HK1010572A - Multiple furnace controller - Google Patents

Description

Multi-furnace controller

The present invention relates to power supply control systems for supplying power to a plurality of furnaces simultaneously, and more particularly to a control system for distributing power in a controlled, proportional manner-two electric furnaces are supplied simultaneously from a single power supply and a single reactive capacitor station.

Power supplies for selectively or alternately heating a plurality of induction furnaces are known. Systems that have been used frequently in the past to alternately power two furnaces are referred to as "butterfly operations". In this operation, a single power supply alternately supplies power to two furnaces, which are used as a self-holding (holding) furnace and a melting furnace. The first furnace holds the molten metal and only requires power sufficient to control the temperature of the metal so that it remains molten. The second furnace allows the metal to melt as quickly as possible. The power source is typically located to rapidly switch its output from one oven to another. Initially, a power supply is connected to the furnace and supplies as much power as possible to the load. The temperature of the metal within the self-sustaining furnace is monitored. When the temperature of the molten metal in the self-protection furnace reaches the minimum value, the power supply to the smelting furnace is cut off, the power output is connected to the self-protection furnace, and meanwhile the self-protection furnace is electrified. And maintaining power supply for the self-protection furnace until the metal temperature reaches the maximum limit value. At this time, the power supply to the self-sustaining furnace is cut off, and the power supply output is connected to the melting furnace, i.e., the melting furnace is energized. This operation is repeated throughout the melting cycle whenever power is required for temperature control of the self-sustaining furnace.

The result of the butterfly operation is poor temperature control of the self-sustaining furnace and poor utilization of the furnace power. Furthermore, the power supply must be switched off/on each time the output connection is switched, which means that none of the ovens can receive power during the switching.

An improved system in terms of power utilization efficiency is shown in US patent 5,272,719 which discloses a power supply system that utilizes a single power supply for both melting and holding molten metal between casting operations. The power supply is connected to the furnaces via a switching network, wherein a power supply having a plurality of outputs includes at least one rectifying section having an output and a plurality of high frequency inverter sections equal in number to the number of independent induction furnaces.

A particular problem with such systems is that each furnace requires its own high frequency inverter device which must include expensive tank circuits and filter capacitors and associated switching circuitry to control the power supply to each furnace. Furthermore, the power consumption to start up each individual capacitor tank circuit for each furnace is increased to avoid the need for multiple tanks for a single system.

The present invention contemplates a new and improved multiple furnace control system which overcomes the above-referenced problems and others to provide a furnace control system for simultaneously powering multiple furnaces, such as self-sustaining and melting furnaces, at preselected furnace power levels from the same capacitor bank during which all operations of the power supply and furnace capacitors are completed within safe limits.

In accordance with the present invention, a multiple furnace control system is provided to supply an operator with selected power levels to optimally melt or maintain products contained within the system furnace. Typically, the first and second furnaces are associated with a power supply to supply power to the furnaces, while the capacitor banks connected in parallel to the power supply and the furnaces form a tank circuit. The switch for selectively controlling the supply of power to the furnaces comprises control means for controlling the supply of a first portion of power to maintain molten product in a first or "self-sustaining" furnace as a main control, and the supply of the remaining portion of power to melt product in a second or "melting" furnace, thereby using the capacitor bank as a reactive tank for both furnaces.

According to another aspect of the invention, the switching circuit includes a solid state control Switch (SCR) for limiting power to the self-sustaining furnace and a plurality of selector switches for controlling which furnace will receive self-sustaining power and which furnace will receive melting power. The power supply includes a conventional inverter circuit in addition to special feedback loop control responsive to the operator selected power stage. When the first furnace is switched from a self-sustaining furnace to a melting furnace, it is switched from series with the SCR to being directly connected in parallel with the capacitor bank. When the second furnace is converted from a melting furnace to a self-sustaining furnace, it is converted to series with the SCR so that the power level can be adjusted as needed. The system can be configured to always have the SCR in series with the furnace having the lower required stage.

