Method for diagnosing an induction furnace

Abstract

A diagnostic method of an induction furnace is provided. In the induction furnace, even if the resistance value of an insulator between a coil provided outside the furnace and the ground decreases due to moisture adsorbed on the insulator, a relatively large current does not flow in the coil. Until the resistance value, which is calculated from the current detected by a current detection section (9) when an intermittent switch (6) is always turned on and an off switch (7) is always turned off, and the voltage detected by a voltage detection section (10) becomes greater than a threshold value, the operation of turning on and off the intermittent switch (6) and always turning on the off switch (7) is repeated, and after that, the operation of always turning on the intermittent switch (6) and always turning off the off switch (7) is repeated, and when the above resistance value becomes greater than the threshold value, an alternating current is caused to flow in the coil (L).

Description

[0001] This application is a divisional application of Chinese invention patent application filed on July 27, 2020, with application number 202010731546.X and entitled "Induction Furnace and Method of Operating Induction Furnace". Technical Field

[0002] This invention relates to an induction furnace and a method for operating the induction furnace. Background Technology

[0003] As an induction furnace, there exists an induction furnace that melts metal by heating the metal inside the furnace using a magnetic field generated by allowing alternating current to flow through a coil located on the outside of the furnace.

[0004] In the aforementioned induction furnace, there is a concern that when the resistance of the insulator decreases due to moisture adsorbed on the insulator between the coil and ground, the current flowing in the coil increases, causing the coil to heat up and deteriorate.

[0005] Therefore, as another type of induction furnace, there exists one induction furnace that, when the temperature detected by a temperature sensor located near the coil exceeds a protection temperature, reduces the current flowing through the coil to suppress coil deterioration. Patent Document 1 is related to this technology.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 2009-289545 Summary of the Invention

[0009] The problem the invention aims to solve

[0010] However, in the other induction furnaces mentioned above, there is a concern that when the resistance of the insulator decreases due to moisture adsorbed on it, a relatively large current flows through the coil from the start of operation of the induction furnace until the temperature detected by the temperature sensor exceeds the protection temperature, causing coil deterioration.

[0011] Therefore, one aspect of the present invention relates to the objective of preventing a large current from flowing through the coil even if the resistance of the insulator decreases due to moisture adsorbed on the insulator between the coil and ground.

[0012] Solution for solving the problem

[0013] A diagnostic method for an induction furnace, wherein the induction furnace generates a magnetic field on the inner side of the furnace by flowing an alternating current through a coil disposed on the outer side of the furnace, the method comprising the following steps: a first step of applying a pulsed voltage to the coil; a second step of applying a direct current voltage to the coil; and a third step of determining the resistance value of an insulator between the coil and ground based on the voltage applied to the coil in the second step and the current flowing through the coil, wherein the remaining lifetime of the insulator is estimated based on the resistance value determined by the third step.

[0014] An induction furnace, as one aspect of the present invention, generates a magnetic field inside the furnace by circulating an alternating current in a coil disposed on the outside of the furnace. The induction furnace includes: an intermittent switch connected between one terminal of the coil and the positive terminal of a power supply; a cut-off switch connected between the other terminal of the coil and the negative terminal of the power supply; a current detection unit that detects the current flowing in the coil; a voltage detection unit that detects the voltage of the power supply; and a control unit that controls the operation of the intermittent switch and the cut-off switch respectively.

[0015] Before the resistance value, calculated by the current detected by the current detection unit and the voltage detected by the voltage detection unit when the intermittent switch is always on and the cut-off switch is always off, becomes greater than a threshold, the control unit repeatedly performs the operation of turning the intermittent switch on and off, and then turning the cut-off switch on and off repeatedly. When the resistance value becomes greater than the threshold, the control unit allows alternating current to flow in the coil. Furthermore, when the intermittent switch is on, it conducts; when it is off, it is off. Similarly, when the cut-off switch is on, it conducts; when it is off, it is off.

