Overview The 2200 cubic meter blast furnace 1NBA slag treatment system in the ironmaking plant is an important part of blast furnace production. It converts the slag produced from ironmaking into water slag, which is then transferred to the main slag conveyor belt at the transfer station and transported to the water slag storage yard. There are four rubber belts used for transporting water slag. The following analysis and discussion will focus on the relay coil failure of the conveyor motor electrical control system as an example. [b]1 Electrical Control System of Conveyor 1.1 Principle of Conveyor Electrical Control System[/b] The electrical control principle diagram of the conveyor is shown in Figure 1: As can be seen from the principle diagram, when the coils of relays K12 and K13 are closed, the coil of the main contactor K11 of the conveyor motor can be energized, and the belt will run. Conversely, when the coils of relays K12 or K13 are released, the coil of the main contactor K11 of the conveyor motor is de-energized and the belt stops. 1.2 Working Status of Conveyor Electrical Control A conveyor belt is about 80 meters long. Every 20 meters, a pull rope switch is installed on each side of the belt for the operator to stop the belt operation in an emergency. The emergency stop switches, pull rope switches B11-B18 in Figure 1, and emergency switches S11 and S12 are installed in the transfer stations at both ends of the conveyor belt. All other components are installed in the main control cabinet in the control building, with a distance of approximately 350 meters between the production site and the control cabinet. The coil operating voltage of the contactors and relays is AC 220V. The control live wires of relays K12 and K13 use three cables from a single KVVR4*1.5 control cable, which draw power L (i.e., the control live wire) from the control cabinet, passing through the pull rope switches B11-B18 and the emergency switches, and connecting to the coils of K12 and K13. The power N for the coils of relays K12 and K13 is drawn from the control cabinet. 2. Fault Phenomenon When the conveyor belt is running, disconnecting the emergency switch S11 or S12 releases the coils of relays K12 and K13. The main contactor K11 coil also releases, stopping the conveyor belt. However, when relays K12 and K13 are simultaneously energized and engaged... Disconnecting any one of the pull rope switches B11-B14 on the left side of the conveyor belt will prevent the relay K12 coil from releasing. Similarly, disconnecting any one of the pull rope switches B15-B18 on the right side of the conveyor belt will prevent the relay K13 coil from releasing, thus preventing the main contactor K11 coil from releasing and the conveyor belt from stopping. This clearly does not meet the requirements for safe production. It fails to achieve the goal of quickly stopping the conveyor belt in an emergency to avoid sudden safety and equipment accidents, and thus fails to ensure the safety of personnel and equipment. [b]3 Cause Analysis and Judgment 3.1 Cause of Induced Voltage[/b] According to the electrical control schematic diagram before the modification, one end of the coils of relays K12 and K13 is connected to the neutral wire (and in parallel to the neutral wire of the control cabinet), and the other end is connected to the live wire of the control cabinet through the pull rope switch and emergency switch. During normal operation, the coils of relays K12 and K13 are always in a closed state. Because relays K12 and K13 control the live wire using the same ordinary control cable, and only one phase of AC power is supplied to the multi-core cable, the parallel arrangement of the cable cores creates a capacitive effect, easily inducing voltage in the unenergized cores. Moreover, the longer the cable or the larger the current supplied, the higher the induced voltage. Due to the excessive length of the cable controlling the belt conveyor relays, the influence of induced voltage must be considered. Relays K12 and K13 are miniature relays with smaller release spring tension, resulting in a lower holding voltage after coil activation. The presence of induced voltage can usually be determined by measurement. Theoretically, assuming normal cable insulation, the voltage across the relay K12 coil should be 0V when the left pull rope switch is disconnected. If a voltage is present across the relay K12 coil, the measured voltage is due to induction. In actual operation, after disconnecting the left pull rope switch, a voltage of 150V was measured across the relay K12 coil. The technical parameters of the relay used state that the relay's release voltage is less than 30% of the coil's rated voltage, which is 220V. Therefore, the relay will only release when the voltage is less than 66.6V. Similarly, when relays K12 and K13 are both engaged, the right pull-rope switch is disconnected. The voltage across the coil of relay K13 is measured to be around 150V. The coil still does not release. 3.2 Causes of Transient Voltage Because the inductor is a coil, it is an energy storage element. According to Lenz's law, when a coil changes from one state to another, it always tries to maintain its original state, hindering the increase or decrease of current. For the coils of relays K12 and K13, there is a certain inductance. At the moment the pull-rope switch is disconnected, the sudden current in the coil will generate a large back electromotive force transient voltage. When the transient voltage value is greater than the relay's release voltage, the coil cannot release normally. [b]4 Elimination Measures 4.1 Elimination of Induced Voltage[/b] In view of the actual situation of induced voltage generation on site, we can take the following measures: First. Twisted-pair cables can be used for control circuits to avoid parallel cable arrangement, thus eliminating induced voltage. However, twisted-pair cables are 2-4 times more expensive than ordinary control cables. Second: When designing electrical control wiring, the current direction in the cable cores should be different in the same cable to cancel induction; or different phase sequence currents can be wired in the same cable to eliminate induced voltage. Third: Connect an appropriate resistor in parallel with the relay coil, usually 2-4 times the coil resistance value, to absorb induced voltage. The wiring method is shown in Figure 2(a). 4.2 Elimination of Transient Voltage For the generation of transient voltage, measures can be taken to suppress it. An energy release path can be provided for each of the K12 and K13 coils, so that the stored energy is consumed in the release circuit. The energy release circuit can be composed of resistors and capacitors. The specific implementation may vary slightly depending on the situation. The wiring method is shown in Figure 2(a) and (b). During normal operation, the resistor R consumes a certain amount of power. The component parameters of R and C can be selected through simple calculation. Alternatively, a varistor can be connected in parallel across the coil. Because the varistor works normally... The resistance is very high, and the power consumption is almost zero. When the circuit is broken, it can limit overvoltage to a safe range. Its wiring method is shown in Figure 3. When selecting a varistor, pay attention to the varistor voltage value and rated power. 5 Conclusion In summary, the simplest method is to connect a resistor R in parallel across each of the coils of relays K12 and K13. This can eliminate induced current and suppress transient voltage. This method has been used to effectively handle the failure of belt conveyor relays to release, achieving satisfactory results. Analysis shows that in power systems, reasonable equipment selection and correct wiring are crucial. This summary aims to provide successful experiences for the improvement of similar equipment. [b][align=center]For detailed content, please click: Fault Classification and Handling of Relay Coil Failure to Release[/align][/b]