HIGH FREQUENCY INDUCTIVE MELTING FURNACE SYSTEM FOR ALARMING POOR FEEDING AND EXCESSIVE GENERATION OF AIR GAP Technical field Thre present invention relates to a high frequency inductive melting furnace system for alarming poor feeding and excessive generation of air gap. More particularly, the present invention relates to a high frequency inductive melting furnace system, in which a proper quality factor (hereinafter, referred to as"Q value") is set in advance for each high frequency inductive melting furnace, and is used to alarm poor feeding and excessive generation of air gap in the furnace by turning a red flashing light on and off or making an alarm sound when the Q value calculated by using detected current and voltage outputted from an inverter exceeds the preliminarily set Q value.

Background Art FIG. 1 is a diagram showing an exemplary construction for describing the principle of high frequency induction heating.

Induction heating is a method of heating a conductive object by placing a conductive object in a varying electro magnetic field. Therefore, an object must be conductive in order to be heated by induction heating. Especially in high

frequency induction heating, high frequency current is supplied from a high frequency power supply 100 and flows through a heating coil 110. Then, electromagnetic induction occurs on an object for heating 120, which is located inside of and wound by the heating coil 110, and causes eddy current and hysteresis core loss, which rapidly heat the surface of the object for heating 120.

Induction heating as described above enables either rapid heating or selective heating of a local surface area of an object. Moreover, the induction heating can realize heating of such a high temperature which cannot be achieved a by combustion heating. In addition, the induction heating can improve productivity, produce high quality goods, provide pollution-protective and agreeable labor environment, save energy, achieve automation, facilitate reutilization of goods, and has various other advantageous. Therefore, such induction heating is being widely used now.

The configuration of the induction heater construction shown in FIG. 1 is similar to that of a transformer having a primary winding of N turns and a secondary winding of a single turn. However, the configuration shown in FIG. 1 is different from that of a typical transformer, in that the secondary winding of the induction heating construction is short-circuited. Further, configuration employs an air core, while a typical transformer employs an iron core in order to boost a magnetic coupling between the primary winding and the secondary winding.

FIG. 2 is an equivalent circuit of an induction heater, which is expressed by using a transformer.

Referring to FIG. 2, a melting furnace 240 has a primary winding 210 and a secondary winding 220. Primary current Il applied from a high frequency power supply 200 flows through the primary winding 210, and secondary current Is induced by the primary current I1 flows through the secondary winding 220. When the primary winding 210 has N turns and the secondary winding 220 has only one turn, the secondary current Is can be calculated by an equation, Ils = N x Il, according to a principle of the transformer.

Therefore, when the current Is flows through a resistor 230 (R2) of the secondary side which is equivalent to the object to be heated, the Joule heat generated in the object can be calculated by an equation, J2 = (I2) 2 x R2.

In the meantime, high frequency induction heating may happen even in a non-ferrous metal, as well as in a magnetic material such as iron or nickel. Therefore, high frequency induction heating may be used in melting or heat treatment of a non-ferrous metal such as copper, aluminum, etc.

However, in a process in which a predetermined cold material is put into and molten in a melting furnace containing molten metal as a solvent, which is heated by the high frequency induction as described above, if the cold material contains moisture or volatile material, the high temperature heat applied to the cold material may either boil the moisture and generate vapor or burn the volatile

material and generate gas. Such vapor or gas generated in the furnace forms a great deal of large or small air gaps.

In the conventional melting process in a melting furnace, a worker eliminates air gaps produced in the molten metal in the melting furnace by directly sticking or piercing the air gaps with a long iron rod. Although this way of working typically induces few problems when small air gaps are involved, when a large air gap is stuck by an iron rod, however, the large air gap may explode and cause the molten metal to splash even up to the worker. Then, the worker may be hit by the hot molten metal and may get severely burnt or even suffer death. What is worse, these kinds of accidents frequently happen.

Also, such a worker near the melting furnace should continuously observe the molten state of the object in the melting furnace and timely supply additional cold material into the melting furnace, according to the quantity of the molten object in the furnace, so as to reduce unnecessary heat loss as much as possible during the induction heating.

However, in fact and in general, a worker for the melting furnace simultaneously does various other jobs as well as this task of watching. Therefore, it is difficult for such a worker to continuously observe the melting furnace and to timely supply the cold material into the furnace. Even when a worker can continuously observe the melting furnace, it is still difficult for the worker to timely supply an appropriate amount of cold material into the furnace. When

an appropriate amount of cold material is not opportunely supplied into the furnace, the thermal energy generated by a heating coil may be wasted. Therefore, the conventional melting process is problematic in both aspects of working efficiency and energy loss.