[0016] Furthermore, as one aspect of the present invention, the operation method of the induction furnace is as follows: the induction furnace includes: an intermittent switch connected between one terminal of a coil disposed on the outside of the furnace and the positive terminal of a power supply; a cut-off switch connected between the other terminal of the coil and the negative terminal of the power supply; a current detection unit that detects the current flowing in the coil; and a voltage detection unit that detects the voltage of the power supply. In the operation method of the induction furnace, before the resistance value obtained by the current detected by the current detection unit and the voltage detected by the voltage detection unit when the intermittent switch is always on and the cut-off switch is always off becomes greater than a threshold, the operation of turning the intermittent switch on and off and turning the cut-off switch on continuously, and then turning the intermittent switch on continuously and turning the cut-off switch off continuously is repeated. When the resistance value becomes greater than the threshold, an alternating current is allowed to flow in the coil to generate a magnetic field inside the furnace.

[0017] The effects of the invention

[0018] According to the present invention, even if the resistance of the insulator decreases due to moisture adsorbed on the insulator between the coil and ground located outside the furnace, a relatively large current will not flow in the coil. Attached Figure Description

[0019] Figure 1 This is a diagram illustrating an example of an induction furnace implementation.

[0020] Figure 2 This is a flowchart illustrating an example of the operation of the control unit.

[0021] Figure 3 This is a timing diagram representing an example of the control signal for the intermittent switch, the control signal for the switch to be turned off, the voltage applied to the coil, and the current flowing in the coil.

[0022] Figure 4 This is a graph showing an example of the increase in the resistance of the insulator associated with an increase in the number of pulse voltage mode implementations, and an example of the decreasing trend in the resistance of the insulator associated with the passage of time.

[0023] Figure 5 This is a diagram illustrating a modified example of an induction furnace according to the implementation method.

[0024] Explanation of reference numerals in the attached figures

[0025] 1: Induction furnace; 2: Inverter circuit; 3, 4: Switching switch; 5: Furnace; 6: Intermittent switch; 7: Cut-off switch; 8: Low-pass filter; 9: Current detection unit; 10: Voltage detection unit; 11: Storage unit; 12: Control unit. Detailed Implementation

[0026] Figure 1 This is a diagram illustrating an example of an induction furnace implementation.

[0027] Figure 1 The induction furnace 1 shown includes an inverter circuit 2, a switching switch 3 (first switching switch), a switching switch 4 (second switching switch), a coil L, a furnace 5, an intermittent switch 6, a cut-off switch 7, a low-pass filter 8, a current detection unit 9, a voltage detection unit 10, a storage unit 11, and a control unit 12.

[0028] During the operation of the induction furnace 1, the inverter circuit 2 converts the direct current flowing from the power supply P into alternating current, which then flows through the coil L located outside the furnace 5. When the alternating current flows through the coil L, the magnetic field generated inside the furnace 5 heats up the metal inside the furnace 5, causing the metal to melt. Furthermore, the power supply P may also include a system power supply, a rectifier circuit, and a smoothing capacitor, configured such that the alternating current flowing from the system power supply is rectified by the rectifier circuit and smoothed by the smoothing capacitor, thereby converting the alternating current from the system power supply into direct current, which is then output to the inverter circuit 2.

[0029] That is, inverter circuit 2 includes switching elements SW1 to SW4, such as IGBTs (Insulated Gate Bipolar Transistors). The collector terminal of switching element SW1 is connected to the collector terminal of switching element SW3, and is connected to switch 3 via one input terminal IN+ of inverter circuit 2. The emitter terminal of switching element SW2 is connected to the emitter terminal of switching element SW4, and is connected to switch 4 via another input terminal IN- of inverter circuit 2. The emitter terminal of switching element SW1 is connected to the collector terminal of switching element SW2, and is connected to one terminal of coil L via one output terminal OUT+ of inverter circuit 2. The emitter terminal of switching element SW3 is connected to the collector terminal of switching element SW4, and is connected to another terminal of coil L via another output terminal OUT- of inverter circuit 2.

