Conversion of ladder diagrams between electrical control circuits and PLCs – an essential skill for electrical engineers

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This post was last edited by yunrun on 2018-8-6 at 22:11. The principles behind electrical circuit control and PLC control are similar; naturally, electrical control can be implemented more conveniently and efficiently using PLCs. So, how exactly are electrical control circuits created? And how is it converted into a ladder diagram, and further into logical instructions controlled by a PLC? To illustrate these changes in a clearer and more intuitive manner, Changhui Instruments uses the simplest example: a button interlock for forward and reverse control circuit. 1. Reason for introducing a button-interlocked forward/reverse control circuit: Motor forward/reverse control circuits generally use the auxiliary contacts of contactors for interlocking; more details on such control circuits can be found in the article \"Principles and Applications of Motor Forward/Reverse Control Circuits That Instrument Technicians Must Know.\" This interlock circuit requires pressing the stop button first when the motor changes direction. The advantage is that it prevents power short circuits in case the contactor gets stuck, while the disadvantage is that it is inconvenient to use, as the stop button must be pressed first before the direction can be changed. http://yunrun.com.cn/upload/201808/05/201808050103206188.png Explanation: In the diagram, FU1-FU5 are fuses; SB1 is the red stop button, SB2 is the green forward start button, SB3 is the green reverse start button; FR is the thermal relay, while KM1 is the forward contactor and KM2 is the reverse contactor. So how can we solve the problem of the inconvenience associated with using the interlock circuit for the motor’s forward and reverse rotation auxiliary contacts? The method suggested by Changhui Instruments is to switch to a button-interlocked forward/reverse control circuit, which makes operation more convenient. http://yunrun.com.cn/upload/201808/05/201808050053328183.png Explanation: In the diagram, FU1-FU5 are fuses; SB1 is the red stop button, SB2 is the green forward start button, SB3 is the green reverse start button; FR is the thermal relay, while KM1 is the forward contactor and KM2 is the reverse contactor. 2. Protection measures for the button interlock forward/reverse control circuit: ① Short-circuit protection. The short-circuit protection is provided by FU1-FU5; fuses FU1-FU3 ensure short-circuit protection for the main circuit, while FU4-FU5 provide short-circuit protection for the control circuit. ②Voltage loss protection: The voltage loss protection is implemented through the self-locking function of the contactor. When the power supply voltage is lost, the contacts of the contactor automatically return to their original position, thereby breaking the circuit. ③Overload protection: Conversion of ladder diagrams between electrical control circuits and PLCs. yunrun.com.cn/tech/2126.html Overload protection is achieved using the normally closed contacts of a thermal relay. When the current in a three-phase motor exceeds the set rated value during operation, the normally closed contacts of FR open, thereby cutting off the control circuit and causing the KM1 or KM2 coils to lose power, which in turn shuts down the main circuit. ④Interlock protection is composed of the normally closed contacts of the forward start button and those of the reverse start button. When the forward-running contactor KM1 circuit is activated, the normally closed contact of the forward-start button opens, preventing the reverse-running contactor KM2 circuit from being powered, thereby achieving interlocking of the KM2 circuit ； The same is true in reverse. 3. Advantages and disadvantages of the button interlock control circuit for forward and reverse rotation ① Advantages of the button interlock motor control circuit for forward and reverse rotation: There is no need to press the stop button when changing direction. ②Disadvantages of the button interlock motor forward/reverse control circuit: When the contactor gets stuck, attempting to change the direction of operation can cause a power short circuit. To achieve forward and reverse control of the motor via hard wiring, Changhui Instruments still recommends using the auxiliary contacts of contactors for interlocking, in order to prevent power short-circuit faults that may occur if the contactors get stuck. 4. PLC control design based on the button interlock forward/reverse control circuit ① PLC I/O allocation design: http://yunrun.com.cn/upload/201808/05/201808050018358779.png The stop button SB1 has its signal input from X1 ； The start buttons SB2 and SB3; signals are input from X2 and X3 ； The signal from the thermal relay FR is input at X0 ； KM1 and KM2 are contactor coils; the corresponding programming elements in the PLC are Y1 and Y2, with signals being output from the Y1 and Y2 terminals. ②PLC hardware connections and external wiring: http://yunrun.com.cn/upload/201808/05/201808050051503795.png ③ PLC ladder diagrams: http://yunrun.com.cn/upload/201808/05/201808050112018855.png ④ PLC instruction statements: http://yunrun.com.cn/upload/201808/05/201808050126513458.png The electrical control circuit is the foundation upon which PLCs carry out their functions; being familiar with circuit diagrams is essential for getting started with PLCs! The conversion of circuit diagrams into logical instructions can be analyzed step by step during learning, starting from the basics and moving to more complex aspects, with a similar approach to analysis.