However, up to now, there has been disclosed no such a system, which can check a quantity of a cold material molten in a melting furnace or a quantity of air gaps generated in a melting furnace and can report it to the worker by a red flashing light or an alarm sound.

Disclosure of the Invention Therefore, the present invention has been made in view of the above-mentioned problems, and it is an object of the present invention to provide a high frequency inductive melting furnace system, in which a proper limit quality factor is set in advance for each high frequency inductive melting furnace, and a red flashing light is turned on and off or an alarm sound is made when a quality factor calculated using detected current and voltage outputted from an inverter exceeds the limit quality factor set in advance, so that the system can signal poor feeding and excessive generation of air gap in the furnace by means of the quality factor.

According to an aspect of the present invention, there is provided a high frequency inductive melting furnace

system, for melting a cold material by means of electromagnetic induction by high frequency electric current and for signaling poor feeding and excessive generation of air gap in the high frequency inductive melting furnace system, the high frequency inductive melting furnace system comprising: an input transformer for transforming voltage fed from a high frequency power supply ; a rectifier, connected to the input transformer, for rectifying AC voltage into DC voltage; a resonant capacitor connected in parallel to the rectifier; an inverter, connected in parallel to the resonant capacitor, for converting the DC voltage into single-phase AC voltage; a heating coil, connected to both the resonant capacitor and the inverter and winding a melting furnace, for receiving the single- phase AC voltage thereby generating AC magnetic flux; and a controller for controlling an operation frequency of the inverter, calculating a Q value by using instantaneous voltage and instantaneous current output from the inverter, and generating an alarm signal when the Q value exceeds a prescribed threshold Q value.

Brief Description of the Drawings The foregoing and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:

FIG. 1 is a diagram showing an exemplary construction for describing the principle of high frequency induction heating ; FIG. 2 is an equivalent circuit of an induction heater, which is expressed using a transformer; FIG. 3 illustrates a circuit of a melting furnace system for alarming poor feeding and excessive generation of air gap according to a preferred embodiment of the present invention ; FIG. 4 illustrates an RLC series resonance circuit for calculating a Q value for a high frequency melting furnace according to the present invention; and FIGs. 5A through 5D are diagrams for describing a method of detecting poor feeding and excessive generation of air gap by means of a Q value according to the present invention.

Best Mode for Carrying Out the Invention Reference will now be made in detail to the preferred embodiments of the present invention. In the following description, the same elements will be designated by the same reference numerals although they are shown in different drawings. Further, in the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention

rather unclear.

FIG. 3 shows a circuit of a melting furnace system for alarming poor feeding and excessive generation of air gap according to a preferred embodiment of the present invention A melting furnace system according to a preferred embodiment of the present invention comprises a power supply section, a heating coil 370, an object for heating 360, an output voltage sensor 380, and a controller 390. The power supply section includes an input transformer 310, a rectifier 320, a smoother 330, a resonant capacitor 340, and an inverter 350. The heating coil 370 receives the electric current from the inverter 350. The object for heating 360 is heated by the heating coil 370 wound around the object for heating 360.

The input transformer 310 transforms a given input voltage applied by a high frequency power supply to a proper voltage and supplies the transformed voltage to the rectifier 320. Usually, a three phase electric power of output voltage of at least 380 volt is supplied to the input transformer 310.

The rectifier 320 generates a DC voltage by rectifying the AC voltage supplied from the input transformer 310.

Additionally, when the input current exceeds a predetermined limit, the rectifier 320 instantly blocks the input current.

The smoother 330 reduces voltage ripple produced in the course of the rectification by the rectifier 320, thereby enabling the DC current to be more steady.

The resonant capacitor 340 generates resonance energy, in cooperation with the heating coil 370 which will be described later.

The inverter 350 converts the DC voltage supplied from the smoother 330 into a single phase alternating current which can be used in the heating coil 370. As the operation frequency of the inverter 350 approaches the resonance frequency of the LC resonance circuit consisting of the resonant capacitor 340 and the heating coil 370, the electric power supplied to the object for heating 360, a melting furnace, increases.

The output voltage sensor 380 detects a voltage of an output of the inverter 350 and reports it to the controller 390.

