During the operation of an induction furnace, because users (especially new users) do not grasp the rules, do not have enough knowledge of the use characteristics of furnace lining, and various misjudgments occur, it is easy to cause serious accidents such as short service life of furnace lining, difficult to use, difficult to master, and even furnace leakage. Of course, there are many reasons for furnace leakage, and there are many ways to prevent furnace leakage. One of the effective ways is to know the furnace lining thickness in time, master the best furnace lining thickness from practice, and there are many ways to know the furnace lining thickness, such as using the naked eye, measuring with a ruler, etc. Among them, the induction furnace ground leak detector can also help users detect the furnace lining thickness, which is one of the auxiliary detection devices to prevent induction heating furnace leakage and reduce the occurrence of furnace leakage accidents.
1. Use and Inspection of Induction Furnace Ground Leak Detector
(1) To correctly understand the furnace lining thickness detection device, it is necessary to connect the furnace lining analog resistance and observe whether the corresponding number (or indication value) is displayed normally. If the corresponding number (or indication value) shows the same or reaches the set alarm value for several times, the action alarm indicates that the product works normally and is qualified. The newly sintered furnace lining has low grounding resistance due to its high moisture content. The furnace lining thickness detection device can only be used after 3-5 furnaces have been melted normally. After 2-5 furnaces have been melted normally, the furnace lining thickness detection device can work normally and can be put into normal use after passing the user’s inspection.
(2) Each time the furnace lining is newly built, the induction furnace ground leak detector should be installed by full-time engineering and technical personnel. After installation, it should be strictly checked and judged to be correct and intact before it can be put into use. It should sign on the boiler opening sheet and record the inspection results. If the installation is not reasonable or the contact is not good (such as the bottom electrode and steel wire are not in good contact, or the steel wire and liquid charge are not in good contact), the grounding leakage detector of the induction furnace will fail to work properly.
(3) According to the difference between large and small furnaces, the reference data are as follows: the furnace lining should be checked frequently to prevent “depletion”, and the furnace must be repaired when the lining thickness (excluding asbestos plate, mica plate…) is worn less than 60mm-80mm (medium and large capacity furnaces), and 40mm-60mm (medium and small capacity furnaces). The grounding leakage detector of the induction furnace shall make corresponding display data, determine the alarming size of furnace lining thickness, which shall be determined by the user according to the user level and experience, and adjust the corresponding parameters displayed (because each furnace lining has different resistivity and leakage current parameters under the same thickness, which must be adjusted), adjustment method, The user shall measure the resistance value of 40mm ～ 50mm ～ 60mm ～ 70mm ～ 80mm ～ 90mm ～ 100mm ～ 110mm ～ 120mm ～ 160mm on the furnace lining used by himself (after sintering ≥ 5 heats), and measure the corresponding parameters displayed by the corresponding leakage current. It is normal that the parameters displayed by the medium frequency power supply are different when it is in normal use and when it is shut down. The two parameters in use are used as test data to determine whether the furnace lining wall thickness can be used normally.
(4) There are many reasons for the leakage of medium frequency induction furnaces during operation, such as poor quality of furnace lining material, nonstandard masonry, and sintering of furnace lining material, the incorrect sintering process of furnace lining material and unreasonable charging method, damage of furnace lining, failure of full-time personnel to inspect the erosion and damage of furnace lining before charging or daily opening, and improper use of grounding leakage detector of induction furnace. The grounding leakage detector of the induction furnace is only used to detect the wall thickness of the furnace lining. The thickness of the furnace lining can prevent furnace leakage, which is determined by the user according to various factors such as furnace lining material and molten iron temperature to avoid furnace leakage.
(5) Before the formal melting of the electric furnace every day, the electrician should regularly check whether the furnace lining thickness detection device works normally, whether the grounding wire and the molten metal are in good contact, and whether various display data are normal. Both shut down and startup data should be included in the work and recorded. If there is a problem, the instrument can be put into operation only after it is repaired in time, otherwise, it cannot be started.