The invention also includes a method of operating a multiple furnace system including a melting furnace and a self-sustaining furnace, in which a power supply and a capacitor station are respectively placed in parallel connection with the furnaces, and a switching circuit controls the power supplied to the furnaces, respectively. The method comprises the following steps:

the first furnace is adjusted as a self-sustaining furnace, including identifying the portion of power necessary to maintain the product contained within the self-sustaining furnace in a molten state. The second step is to supply the identified portion of power from the power source to the self-sustaining furnace using a power control switch disposed in series with the self-sustaining furnace. The remaining portion of the power may then be supplied directly to the melting furnace to melt the product contained therein. The invention includes selectively switching the furnace from a holding furnace to a melting furnace or vice versa as required by the state of the product.

The subject of the invention provides the advantage that any furnace in the system is continuously supplied with the appropriate power in order to precisely control the temperature of the product therein, while at the same time being able to supply as much of the remaining power as the operator chooses for the other furnaces of the system.

Another advantage obtained by the present invention is that: the same power supply and capacitor station are used to simultaneously power both furnaces.

Other benefits and advantages of the novel multiple oven control system of the present invention will become apparent to those skilled in the art upon reading and understanding the present specification.

The invention may take physical form in certain parts and arrangement of parts, the preferred embodiment of which will be described in detail in this specification and illustrated in the accompanying drawings which form a part hereof, and wherein:

FIG. 1A includes a schematic block diagram of a multiple furnace system formed in accordance with the present invention;

FIG. 1B shows a control panel that an operator may face according to the embodiment of FIG. 1A;

FIG. 2A includes a schematic block diagram of the multi-furnace system of FIG. 1A in an alternative circuit configuration;

FIG. 2B shows a control panel for the embodiment of FIG. 2A;

FIG. 3 shows an alternative circuit configuration to the system of FIGS. 1A and 2A;

FIG. 4 shows another alternative embodiment, except that each furnace in the system includes a solid state switch in series therewith;

FIG. 5A shows a state diagram illustrating the alternating states of the system and the changes in the elements and their circuits under different state conditions;

FIGS. 5B and 6 show detailed state diagrams for the selection switch of FIGS. 1A and 2A in the open state; and

FIG. 7 illustrates a typical melting cycle of the system of FIGS. 1A and 2A, showing the percentage of power supplied to each furnace simultaneously.

Referring now to the drawings wherein the showings are for the purpose of illustrating preferred embodiments of the invention only and not for the purpose of limiting the same, there is shown a multiple furnace control system comprising first and second induction furnaces a and B which induce heat in products contained within the furnaces by means of induction coils 10, 12 powered by a power supply 14 and a reactive capacitor tank station (capacitor tank station) 16. The power supply 14 is a conventional inverter and is well known for supplying the coils 10, 12 with a suitable alternating current to power the furnaces a and B. The power supply 14 and the capacitor station 16 are connected in parallel to the furnaces a and B such that the same capacitor station is sufficient to serve as a reactive energy storage circuit for both melting and self-sustaining operation of both furnaces a and B, as is conventionally required for a melting furnace. As will be explained in more detail later, the operation of the power supply, the capacitor station and the furnace is done so that power is supplied while operating the power supply and the capacitors within safe limits.

In the embodiment of fig. 1A and 2B, the switching means for controlling the feed furnaces a and B comprise a plurality of selector switches 1, 2, 3, 4 and a solid state control Switch (SCR) 20. The controller 30 controls the switch operation based on an operator input from the control panel of fig. 1B. Safety disconnect switches 22, 24 are provided to manually disconnect the furnace from the power source. Selector switches 1-4 are operated such that one of the furnaces is connected directly across the capacitor platform and the other furnace is connected in series with SCR20 across the capacitor. A micro switch (not shown) is also provided on all the selector switches to report its status to the controller 30. When the safety disconnect switches 22, 24 are opened, the control system will also open the corresponding selector switches 1-4 to completely isolate the furnace.

With particular reference to FIG. 1B, this figure shows the control panel as it is operated and observed by the operator of the system of FIG. 1A. A selector switch, such as potentiometers 32, 34, allows the operator to select a fraction of the available power that can be supplied to each furnace through the power supply and capacitor stations. As can be seen, oven a has been set to accept eighty percent (80%) of the available power and oven B has been set to accept twenty percent (20%) of the available power. The digital readers 36, 38 inform the operator of the true percentage of the power being supplied to the furnace.