The controller 390 calculates the input voltage supplied to the heating coil 370, and controls the operation frequency of the inverter 350 so as to prevent the calculated input voltage from exceeding a limit value set in advance by the worker. Further, the controller 390 monitors whether voltages and currents of various devices in the power supply section exceed limit values, receives interlock signals from various monitor devices including a cooling water sensor, and controls the system so that the power supply section to stably operate in order.

According to the present invention, the controller 390 conducts a real-time detection of a molten degree of the cold material fed into the object 360 or the melting furnace

and a quantity of generated air gaps, and turns a red flashing light 392 on and off or produces an alarm sound by a buzzer 394. In order to perform such an alarm function, the controller 390 detects output current I and output voltage V outputted from the inverter 350, calculates a Q value by using the detected current and voltage, and determines by using a comparator (not shown) whether the calculated Q value exceeds a preset limit or not. When it is determined that the calculated Q value exceeds the preset limit, the controller 390 generates an alarm control signal, thereby operating the red flashing light 392 and/or the buzzer 394.

A principle in which a Q value for the high frequency melting furnace of FIG. 3 according to the present invention is calculated will be described in detail hereinafter with reference to FIG. 4.

FIG. 4 shows an RLC series resonance circuit for calculating a Q value for the high frequency melting furnace according to the present invention.

The resonant capacitor 340, the inverter 350, the object 360, and the heating coil 370 in the high frequency melting furnace of FIG. 3 described above can be equivalently expressed by an RLC series resonance circuit connected to a variable frequency power source. In FIG. 4, reference numerals 410, 420,430, and 440 designate an inverter, a capacitance of the resonant capacitor 340, an equivalent inductance of the heating coil 370, and an equivalent resistance of the object 360, respectively.

In general, a characteristic impedance in a lossless- uniform line such as an RLC series resonance circuit is expressed as equation 1.

Equation 1 In equation 1, Zo represents a characteristic impedance. Further, a Q value in an RLC series resonance circuit is expressed as equation 2.

Equation 2 In equation 2, R represents a reactive power of the capacitance 420 or the equivalent inductance 430, and P represents an average power of Ro. Electric current flowing through an AC circuit includes an effective current portion and a reactive current portion, which contributes and does not contribute to transfer of electric power, respectively.

An effective power is calculated by multiplying the voltage to the effective current portion, and a reactive power is calculated by multiplying the voltage to the reactive current portion. Here, both the reactive current and voltage for calculating the reactive power are root mean square values.

In general, the Q value presents a scale factor indicating how well a resonance occurs in a series or parallel resonance circuit. That is, the larger the Q value is, the smaller the resonance range is and the better the resonance occurs. As apparent from equation 2, such a Q value is proportional to the magnitude of reactive power, which suggests that most of the loss due to the reactive power of the high frequency melting furnace system shown in FIG. 3 is consumed by the heating coil 370. That is, the smaller the loss due to the reactive power consumed by the heating coil 370 is, the larger the Q value is.

Meanwhile, the heating coil 370 is wound around the object 360 or the melting furnace. Therefore, the loss of the reactive power in the heating coil 370 varies according to the quantities of cold material and air gaps in the object 360. This will be described in detail hereinafter with reference to FIGs. 5A through 5D.

FIGs. 5A through 5D are diagrams for describing a method of detecting poor feeding and excessive generation of air gap by means of a Q value according to the present invention.

FIG. 5A shows an initial state of the furnace wound by the heating coil, into which a cold material is supplied, and FIG. 5B shows the furnace when a considerable amount of the cold material in the furnace has been molten by the heat generated by the heating coil. As described with reference to FIG. 3, the input voltage supplied to the heating coil

370 generates reactive power. Here, the more the cold material contained in the melting furnace wound by the heating coil 370 is, the more the quantity of the reactive power becomes.

That is, in the initial state shown in FIG. 5A, the cold material is surrounded by all of the heating coil 370, so that the heat generated by the heating coil 370 is used to melt a larger quantity of cold material, thus increasing the loss due to the reactive power. Therefore, as noted from equation 2, the larger the loss or the reactive power is, the smaller the Q value is. However, in the state as shown in FIG. 5B, in which a considerable amount of the cold material has been molten, the heat generated by the heating coil 370 is used to melt a smaller quantity of the cold material than that shown in FIG. 5A, so that the loss or the reactive power is reduced. Therefore, the numerator value in equation 2 is larger in the case shown in FIG. 5A than that shown in FIG. 5B.