[0030] Furthermore, when the input terminal IN+ of inverter circuit 2 is electrically connected to the positive terminal of power supply P via switch 3, and the input terminal IN- of inverter circuit 2 is electrically connected to the negative terminal of power supply P via switch 4, when switching elements SW1 and SW4 are turned on and switching elements SW2 and SW3 are turned off, current flows from the positive terminal of power supply P through switch 3, switching element SW1, coil L, switching element SW4, and switch 4 to the negative terminal of power supply P. Conversely, when switching elements SW1 and SW4 are turned off and switching elements SW2 and SW3 are turned on, current flows from the positive terminal of power supply P through switch 3, switching element SW3, coil L, switching element SW2, and switch 4 to the negative terminal of power supply P. That is, when switching elements SW1 and SW4 are alternately turned on and off with switching elements SW2 and SW3, alternating current flows in coil L.

[0031] Intermittent switch 6 is composed of semiconductor relays such as IGBTs and MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) or electromagnetic relays. One terminal of intermittent switch 6 is connected to changeover switch 3, and the other terminal of intermittent switch 6 is connected to one terminal of coil L via low-pass filter 8. Furthermore, when intermittent switch 6 is turned on, it conducts, and changeover switch 3 is electrically connected to low-pass filter 8. Conversely, when intermittent switch 6 is turned off, it disconnects, and changeover switch 3 is no longer electrically connected to low-pass filter 8.

[0032] The disconnect switch 7 is composed of semiconductor relays such as IGBTs and MOSFETs, or electromagnetic relays. One terminal of the disconnect switch 7 is connected to the changeover switch 4, and the other terminal of the disconnect switch 7 is connected to the other terminal of the coil L. Furthermore, when the disconnect switch 7 is closed, it conducts, and the changeover switch 4 is electrically connected to the other terminal of the coil L. Conversely, when the disconnect switch 7 is open, it disconnects, and the changeover switch 4 is no longer electrically connected to the other terminal of the coil L.

[0033] The low-pass filter 8, composed of resistors, capacitors, etc., smooths the current flowing from the intermittent switch 6 to the low-pass filter 8 before outputting it to the coil L when implementing the pulse voltage mode (described later). Alternatively, if smoothing the current flowing from the intermittent switch 6 to the low-pass filter 8 is not required when implementing the pulse voltage mode, the low-pass filter 8 can be omitted. In this configuration, the other terminal of the intermittent switch 6 is directly connected to one terminal of the coil L.

[0034] The current detection unit 9, composed of a Hall element, a shunt resistor, etc., detects the current flowing in the coil L and sends the detected current to the control unit 12. Furthermore, the connection position of the current detection unit 9 is not particularly limited.

[0035] The voltage detection unit 10, composed of voltage divider resistors, operational amplifiers, etc., detects the voltage of the power supply P and sends the detected voltage to the control unit 12. Furthermore, the connection location of the voltage detection unit 10 is not particularly limited.

[0036] The storage unit 11 is composed of RAM (Random Access Memory) or ROM (Read Only Memory), etc., and is used to store the resistance value of the insulator RL between the coil L and ground.

[0037] The control unit 12 consists of a CPU (Central Processing Unit), programmable devices (FPGA (Field Programmable Gate Array), PLD (Programmable Logic Device)), etc. Before the induction furnace 1 starts operating, the control unit 12 controls the operation of the switching switches 3 and 4, the intermittent switch 6, and the cut-off switch 7 to allow a direct current equivalent to the rated current to flow through the coil L. When a direct current flows through the coil L, the moisture adsorbed on the insulator RL evaporates due to the heat generated by the resistive component of the coil L (copper loss), and the resistance of the insulator RL increases. Furthermore, during the operation of the induction furnace 1, the control unit 12 controls the operation of the switching switches 3 and 4 and the switching elements SW1 to SW4 to allow an alternating current to flow through the coil L, thereby generating a magnetic field inside the furnace 5. When a magnetic field is generated inside the furnace 5, the moisture adsorbed on the insulator RL evaporates due to the heat generated from the metal inside the furnace 5, and the resistance of the insulator RL increases. Therefore, before and during the operation of the induction furnace 1, the current flowing from the power source P through the coil L and the insulator RL to the ground can be suppressed. Thus, the overall current flowing in the coil L can be suppressed from exceeding the rated current of the coil L, thereby suppressing the deterioration of the coil L.