One feature of the invention is: whichever of the a, B furnaces is selected by the operator to receive a lesser amount of available furnace power is connected in series with the thyristor 20, while the furnace selected to receive a higher percentage of the available power is connected directly across the power supply and capacitor stations 14, 16. Traditionally, the furnace selected for lower power requirements would typically be a self-sustaining furnace, while the other furnace would typically comprise a melting furnace; today (in availability), however, it is irrelevant which furnace is the melting furnace and which furnace is the self-sustaining furnace, since the control scheme is based on the selected power to be supplied and not on the actual use of the power, i.e. holding or melting.

Another important feature of the present invention is: the scheme uses the lower power requirement as the primary control, i.e. the furnace is always made to meet its selected power requirement, while another furnace that is selected to accept a higher percentage of the available power is anyway limited to accept the balance of the available power.

More specifically, with continued reference to fig. 1A and 1B, furnace B has been selected to receive twenty percent (20%) of its rated power from the power supply and capacitor stations 14, 16. Furnace a has been selected to accept eighty percent (80%) of the available rated power. In a conventional modification, furnace B is then a self-sustaining furnace and furnace a is a melting furnace, but as noted above, the actual function of the furnaces is irrelevant. However, since furnace B has been selected to receive a lower percentage of power, thyristor 20 is switched by selector switches 1-4 into series with furnace B due to the closing of switch 3 and the opening of switch 4. Furnace a is directly connected across the capacitor station by closing switch 1 and opening switch 2. In this case, one hundred percent (100%) of the available power from the power supply is routed to both furnaces for efficient melting and holding operations. All switching is effected by the controller 30 in response to operator adjustment of the selection of the respective oven on/off buttons, not shown in fig. 1B or 2B but illustrated in fig. 5A and 5B, by the potentiometers 32, 34. Today, the controller will operate the converter 14 such that it can seek to provide eighty percent (80%) of the rated power to furnace a, provided it can meet the selected twenty percent (20%) requirement for furnace B. This is accomplished by the operation of the thyristors to reduce the available power from the power supply and capacitor station 16 to a selected level of twenty percent.

Fig. 2A and 2B show the situation where the operator has reversed the operating conditions so that furnace a now accepts twenty percent (20%) of the rated power and furnace B accepts eighty percent (80%). In this case, the selector switches are reversed such that after a short no-load switching operation, switches 1 and 3 are opened and switches 2 and 4 are closed.

In case the operator has selected two different power levels, the sum of which is higher than one hundred percent (100%) of the power that can be supplied by the power supply, the system will first meet the lower power requirement, since this is the main control, and then supply the other furnace with the rest of the available power. For example, if the operator selects the self-sustaining furnace to accept thirty percent (30%) of the power available from the power source and selects the melting furnace to accept eighty percent (80%) of the available power. The sum of the two is one hundred and ten percent (110%) higher than the ten percent (10%) of the power supply that would be established if it were only capable of supplying its rated power. In this case, the melting furnace only accepts seventy percent (70%) of the available power, although the operator has requested that the melting furnace be supplied with eighty percent (80%) of the rated power. The control panel indicates that the self-sustaining furnace is selected to accept thirty percent (30%), the display indicates that it is accepting this percentage of available power, and the melting furnace is selected to accept eighty percent (80%) at the potentiometer, but the display only indicates: seventy percent (70%) of the available power is supplied to the furnace.

Several advantages are derived from this control scheme. First, the power supply and capacitor stations 14, 16 will tend to operate at maximum efficiency so that one power supply and capacitor station 14, 16 can power multiple furnaces. Second, since the control scheme sets the lower selected power level as the primary control, the furnace always accepts its selected power while the other furnace can accept either some or all of the remaining available power. Third, the thyristor pair 20 can control the power of either furnace by being connected in series with one of the furnaces via selector switches 1-4. Thus, with a single thyristor, a single capacitor station and a single power supply can power multiple furnaces.

Referring to fig. 3, an alternate embodiment of the system is shown in which only two selector switches 40, 42 are used to selectively connect thyristor pair 20 to either furnace a or furnace B. In the embodiment shown in fig. 3, furnace a is designated as the melting furnace and is therefore directly connected to capacitor station 16 as switch 40 is closed, while furnace B is a self-sustaining furnace because it is in series with thyristor pair 20 as switch 42 is opened.

