The diagram shows the circuit for switching on or off an electric motor. We can redraw this diagram in a different way, using two vertical lines to represent the input power rails and stringing the rest of the circuit between them. Figure 1. Both circuits have the switch in series with the motor and supplied with electrical power when the switch is closed.
With such a diagram the power supply for the circuits is always shown as two vertical lines with the rest of the circuit as horizontal lines. The power lines, or rails as they are often termed, are like the vertical sides of a ladder with the horizontal circuit lines like the rungs of the ladder. The horizontal rungs show only the control portion of the circuit; in the case of Figure 1. Circuit diagrams often show the relative physical location of the circuit components and how they are actually wired.
With ladder diagrams no attempt is made to show the actual physical locations and the emphasis is on clearly showing how the control is exercised. In the normal state, push button 1 is open and push button 2 closed. When button 1 is pressed, the motor circuit is completed and the motor starts. Also, the holding contacts wired in parallel with the motor close and remain closed as long as the motor is running.
Thus when the push button 1 is released, the holding contacts maintain the circuit and hence the power to the motor. To stop the motor, button 2 is pressed. This disconnects the power to the motor and the holding contacts open.
With the use of a retentive timer to control a motor with an automatic lubrication system, they have made a great practical example. Ladder logic program for automatc lubrication. Feel free to watch this video for more information about PLC timers in ladder logic.
The video illustrates some great examples and the basics of PLC timers. Finally I will encourage you to take a look at my video about the pulse timer. The pulse timer is used for generating pulses of a specific length and is very useful in ladder logic:. Motor control can be done with a PLC program.
Here are some examples of ladder diagrams for motor control. One of the most common ways to start an AC motor is by first starting the motor in star connection. When the motor speed is sufficient, the connection is switched to delta.
This is due to the high current AC motors use when starting. To switch between the star and the delta relay, a timer is used. It includes a lot explination and a lot of great power and control circuit diagrams. Example 5 is on page Still commonly used in many factories the DOL or direct on line motor starter is another way of starting AC motors.
The DOL is made of a contactor usually 3-phase contactor , an overload relay like the thermal relay, and some connections in between. Sometimes you might have to use a smaller relay between the PLC output and the coil of the contactor. Make sure you always check the ratings of the PLC outputs you are using. All examples of how to use PLC programming and ladder logic to solve real problems. One of the most used applications for a PLC is the traffic lights. At many schools, universities and even companies you will get the challenge to make a traffic light ladder logic diagram.
The traffic light PLC program is a combination of timers to control which lights are turned on and for how long time. But some sort of interlock must be there to prevent the green light to be on in multiple directions.
A PLC program like the traffic light is a little more complicated and therefore are a lot more solutions to. For inspiration you can look at these good examples of traffic light ladder diagrams:.
The first ladder logic example is from Engineer On A Disk, which is a marvelous site full of great articles. In the example you will get all the ladder diagrams and step-by-step instructions and explanation. PLC program example of a traffic light. If you are using LogixPro Simulator, then you should absolutely take a look at this great example video:. Detailed example from Electrical Engineering Portal. This is a great example because of all the explanation it gives. You will be introduced to the actuators motors , sensors and switches and a step-by-step guide to how to make the PLC program.
At last you will see the example ladder logic for the bottle filling application. Before you check out this application, be sure to include counters in your program. For a bottle filling plant, it is very important to keep track of how many bottles that went through the system. For this you can use the PLC counter instructions. PLC implementation of bottle filling application. If you are using LogixPro Simulator from Allen Bradley, then you can learn a lot from this example video:.
Elevators are often controlled by a PLC or a similar controller sometimes even relay controllers. In fact a PLC program is a great way to make an elevator control. But before you start looking at ladder diagrams and PLC program examples for elevator control, some safety issues are important to know about. You might want to incorporate a safety relay in the system.
Elevator Control System: How they work. And here is an example of a ladder diagram for elevator control from circuit4hobby :. Rate this article below or write a comment if you found this article helpful or you have any questions. If you would like to share your ladder logic examples, please write a comment below or send me a message. You can also find us on Facebook. I been using Plcs for over 20 yrs..