[0038] Figure 2 This is a flowchart illustrating an example of the operation of the control unit 12. Figure 3 (a) is a timing diagram showing an example of the control signal S1 used to control the on and off of the intermittent switch 6. Figure 3 (b) is a timing diagram showing an example of the control signal S2 used to control the on / off state of the cut-off switch 7. Figure 3 (c) is a timing diagram representing an example of the voltage applied to coil L. Figure 3 (d) is a timing diagram showing an example of the current flowing in coil L. Furthermore, Figure 3 In the two-dimensional coordinate system shown in (a), the horizontal axis represents time, and the vertical axis represents the magnitude (voltage) of the control signal S1. Additionally, Figure 3 In the two-dimensional coordinate system shown in (b), the horizontal axis represents time, and the vertical axis represents the magnitude (voltage) of the control signal S2. Additionally, Figure 3 In the two-dimensional coordinate system shown in (c), the horizontal axis represents time, and the vertical axis represents voltage. Additionally, Figure 3 The horizontal axis of the two-dimensional coordinate system shown in (d) represents time, and the vertical axis represents current. Let's assume... Figure 3 (a)~ Figure 3 The time values ​​of the horizontal axis shown in (d) are consistent with each other.

[0039] First, when the user inputs an operation start instruction for the induction furnace 1 through operation of the start button, the control unit 12 implements the pulse voltage mode (step S11). During the fixed time T1 of implementing the pulse voltage mode, the control unit 12 controls the operation of the switching switches 3 and 4 in the following manner: one terminal of the intermittent switch 6 is connected to the positive terminal of the power supply P via the switching switch 3, and one terminal of the cut-off switch 7 is connected to the negative terminal of the power supply P via the switching switch 4. Additionally, as... Figure 3 (a) and Figure 3 As shown in (b), within a fixed time T1, the control unit 12 alternately switches the voltage of the control signal S1 between a high level and a low level, and keeps the voltage of the control signal S2 always at a high level. Therefore, within the fixed time T1, the intermittent switch 6 repeatedly turns on and off, while the cut-off switch 7 remains on. In other words, within the fixed time T1, the intermittent switch 6 repeatedly turns on and off, while the cut-off switch 7 remains on. Thus, as... Figure 3 (c) and Figure 3 As shown in (d), a DC voltage is applied to coil L during a fixed time T1, and a DC current flows through coil L. As a result, the temperature of the coil L gradually rises due to the heat generated by the resistive component of coil L during the fixed time T1. Furthermore, the duty cycle of the control signal S1 is set such that the DC current flowing through coil L during the fixed time T1 is below the rated current of coil L.

[0040] Next, the control unit 12 stands by until a fixed time T2 elapses (step S12: "No"). During the fixed time T2, the control unit 12 controls the operation of the switching switches 3 and 4 in the following manner: one terminal of the intermittent switch 6 is connected to the positive terminal of the power supply P via the switching switch 3, and one terminal of the disconnect switch 7 is connected to the negative terminal of the power supply P via the switching switch 4. Additionally, as... Figure 3 (a) and Figure 3 As shown in (b), during the fixed time T2, the control unit 12 keeps the voltage of control signal S1 and control signal S2 at a low level at all times. Therefore, during the fixed time T2, the intermittent switch 6 and the cut-off switch 7 are always open. In other words, during the fixed time T2, the intermittent switch 6 and the cut-off switch 7 are always closed. Thus, during the fixed time T2, the temperature rise of the coil L can be used to evaporate the moisture adsorbed on the insulator RL. Furthermore, if the moisture adsorbed on the insulator RL can be sufficiently evaporated in step S11, the standby fixed time T2 can be omitted, and the process can proceed from step S11 to step S13.

