirement in different areas of the crucible is provided by a crucible which is only partially full, as is the case, for example, during the charging of the crucible or, possibly, where the quantity of the material to be melted is less than the maximum possible. Here, those coils which surround an empty part of the crucible cannot transmit any power to the melt.
Where a coil surrounds the filled part of a crucible, the material to be melted acts like a core of the coil, and almost all types of materials to be melted reduce the inductance of the coil. Such a reduction in the inductance effects an increase in the resonant frequency f.sub.0 in the corresponding tuned circuit in accordance with equation (I).
If, in this case, consideration is given, for example, to a crucible induction furnace with two coils of identical construction, the crucible of which is approximately half full, the lowered inductance of the lower coil has the effect of a higher resonant frequency relative to the upper coil. Since, when using a parallel tuned circuit converter, the operating frequency is somewhat above the resulting resonant frequency of the overall tuned circuit system, i.e. above the resonant frequencies of the individual tuned circuits, the lower individual tuned circuit in the above example is operated closer to its resonant frequency than the upper one. Consequently, the voltage present between the input and the output of the coil and hence also the maximum possible power output in the lower individual tuned circuit is higher than in
REFERENCES:
patent: 1631667 (1927-06-01), Beckmann
patent: 1795136 (1931-03-01), Northrup
patent: 1943802 (1934-01-01), Northrup
patent: 3331909 (1967-07-01), Kasper
Frey Thomas
Hurtgen Reinhold
Hoang Tu Ba
Otto Junker GmbH
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