Fig. 4 includes yet another alternative embodiment in which two thyristors 44, 46 are used in series with each furnace. In this case, the thyristors 44, 46 will power the relevant furnace as controlled by the operator and only two additional disconnectors 5, 6 are required.

With particular reference to fig. 5a, 5B and 6, a control scheme for coordinating the functions of the control system is illustrated. The basic coordination function of the control system is to ensure that: regardless of the operator controlled requirements, the power supply is always running or begins to adjust the capacitor banks 16 in parallel. For example, if furnace A and furnace B are both operating, and furnace A is holding (i.e., power controlled by SCR switch 20) and the operator turns furnace B off, the control system must first turn SCR switch 20 off, then power supply 14 off, then Swap (Swap) switches 1-4 to connect furnace A directly to energy storage circuit (tank)16, then turn power supply 14 on, and then bring the power supply power level up to the same power level that furnace A had been operating before furnace B was turned off.

All of these states of the furnace switch transition system are shown in the state diagrams of fig. 5A and 5B. Eleven value states of the system, labeled states a through K, are shown and shown as ellipses. The lines listed from the oval state represent the actions of the operator, and the circles on these lines are the system behavior to go from one state to the next. Using State A as an example, the possible actions and results of the operator may be interpreted. State a can be identified as: furnace a is melting and furnace B is holding (i.e., switches 1 and 3 are closed, switches 2 and 4 are open, and both manual disconnects 22, 24 are closed) the possible actions are:

1) the operator pushes the furnace a-on button or the furnace B-on button as shown by the top two arcs. The system never leaves state a because both furnaces are on.

2) The operator pushes the B furnace-off button as shown by the left line. The system turns off the SCR switch (r) directly and enters state F where furnace a is still melting and furnace B is off, but all selector switches are left in their current positions while furnace B is ready to operate as a self-sustaining furnace.

3) The operator pushes the furnace a-off button as shown by the lower left diagonal line. The system turns off the SCR switch (r) first, then turns off the power supply (r), and then interchanges the selector switch positions to switch from furnace a melting to furnace B melting, since furnace B will become the only operating furnace, the power supply requiring an energy storage circuit. (the control system opens selector switches 1 and 3 and closes selector switches 2 and 4). After the power is turned on, the system enters state D.

4) The operator rotates the control potentiometer of furnace a to be raised even higher than the control potentiometer of furnace B, as indicated by the bottom arc. The system never exits state a because furnace a is already a melting furnace.

5) The operator rotates the control potentiometer of furnace B above the control potentiometer of furnace a as shown by the bottom vertical line. The system first turns off the SCR switch (c), then turns off the power supply (c), then switches the selector switch so that furnace B is the melting furnace and furnace a is the self-sustaining furnace, so after turning on the power supply and the SCR switch (c and c) the system enters state B, at which time furnace B is the melting furnace and furnace a is the self-sustaining furnace.

6) The operator turns on the furnace manual disconnect switch as shown in one of the two right side lines at the top of fig. 6A from state a. The system immediately stops triggering the SCR switch and turns off the power supply and enters state J or state H after opening the appropriate switch to fully isolate the furnace to which the disconnect switch was opened.

The action described as rotating one control potentiometer above the other represents a control scheme for determining which furnace is a melting furnace and which furnace is a self-sustaining furnace.

With particular reference to fig. 7, this figure illustrates the advantageous power usage (usage) of the system. It follows that when the self-sustaining furnace power requirement is reduced to zero percent (0%) of power by virtue of the molten product therein being poured (pour off), the power available to the melting furnace is increased.

As noted above, the multi-furnace controller of the present invention enables the continuous application of appropriate power to the self-sustaining furnace to precisely control the temperature of the molten metal while also continuously supplying up to the maximum remaining available power to the melting furnace. The self-protection furnace is the main body of the scheme. Its power requirements are always satisfied first. Melting furnaces accept as requested up to a maximum available power value determined by the power supply rating and the self-sustaining furnace power requirement. That is, the maximum power to the melting furnace is equal to the nominal rated power value of the power supply minus the power supplied from the holding furnace.