[0041] Next, after a fixed time T2 elapses (step S12: "Yes"), the control unit 12 implements the DC voltage mode (step S13). During the fixed time T3 of implementing the DC voltage mode, the control unit 12 controls the operation of the switching switches 3 and 4 in the following manner: one terminal of the intermittent switch 6 is connected to the positive terminal of the power supply P via the switching switch 3, and one terminal of the disconnect switch 7 is connected to the negative terminal of the power supply P via the switching switch 4. Additionally, as... Figure 3 (a) and Figure 3 As shown in (b), during the fixed time T3, the control unit 12 keeps the voltage of control signal S1 at a high level and the voltage of control signal S2 at a low level. Therefore, during the fixed time T3, intermittent switch 6 is always on and cut-off switch 7 is always off. In other words, during the fixed time T3, intermittent switch 6 is always on and cut-off switch 7 is always off. Thus, as... Figure 3 (c) and Figure 3 As shown in (d), a DC voltage is applied to the coil L during a fixed time T3, and a DC current flows through the coil L. Additionally, during the fixed time T3, the control unit 12 measures the resistance value of the insulator RL by dividing the voltage detected by the voltage detection unit 10 by the current detected by the current detection unit 9.

[0042] Next, if the resistance of the insulator RL is below the threshold value (step S14: "No"), that is, if current easily flows from the power source P through the coil L and the insulator RL to ground, the control unit 12 again performs the processing of steps S11 to S13 to evaporate the moisture adsorbed on the insulator RL, thereby further increasing the resistance value of the insulator RL. Here, Figure 4 Figure (a) is an example of the increase in the resistance of the insulator RL associated with an increase in the number of pulse voltage mode implementations. Furthermore, Figure 4 The horizontal axis of the two-dimensional coordinate system shown in (a) represents the number of times the pulse voltage mode was executed before the induction furnace 1 was started, and the vertical axis represents the resistance value of the insulator RL. Figure 4 In the example shown in (a), before the induction furnace 1 is started, after the third pulse voltage mode is implemented, the resistance value of the insulator RL is still below the threshold. After the fourth pulse voltage mode is implemented, the resistance value of the insulator RL becomes greater than the threshold.

[0043] On the other hand, when the resistance of the insulator RL is greater than the threshold (step S14: "Yes"), that is, when current is difficult to flow from the power supply P through the coil L and the insulator RL to ground, the control unit 12 starts the operation of the induction furnace 1 (step S15). During the operation of the induction furnace 1, the control unit 12 controls the operation of the switching switches 3 and 4 in the following manner: the input terminal IN+ of the inverter circuit 2 is connected to the positive terminal of the power supply P via the switching switch 3, and the input terminal IN- of the inverter circuit 2 is connected to the negative terminal of the power supply P via the switching switch 4. In addition, during the operation of the induction furnace 1, the control unit 12 repeatedly performs the operation of turning on the switching elements SW1 and SW4 and turning off the switching elements SW2 and SW3, and then turning off the switching elements SW1 and SW4 and turning on the switching elements SW2 and SW3, so as to allow the alternating current to flow in the coil L.

