As a necessary certificate for electricians, as the safety supervision requirements for employers become more and more stringent, the gold content is getting higher and higher, and the exams are becoming more and more stringent!
Some of the general textual research questions are compulsory, and the difficulty is mixed. The automatic reciprocating loop control of the travel switch is also one of the more difficult questions in the mandatory exam! Practical work has also proved that if you are a maintenance electrician, this Circuit is also very common in maintenance. Lathes, milling machines, and winding equipment all have the shadow of its transformation and use. Today, I will talk about the principle of this circuit and its troubleshooting techniques.
In fact, no matter how the circuit changes, these seven major parts are indispensable: circuit breakers, mainly responsible for over-current protection, button switches, travel switches, thermal relays, terminals, AC contactors, and motors. Because they are more common, each control plays a role in the circuit. I won’t explain it much, but mainly talk about its operating principle and how to overhaul it during actual operation.
Let me talk about the working principle first: ST1-ST2-ST3-ST4 is the limit switch in the fixed position. When it is pressed by the impact block during operation, its internal dynamic contact is closed, the dynamic breaking contact is opened, and the circuit is cut off. The circuit at this time is equivalent to being tapped, and it stops running instantly.
When the start button SB2 is pressed, the contactor KM1 is energized and self-locked. The motor is in the forward transmission state. When it runs to the ST2 position, the impact block presses ST2, and its internal moving-break contact makes SM1 de-energize. However, the moving and closing contacts of ST2 make KM2 energized and self-locked, and the motor enters the reverse state. When ST1 is pressed again, KM2 is de-energized, KM1 is energized again, and the motor enters the forward rotation state again. The motor will automatically reciprocate forward and backward, and enter the cycle mode!
It is worth noting here that SB1 is used as a stop button, and SB2-SB3 are composite buttons with different directions. The reason for being a composite button is to meet the requirement of direct operation without pressing the stop button when changing the direction.
ST3-ST4 is installed in the limit position to prevent limit protection when over-position is not stopped!
When the circuit fails, if the power is turned on, directly press the AC contactor (because this is a live operation, pay attention to personal safety), the motor should rotate, if the motor does not move, it means that the fault is in the main circuit. Power off, check and repair the main circuit .
If the motor rotates, it means that the fault lies in the control circuit. Cut off the power and check and repair the control circuit. The troubleshooting method used here is for the situation that the circuit is not too familiar. Use the method of gradually narrowing down the range. If the circuit is integrated, the voltage follow method can be used to check faster, that is, follow the voltage, and where there is no electricity, the fault will be there. , This is more simple and convenient.