Blast furnace charging car overtravel backup protection circuit

The disadvantage of the limit switch SA as a limit element is that it can only disconnect the control circuit of the contactor. When the main contact of the contactor encounters welding or delayed release, the SA limit protection will fail. Therefore, in some important equipment, such as blast furnace feeders, overtravel backup protection must be installed. Usually, overtravel backup protection is provided with a main control contactor KM3 to cut off the main circuit, as shown in the figure. Since the motor of the blast furnace trolley starts frequently, the starting current is large and the environment is dusty. Contactors KM1 and KM2 have been running for a long time, especially the uplink contactor KM1, which has greater wear and tear, and sometimes has singeing and welding phenomena. In addition, the core of the contactor is often stained with sludge, and frequent movements cause the release spring to become soft. The main contact of the contactor is still connected to the main power supply after the coil loses power, and the motor M continues to run. For this purpose, a contactor KM3 of the same capacity is connected in series before KM1 and KM2. Normally, KM3 is in the pull-in state; only when KM1 or KM2 cannot be cut off by SA and motor M is still running causing over-travel operation, SA's upward limit contacts 9-10 or downward limit contacts 11-12 are broken. When it is turned on, the KM3 coil loses power and cuts off the power supply to the motor M, thus serving the purpose of backup protection.