[0044] Furthermore, the control unit 12 can also be configured such that, when the resistance value of the insulator RL is greater than a threshold (step S14: "Yes"), in step S15, the remaining lifespan of the insulator RL is estimated based on the decreasing trend of multiple resistance values ​​obtained each time the induction furnace 1 is started. Here, Figure 4 Figure (b) is an example of a graph showing the decreasing trend of the resistance value of the insulator RL over time. Furthermore, Figure 4 In the two-dimensional coordinate system shown in (b), the horizontal axis represents time, and the vertical axis represents the resistance value. Figure 4 The points shown in (b) represent the resistance values ​​of the insulator RL stored in the storage unit 11 each time the induction furnace 1 is started. Furthermore, the resistance value stored in the storage unit 11 is, for example, the last calculated resistance value when the resistance value of the insulator RL is below a threshold value. Figure 4 As shown in (b), within a fixed period, the control unit 12 uses the lowest resistance value (and) among the multiple resistance values ​​stored in the storage unit 11 that correspond to each specified time. Figure 4 Using the lowest resistance values ​​r1 to r6 corresponding to times t1 to t6 shown in (b), an approximate straight line is obtained. The difference between the time corresponding to the intersection of this approximate straight line and the threshold and the current time is taken as the remaining lifetime of the insulator RL. Furthermore, Figure 4 The threshold shown in (b) is... Figure 4 The thresholds shown in (a) can be either the same or different values.

[0045] Thus, in the induction furnace 1 of this embodiment, before the resistance value calculated by the current detected by the current detection unit 9 and the voltage detected by the voltage detection unit 10 when the intermittent switch 6 is always on and the cut-off switch 7 is always off becomes greater than a threshold, the operation of turning the intermittent switch 6 on and off, and turning the cut-off switch 7 on continuously, and then turning the intermittent switch 6 on continuously and the cut-off switch 7 off continuously is repeated. When the resistance value becomes greater than the threshold, the induction furnace 1 starts operating. Therefore, before the induction furnace 1 starts operating, the moisture adsorbed on the insulator RL between the coil L and ground can be evaporated, increasing the resistance value of the insulator RL, and then the induction furnace 1 starts operating. That is, just before the induction furnace 1 is about to start, the self-heating of the coil L caused by the direct current evaporates the moisture adsorbed on the insulator RL. Then, the insulation performance of the insulator RL is evaluated by the resistance value calculated using the direct current and the direct current. After ensuring the insulation performance of the insulator RL, the induction furnace 1 is started. Therefore, during the operation of the induction furnace 1, the current flowing from the power supply P through the coil L and the insulator RL to ground can be suppressed. This prevents the current flowing through the coil L from exceeding its rated current, thereby suppressing the deterioration of the coil L. In other words, according to the embodiment of the induction furnace 1, even if the resistance value of the insulator RL decreases, a relatively large current will not flow through the coil L.

[0046] Furthermore, in the induction furnace 1 of the embodiment, before the induction furnace 1 is operated, the power supply P used to allow alternating current to flow in the coil L during the operation of the induction furnace 1 is also used as the power supply to allow direct current to flow in the coil L. Therefore, there is no need to set up a new power supply other than the power supply P, thus suppressing the manufacturing cost of the corresponding part.

[0047] This invention is not limited to the embodiments described above and can be implemented with various modifications. In the above embodiments, the size, shape, function, etc., of the structural elements illustrated in the drawings are not limited thereto, and appropriate modifications can be made within the scope of achieving the effects of this invention. Furthermore, modifications can be made appropriately as long as they do not depart from the purpose of this invention.

[0048] Figure 5 This is a diagram showing a modified example of the induction furnace 1 according to the embodiment. Furthermore, in Figure 5 In the middle, to and Figure 1 Structures with the same structure shown are labeled with the same markings and their descriptions are omitted.

[0049] exist Figure 5 In the induction furnace 1 shown, with Figure 1The difference in the induction furnace 1 shown is that: the switching switch 4, the intermittent switch 6, and the cut-off switch 7 are omitted; the switching element SW1, which is used to allow the AC current to flow in the coil L during the operation of the induction furnace 1, is also used as the intermittent switch 6, and the switching element SW4, which is used to allow the AC current to flow in the coil L during the operation of the induction furnace 1, is also used as the cut-off switch 7; and the switching switch 3 and the low-pass filter 8 are connected between the output terminal OUT+ of the inverter circuit 2 and one terminal of the coil L.