(6) If a large area of molten iron leakage occurs, the load current of the furnace rises rapidly and exceeds the protection current value, the frequency converter will automatically stop to protect the equipment components from damage. The furnace can operate as usual after repair. If the crack in the furnace lining leaks molten iron, causing the induction coil to be local, without causing the inter-turn short circuit or local ignition of the induction coil, the load current does not exceed the current protection value of the equipment, and the equipment cannot be stopped, which is a normal phenomenon. Therefore, users should always check whether the rated current protection value of the equipment works normally. The alarm signal set by the induction furnace grounding leakage detector can also be linked to the shutdown of the frequency conversion power supply device. The size of the alarm signal is carefully determined by the user. Because the signal is too large, it is easy to cause equipment misoperation, the signal is too small, and the action is not sensitive, the above two methods can achieve the shutdown of variable frequency power supply in the work, but the principle of the shutdown is different, which needs to be treated separately, find out the reason, and cannot be confused.
2. Working Principle of Induction Furnace Ground Leak Detector
The power supply is reduced by the transformer, rectified by the bridge, and filtered by a capacitor into a DC-safe low-voltage U, which is added between the two electrodes through current-limiting resistance. Under normal conditions, the furnace lining resistance is large, and the ammeter indicates a small current range. When the furnace lining becomes thinner, the resistance decreases, the current increases, and the current of the meter is greater when the furnace leakage occurs. Thus, the originally set operational amplifier acts, the transistor is on, and the relay of the collector act. On the one hand, the contact of the relay gives an alarm with a bell, and on the other hand, it controls the intermediate frequency power control board to stop it. The two potentiometers set the indication range of the ammeter respectively.
The bottom electrode shall be installed for the induction furnace ground leak detectors. DC low voltage U is applied between the stainless steel mesh and the bottom electrode between the induction coil cement and the furnace lining. When the furnace lining is normal, the resistance is large, the current is small, and the corresponding parameters on the device display screen (or meter head) are small. In case of low resistance of furnace lining (furnace lining thinning and large area leakage of molten metal will cause low resistance of furnace lining), the current is large, and the corresponding parameters on the display screen (or meter head) of the device are large. If the data is large enough to exceed the alarm parameters set by the user, the alarm device will work and alarm. This signal can also be linked to the shutdown of the frequency conversion device, and whether the linkage is determined by the user.
3. Installation of Alarm Device
For furnace bottom electrode Φ It is made of 1-2mm non-magnetic stainless steel wire, with a total of 8-18 pieces (according to the size of the furnace). The stainless steel wire is introduced through the special hole at the furnace bottom and arranged radially. The small hole through which the steel wire passes shall be blocked and sealed with fire-resistant slurry. The stainless steel wire shall be of sufficient length. After the quartz sand layer at the furnace bottom is knotted, the stainless steel wire shall be able to expose the sand layer for 5-10 mm. Cut off the part that is too long and bend it so that the exposed part is parallel to the surface of the quartz sand layer. In this way, when the crucible is hoisted in, it can ensure that the stainless steel wire and the iron crucible are in good contact. After the iron crucible is melted, the steel wire is always in contact with the molten iron.
During medium repair of the furnace lining, the original laid stainless steel wire can be found on the non-sintering layer at the furnace bottom and extended. The installation method is the same as the above. The stainless steel wire from the small hole at the bottom of the furnace shall be combined into one strand and covered with a heat-resistant porcelain tube to prevent the stainless steel wire from contacting the steel plate at the bottom of the furnace and causing a short circuit to the ground. Before each melting, it is necessary to check that the four links of the bottom electrode stainless steel wire, liquid charge and the input end of the lining wall thickness inspection device are in good contact. If there is any problem, it must be repaired before it can be put into operation. The wiring between the detection device and the furnace is a two-color wire grounding. The red wire is connected to the copper bar connected with the induction coil, and the other wire is connected to the furnace’s bottom electrode. The wire should be double-insulated. The power line is connected to an AC 220V power supply.