Meanwhile, in an air core coil such as the heating coil 370 shown in FIG. 3, in case when magnetic flux is proportional to electric current, a coil has an inductance, a magnitude of which depends on a geometrical shape of a circuit or magnetic properties of surrounding media. In other words, since the heating coil 370 shown in FIG. 5A surrounds too much cold material, the heating coil 370 can hardly be considered to be equivalent to a inherent air core coil (which means that the heating coil 370 has a reduced

inductance). However, as the cold material surrounded by the heating coil 370 is molten and decreases as shown in FIG.

5B, the heating coil 370 approaches the inherent air core coil (which means that the inductance of the heating coil 370 increases). Therefore, in the aspect of either the reactive power or the inductance of the coil, it is noted from equation 2 that the Q value is larger in the case shown in FIG. 5A than in the case shown in FIG. 5B.

Meanwhile, FIG. 5C shows a state in which the cold material has been normally and completely molten in the furnace without an air gap, and FIG. 5D shows a state in which the molten cold material in the furnace has an air gap.

For the states shown in FIGs. 5C and 5C, the change of the Q value can be described in the same principle as that for the states shown in FIGs. 5A and 5B. That is, since the molten material surrounded by the heating coil 370 in the state shown in FIG. 5C has a larger volume than that in the state of FIG. 5D, the loss or the reactive power is larger and the inductance is smaller in'the state shown in FIG. 5C than in the state of FIG. 5D. Therefore, the Q value is also smaller in the state shown in FIG. 5C than in the state of FIG. 5D.

In other words, the smaller the cold material in the melting furnace is and the larger the volume of the generated air gaps is, the larger the Q value is.

Referring again to FIG. 3 and applying this principle to FIG. 3, the controller 390 calculates the reactive power,

the effective power, and the Q value, by using an instantaneous current and an instantaneous voltage outputted from the inverter 350. In such a calculation of the reactive power and the effective power, the controller 390 utilizes equation 3 as follows.

Equation 3 <BR> R=Irms # Vrms<BR> P=mean (IX T In equation 3, I, :. s represents a root mean square value of the instantaneous current I, Vrms represents a root mean square value of the instantaneous voltage V, P represents the effective power, and mean (IxV) represents a mean value of IxV. That is, P has a mean value of the product of I and V for one period of an AC power curve. The controller 390 obtains the Q value by dividing by the effective power the reactive power calculated according to equation 3.

Meanwhile, threshold Q values, each of which is specific to each melting furnace connected to the controller 390, are set in advance in the controller 390. Each of such threshold Q values may be determined by adding a predetermined weight to a mean value of normal Q values obtained through iterated melting experiments for each melting furnace connected to the controller 390. For example, when a normal Q value obtained through experiments is 15, a threshold Q value may be set as 18 which is

obtained by multiplying a weight of 1.2 to 15 in consideration of an allowance. In a real operation of the system, when the Q value calculated by the controller 390 exceeds the limit value, the red flashing light 392 is turned on and off and the buzzer 394 beeps.

In order to perform this job, the controller 390 incorporates programs for calculating Irms. Vrms, R. P, and Q using inputted I and P, comparison modules such as a comparator for comparing the calculated Q value with the threshold Q value, or a microprocessor, etc.

Meanwhile, according to the present invention, a Q value is basically used to alarm poor feeding and excessive generation of air gap in a high frequency inductive melting furnace. However, various additional conditions may be added to the Q value in determining whether to alarm or not.

For example, an alarm may be made when the electric power applied to the high frequency inductive melting furnace exceeds one-third of a rated power and the Q value exceeds a limit value, or a function module reflecting various conditions may be added in determining whether to alarm or not.

Industrial Applicability As described above, there has been disclosed up to now no alarm system capable of reporting poor feeding and excessive generation of air gap to a worker for a high

frequency inductive melting furnace system. However, according to the present invention, poor feeding and excessive generation of air gap can be checked and signaled in real time by means of a quality factor.

Further, such an alarm system according to the present invention can prevent not only deterioration in the working efficiency due to poor feeding of a cold material but also excessive generation of air gaps in advance, thereby preventing accidents which may frequently happen due to unpredicted excessive generation of air gaps.

While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment and the drawings, but, on the contrary, it is intended to cover various modifications and variations within the spirit and scope of the appended claims.