[0050] Furthermore, one input terminal IN+ of inverter circuit 2 is directly connected to the positive terminal of power supply P, and the other input terminal IN- of inverter circuit 2 is directly connected to the negative terminal of power supply P. Additionally, switching element SW3 is designated as the first switching element, and switching element SW2 as the second switching element. That is, one terminal (collector terminal) of switching element SW3, as the first switching element, is connected to the positive terminal of power supply P, and the other terminal (emitter terminal) of switching element SW3 is connected to the other terminal of coil L. Similarly, one terminal (collector terminal) of switching element SW2, as the second switching element, is connected to one terminal of coil L, and the other terminal (emitter terminal) of switching element SW2 is connected to the negative terminal of power supply P. Furthermore, one terminal (collector terminal) of switching element SW1, which serves as the intermittent switch 6, is connected to one terminal of switching element SW3, and the other terminal (emitter terminal) of switching element SW1 is connected to one terminal of switching element SW2. In addition, one terminal (collector terminal) of the switching element SW4, which is the cut-off switch 7, is connected to another terminal of the switching element SW3, and the other terminal (emitter terminal) of the switching element SW4 is connected to another terminal of the switching element SW2.

[0051] first, Figure 5 The control unit 12, during the fixed time T1 of the pulse voltage mode, controls the operation of the switching switch 3 in the following manner: one terminal of the coil L is connected to the output terminal OUT+ (the other terminal of the switching element SW1) of the inverter circuit 2 via the low-pass filter 8. Furthermore, during the fixed time T1, the control unit 12 repeatedly turns the switching element SW1 on and off, keeps the switching element SW4 always on, and keeps the switching elements SW2 and SW3 always off. In other words, during the fixed time T1, the switching element SW1 repeatedly turns on and off, the switching element SW4 is always on, and the switching elements SW2 and SW3 are always off. Thus, during the fixed time T1, a DC voltage is applied to the coil L, and a DC current flows through the coil L. Consequently, during the fixed time T1, the temperature of the coil L gradually rises due to the heat generated by the resistive component of the coil L. Furthermore, the duty cycle of the control signal for the switching element SW1 is set such that the DC current flowing through the coil L is below the rated current of the coil L during the fixed time T1.

[0052] then, Figure 5 The control unit 12 shown is in standby mode until a fixed time T2 has elapsed. During the fixed time T2, Figure 5 The control unit 12 shown controls the operation of the switching switch 3 in the following manner: one terminal of the coil L is connected to the output terminal OUT+ of the inverter circuit 2 (the other terminal of the switching element SW1) via the low-pass filter 8. Furthermore, within a fixed time T2, Figure 5 The control unit 12 shown keeps the switching elements SW1 to SW4 always off. In other words, the switching elements SW1 to SW4 are always off during the fixed time T2. As a result, during the fixed time T2, the temperature rise due to the heat of the coil L allows the moisture adsorbed on the insulator RL between the coil L and ground to evaporate. Furthermore, if the moisture adsorbed on the insulator RL can be sufficiently evaporated during the fixed time T1, the standby fixed time T2 can be omitted.

[0053] Then, after a fixed time T2, Figure 5 The control unit 12 shown implements a DC voltage mode. During the fixed time T3 of implementing the DC voltage mode, the control unit 12 controls the operation of the switching switch 3 in the following manner: one terminal of the coil L is connected to the output terminal OUT+ (the other terminal of the switching element SW1) of the inverter circuit 2 via the low-pass filter 8. Furthermore, during the fixed time T3, the control unit 12 keeps the switching element SW1 always on and keeps the switching elements SW2 to SW4 always off. In other words, during the fixed time T3, the switching element SW1 is always on and the switching elements SW2 to SW4 are always off. Thus, during the fixed time T3, a DC voltage is applied to the coil L, and a DC current flows through the coil L. Additionally, during the fixed time T3, the control unit 12 measures the resistance value of the insulator RL by dividing the voltage detected by the voltage detection unit 10 by the current detected by the current detection unit 9.

[0054] Next, when the resistance of the insulator RL is below the threshold value, Figure 5 The control unit 12 shown executes the pulse voltage mode and DC voltage mode again to evaporate the moisture adsorbed on the insulator RL, thereby further increasing the resistance value of the insulator RL.