Once you know it its very easy. I still watch all videos on youtube. Type in ladiesandtech. I have been looking for example and videos or notes for using all those symbol in ladder logic and how can i use the in future for my programming.
Got a simmilar problem Chavan…. Thank you. How to make a ladder diagram for oil and gas tank level? All the best! How to enable this button in manager?
Thank you so much for this. You should start a paypal. We use Parker drives. I have to coordinate a sequence of motors to flow at one speed with the ability to draft. Well the post removed my spaces, in the last post.
Hi, My name is Mr. So, I want to help me with a project for school. Can you help me, please. I send my adress e-mail to contact: [email protected]. Most people learn to draw ladder logic diagrams this way — by building them as electrical schematics.
But there are some differences. This is why I will advise you to learn it in a different way. The problem here is that electrical control systems and the PLC work in different ways. Here are the biggest differences:. The first thing you will see when you create a new piece of ladder logic is two vertical lines.
It is in between these two lines your ladder logic goes. When you draw ladder logic, you will draw vertical connections between these two lines. Each of those is called a rung. Just like on a physical ladder.
Ladder Logic with Horizontal Lines called Rungs. In these rungs, you can put any of the ladder logic symbols to create the logic you want. As you can see above, I have put numbers on each rung. This is to understand how the PLC hardware will execute the ladder logic.
You may be familiar with the PLC scan time or scan cycle. Roughly said, the PLC will first scan all its inputs, then execute the program to set outputs. This might be one of the most important rules of ladder logic. The PLC only executes one rung at a time, then executes the next. In fact, the PLC only executes one symbol at a time.
Each symbol in ladder logic is an instruction. This can, in the beginning, be rather confusing. I will explain this with simple examples. Let me start by giving you a simple example. In this first example, you will be introduced to the two first ladder logic symbols. They are basically logic instructions, that make you able to create a piece of logic. That piece of logic is your ladder logic or PLC program. If you take a closer look at the example below, you will see two instructions symbols. You can check out my video tutorial, and see how the basic PLC instructions work.
I would still recommend you to finish this tutorial anyway since the video only gives you basic coverage. The first instruction here is called examine if closed. The symbol for the instruction looks like this:. This is a conditional instruction. It means that you can use it to check if something is true. For example, check if a bit is on. This is the address of the specific bit, this instruction will examine.
In this case, a digital input. It could also just be an internal memory bit or even an output. Examine if closed is also known as normally open. It works basically the same way as a normally open contact in en electrical circuit. Of course, the normally open contact has no memory bit as a condition. The condition is whether the contact is activated or not. So the condition could be a finger pressing a button. Yes, inputs and outputs are also bits of memory in the PLC.
In the example above, the examine if closed instruction has been given memory address I0. This address belongs to the first input of the PLC.
The instruction itself even has a place in the PLC memory. What the PLC will put there is the result of the instruction. To see what the PLC uses that result for, we have to look at the next instruction:. As you can see, the symbol is placed on the right side of the rung. This means, that all the instructions that come before in the same rung act as a condition for that instruction. In our example that will be the result of the examination if closed instruction.
How ladder logic instructions work with the PLC scan cycle. In the animation above you can see that the PLC first scans all its inputs.
The states of these inputs are then saved in a memory byte. A memory byte is just 8 bits next to each other. But placing the bits next to each other is very smart. When the PLC has the states of all inputs saved, the program will start to run. The first instruction to be executed is the examination if closed normally open. The result of this instruction will be the same as the state of the memory bit. It makes sense to call the instruction normally open.
In a normal state where the memory bit is 0 the contact will be open, and the result is 0. But if the memory bit is 1 the contact will close and yield the result 1.
Now, the output coil uses the result of the previous instruction as a condition. The output coil works in a simple way. It simply sets the bit to the same value as its condition RLO.
In the PLC all the digital outputs are also assigned to bits in memory. The result of the output coil will be put in memory bit Q0. When the PLC has executed the whole program, it will set the outputs. The state of each output is set to the same state as the output bits.
Otherwise, your program might act a bit strange. This will be illustrated in the next example. At the same time, you will also learn about 3 other ladder logic instructions.
In the previous example, you learned how to read the state of digital input and set a digital output to the same state.
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