The invention has now been described with reference to these preferred embodiments. It is clear that other circuit configurations, i.e. a single power supply and a single capacitor station to alternately power the self-sustaining and melting furnaces, may be used for the intended purpose of the invention. On the other hand, the same effect can be obtained without controlling the switch 20, such as by connecting the furnace load in series with the variable impedance, and this is within the scope of the present invention.

The invention includes all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

Claims (12)

1. A furnace control system for supplying power for selectively melting or maintaining product contained in a plurality of furnaces of the system, the system comprising:

a first furnace and a second furnace;

a power supply for supplying power to the two furnaces;

a capacitor bank connected in parallel to a power supply and the furnace and forming therewith an energy storage circuit; and

switching means for selectively controlling the power supplied to the two furnaces, the switching means including control means for controlling delivery of a first preselected portion of power for maintaining molten product in the first furnace as a primary control and for controlling delivery of the remaining portion of power for melting product in the second furnace, thereby using the capacitor bank as a reactive energy storage loop for both furnaces.

2. The furnace control system of claim 1, wherein: the first and second furnaces may either comprise a self-sustaining furnace for holding the molten product or a melting furnace for melting the product into a molten state.

3. The furnace control system of claim 2, wherein: the switching means includes a power stage control switch and a plurality of selector switches configured to selectively connect the first and second furnaces to either the continuous melting furnace or the continuous self-sustaining furnace, respectively.

4. The furnace control system of claim 3, wherein: when a first portion of the plurality of selector switches is selected as a self-sustaining furnace, the power stage control switch is switched in series with the first furnace to supply a first preselected portion of power to the first furnace and to switch the remainder of the power to the second furnace when a second portion of the plurality of selector switches is selected as a melting furnace.

5. The furnace control system of claim 1, wherein: also included are first and second control potentiometers associated with the first and second furnaces for selecting first and second percentages thereof from the available power of the power supply to deliver them to the furnaces, respectively, and means for adjusting the lower of the first and second percentages as a primary control for maintaining molten product.

6. A method of operating a multiple furnace system whereby a selected percentage of available power is transferred from a common power supply and common capacitor bank connected in parallel to the power supply and furnaces to the furnaces of the system, the system comprising first and second furnaces, both of which may be selected or operated simultaneously as self-sustaining or melting furnaces; and switching means for controlling the supply of available power to the two furnaces from the power supply and the capacitor station, respectively, the method comprising the steps of:

adjusting the first furnace to a self-sustaining furnace including identifying a portion of power necessary to maintain the product contained in the first furnace in a sustaining state;

providing the identified portion of power to the first furnace; and

the remainder of the power is supplied to the second furnace to melt the product contained therein.

7. The method of operating a multiple furnace system of claim 6, wherein: supplying power to the identification portion and to the remaining portion includes supplying power to both ovens from the power supply and the capacitor station simultaneously.

8. The method of operating a multiple furnace system of claim 6, wherein: the supplying the identified portion of power includes limiting power applied to the first furnace from the power supply and the capacitor station with the phase controlled switch.

9. A multiple furnace control system for sharing power supplied to each furnace among the furnaces as a percentage of available power selected by an operator, comprising:

a plurality of furnaces;

a single power supply and a capacitor station connected in parallel with the power supply and each furnace and adapted to form an energy storage circuit simultaneously;

means for selecting a first desired percentage of available power to supply one of the furnaces; and

switching means for supplying the required percentage to one furnace as a master control of the available power and for supplying the remainder of the available power to the remaining furnaces.

10. The multi-furnace control system of claim 9, wherein: means for selecting a second percentage of available power to supply to a second furnace, wherein said switching means comprises means for determining the lower percentage of said first and second percentages and for setting the lower percentage as master control.

11. The multi-furnace control system of claim 10, wherein: the means for selecting the first and second percentages each comprise means for displaying the percentage of available power to each furnace, respectively.

12. The multi-furnace control system of claim 10, wherein: the means for selecting the first and second percentages each includes a control potentiometer and the switching means includes a power level control switch selectively connected in series with the furnace determined to have the selected percentage lower to adjust the power supplied to the furnace determined to have the lower percentage as the primary control.