[0055] On the other hand, when the resistance of the insulator RL is greater than the threshold, Figure 5The control unit 12 shown starts the operation of the induction furnace 1. During the operation of the induction furnace 1, the control unit 12 controls the operation of the switching switch 3 in the following manner: one terminal of the coil L is directly connected to the output terminal OUT+ of the inverter circuit 2 (the other terminal of the switching element SW1). In addition, during the operation of the induction furnace 1, the control unit 12 repeatedly performs the operation of turning on the switching elements SW1 and SW4 and turning off the switching elements SW2 and SW3, and then turning off the switching elements SW1 and SW4 and turning on the switching elements SW2 and SW3, so as to allow the alternating current to flow in the coil L.

[0056] Furthermore, when implementing pulse voltage mode, if it is not necessary to smooth the current flowing from the switching element SW1 through the switching switch 3 to the low-pass filter 8, the switching switch 3 and the low-pass filter 8 can be omitted. In this configuration, the output terminal OUT+ of the inverter circuit 2 is directly connected to one terminal of the coil L.

[0057] Thus, in a modified embodiment of the induction furnace 1, before the resistance value calculated by the current detected by the current detection unit 9 and the voltage detected by the voltage detection unit 10 when the switching element SW1 is always on and the switching elements SW2 to SW4 are always off becomes greater than a threshold, the operation of turning the switching element SW1 on and off, turning the switching element SW4 on and turning the switching elements SW2 and SW3 off repeatedly, and then turning the switching element SW1 on and turning the switching elements SW2 to SW4 off repeatedly, is repeated until the resistance value becomes greater than the threshold. Therefore, before the induction furnace 1 starts operating, the moisture adsorbed on the insulator RL between the coil L and ground can be evaporated, increasing the resistance value of the insulator RL, and then the induction furnace 1 can start operating. That is, before the induction furnace 1 is about to start, the self-heating of the coil L caused by the direct current evaporates the moisture adsorbed on the insulator RL. Then, the insulation performance of the insulator RL is evaluated by the resistance value calculated using the direct current and the direct current, and the induction furnace 1 is started after ensuring the insulation performance of the insulator RL. Therefore, during the operation of the induction furnace 1, the current flowing from the power supply P through the coil L and the insulator RL to ground can be suppressed. This prevents the overall current flowing through the coil L from exceeding its rated current, thereby suppressing deterioration of the coil L. In other words, according to a variation of the induction furnace 1, even if the resistance of the insulator RL decreases, a relatively large current will not flow through the coil L.

[0058] In addition, with Figure 1 Compared to the induction furnace 1 shown, Figure 5 The induction furnace 1 shown can omit the switching switch 4, intermittent switch 6, and cut-off switch 7, thereby reducing manufacturing costs.

Claims

1. A diagnostic method for an induction furnace, wherein the induction furnace generates a magnetic field on its inner side by circulating an alternating current in a coil disposed on the outer side of the furnace, the method comprising the following steps: The first step is to apply a pulsed voltage to the coil so that the moisture adsorbed on the insulator between the coil and ground evaporates. The second step is to apply a DC voltage to the coil after the first step; as well as The third step involves determining the resistance value of the insulator based on the voltage applied to the coil and the current flowing through the coil in the second step. The remaining lifetime of the insulator is estimated based on the resistance value measured in the third step. The resistance value is the final resistance value obtained by performing the first step, the second step, and the third step multiple times, and when the resistance value is below the second threshold.

2. The diagnostic method for an induction furnace according to claim 1, characterized in that, The remaining lifetime of the insulator is estimated based on the decreasing trend of the plurality of resistance values.

3. The diagnostic method for an induction furnace according to claim 2, characterized in that, Find an approximate straight line for the minimum resistance value at a specified time, and take the time corresponding to the intersection of the approximate straight line and the first threshold as the remaining lifetime of the insulator.

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