Industrial electric heating furnaces – Induction furnace device – With means for manipulation of the charge or melt
Reexamination Certificate
2003-04-02
2004-09-28
Hoang, Tu Ba (Department: 3742)
Industrial electric heating furnaces
Induction furnace device
With means for manipulation of the charge or melt
C373S148000
Reexamination Certificate
active
06798822
ABSTRACT:
FIELD OF THE INVENTION
The present invention is in the technical field of inductively heating and stirring electrically conductive molten materials wherein the heating and stirring can be accomplished simultaneously.
BACKGROUND OF THE INVENTION
It is well known in the art to melt an electrically conductive material, such as a metal, to heat (or melt) the molten metal, and to hold the melt at a temperature by placing the metal in an induction furnace or holding crucible and magnetically coupling the metal to an ac magnetic field. The field is produced in one or more induction coils surrounding the crucible by the flow of ac current from a power source. To maintain sufficient electromagnetic stirring, the electrical frequency of the current is reduced as the furnace capacity increases and the applied ac induction power (and current) increases. For example, a furnace with a melt capacity of 35,000 pounds (16 tonnes) of iron has an optimal power supply frequency of approximately 150 Hz, whereas a furnace with a melt capacity of 5,000 pounds (2¼ tonnes) of steel has an ideal power supply frequency of approximately 600 Hz.
It is also well known that a melt subjected to an ac magnetic field will move when eddy currents generated in the melt by the applied field produce a flux field that opposes the applied magnetic field. Generally, fields produced by higher frequency currents will result in little stirring action and fields produced by lower frequency currents will result in preferred electromagnetic stirring motions with circular-like flow streams through the melt. Further the turbulence of the flow will increase as the magnitude of the applied field (supplied current) is increased.
For some melt compositions and applications, the pre-selected frequency of a single ac power supply may provide both heating and stirring actions that are sufficient for the process. In other applications, separate heat and stir frequencies may be used. There are numerous prior art approaches to applying ac power to a melt at two different frequencies to achieve the heating and stirring functions. Earlier approaches focused on using switching arrangements that alternatively isolated heating and melting power sources from the induction coil sections. Switching arrangements are disadvantageous in that they do not allow for simultaneous heating and stirring of the melt and require additional system components.
Later approaches focused on system topologies that simultaneously applied heating power (operating at a pre-selected heat frequency) and stirring power (operating at a pre-selected stir frequency). A significant technical problem to be overcome in these systems is adequate electrical isolation between the simultaneously connected heating and stirring ac power supplies. Failure to provide this isolation when electronic ac power sources are used can result in component malfunction or failure in a power supply that has its output connected to a second power supply operating at a different output voltage and/or frequency.
One solution to this technical problem is identified in U.S. Pat. No. 5,012,487, entitled Induction Melting (the 487 patent).
FIG. 1
is a simplified schematic that represents the prior art teachings of the 487 patent. In
FIG. 1
an electrostatically screened three-phase transformer
126
, having primary windings
124
and secondary windings
128
, is used to provide stirring power to three coil sections,
114
a
,
114
b
and
114
c
, that make up an induction coil for an induction melting vessel. Stirring power is provided from a 50 Hz, three-phase power source
120
(utility service power). The transformer also uses a tertiary three-phase winding
127
that feeds a three-phase delta-connected power factor correction arrangement (not shown in the simplified schematic). Capacitors
138
a
,
138
b
and
138
c
are connected to the three coil sections as shown in FIG.
1
. The high voltage single-phase output of the heating power source
136
, operating in the frequency range of 150 Hz to 10 kHz, provides heating power to the coil sections through the capacitors. By selecting the impedance of the capacitors, the coil sections and the secondary of transformer, so that the resultant L-C series circuit is at resonance for the operating frequency of the heating power supply, heating power is transferred from the heating power supply to the coil sections. The 50 Hz stirring power source, operating at off-resonant frequency, is impeded from being applied to the input terminals of the heating power source
136
by the tuned series-resonant circuit. Conversely, heating power is blocked from the stirring power source since the secondary windings of transformer
126
are effectively in parallel at the operating frequency of the heating power source.
There are a few disadvantages to the circuit arrangements disclosed in the 487 patent. Power transformer
126
is an expensive component with voltage tap changers (not shown in the simplified schematic) and the tertiary winding as further described in the 487 patent. Further the operating frequency difference between the heat power source and the stir power source must exceed a certain range for the series resonant circuit to operate effectively. This is particularly problematic for large capacity induction melting vessels.
Therefore, there exists the need for apparatus for and method of simultaneously induction heating and stirring a melt from two separate power supplies, without the use of isolation transformers or switches, wherein the frequency of stir power supply (and induced stir field) is less than the frequency of the heat power supply (and induced heat field), particularly when the frequency of the heat power supply is close in frequency of the stir power supply.
BRIEF SUMMARY OF THE INVENTION
In one aspect, the invention is apparatus for and method of simultaneous induction heating and stirring of an electrically conductive material in a vessel having at least one set of three interconnected induction coil sections disposed around the vessel. Inductive heating of the electrically conductive material is accomplished by applying single-phase ac power across the coil sections via one or more tuning capacitors and stirring of the electrically conductive material is accomplished by applying three-phase ac power to the coil sections via one or more inductors. In some examples of the invention, the capacitive heating circuit and the coil sections operate at or near a first resonant point and the inductive stir circuit and the coil sections operate at or near a second resonant point while the tuning capacitors and inductors block power transfer between the sources of the single-phase and three-phase ac power. In other examples of the invention, the capacitive heating circuit and the coil sections operate at or near a resonant point while the tuning capacitors and inductors block power transfer between the three-phase ac induction stirring power source and the single-phase ac induction heating power source.
These and other aspects of the invention are set forth in the specification and claims.
REFERENCES:
patent: 3536983 (1970-10-01), Kennedy
patent: 5012487 (1991-04-01), Simcock
patent: 6542535 (2003-04-01), Fishman et al.
patent: 6546039 (2003-04-01), Fishman et al.
patent: 2002/0122456 (2002-09-01), Fishman et al.
Fishman Oleg S.
Mortimer John H.
Nadot Vladimir V.
Hoang Tu Ba
Inductotherm Corp.
Post Philip O.
LandOfFree
Simultaneous induction heating and stirring of a molten metal does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Simultaneous induction heating and stirring of a molten metal, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Simultaneous induction heating and stirring of a molten metal will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3262076