Dec. 30, 2024
In this article, I will try to explain how to analyze contact control systems based on ready-made wiring diagrams. In contact electrical systems, relays, contactors and pushbuttons are used to transfer, amplify, multiply, invert or combine control signals into logic functions. By configuring the contact connections, the appropriate behavior of the control system is achieved.
The previous lessons described how the contacts work and the actuators that control these contacts. If you are new to electrical schematics, I recommend that you read the previous lessons first. At the bottom of this article you will find the list of all lessons from the series &#;How to read electrical and I&C schematic diagrams&#;.
You will need a sample diagram for the course. On the net I found a diagram of electrical and Control and
Measurement Instruments and Automation of a sewage pumping station. I think enough to start with.
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Documentation also includes description and drawings.
We will be relying on page 6 of the sewage pumping station wiring diagram. Below is a scan of this page with corrections (highlighted in red), as the person drawing this schematic must have overlooked these errors. Such irregularities can happen even in schematics drawn by experienced engineers. Therefore, you should ultimately keep this possibility in mind when analyzing other wiring diagrams.
At first, let&#;s preliminarily analyze the page. The first 3 lines at the top of the schematic are the 400V power supply (L1, L2, L3). Line L3 was used for control therefore it was protected by overcurrent circuit breaker F5. At the very bottom of the diagram, the line marked N is the neutral potential, so the diagram should be read from top to bottom (according to current flow).
In the diagram of Fig .1. we have seven devices / receivers:
All of the elements on page 6 have to do with contactors ST1 and ST2 and we will focus on them. In what follows I will explain what conditions must be met for these contactors to switch on the P1 and P2 pump motors.
At first glance, it is not apparent that the control system has been divided into two functions &#; manual (manual) control and automatic control. If we look at the connections from below, the voltage to the coil of contactor ST1 is divided into two paths on switch S1:
Analogicznie podłączona jest cewka stycznika ST2. Podsumowując powyższe, układ sterowania na tej stronie można podzielić na dwa obszary &#; sterowanie ręczne i automatyczne.
As you can see from Figure 3, the two control areas overlap in some places. The control circuits are built this way because they share common logic conditions built using contact connections. When designing electrical schematics, the goal is to optimize the number of connections and contacts.
Now that we&#;ve preliminarily examined this side of the wiring diagram, we can move on to a more thorough examination. Let&#;s start with the manual control.
In Figure 4, I have placed page 6 of the diagram with a snippet from page 7 to make it easier for us to analyze the relationships between the diagram pages.
The first contact on the way to switch on contactors ST1 and ST2 in manual mode is the contact of relay K1 [1]. To find out which device will connect the 11,14 contact of the relay K1 [2], follow the &#;7.2&#; symbol next to the symbol. The number &#;7.2&#; refers to the device that controls the contact. In this case, it is the coil of relay K1 and can be found on page 7 in column 2 [2] .
On page 7, you can see that the device labeled B1 [3] switches on the coil of relay K1 [2]. Next to the designation B1 [3] is reference 6.2 so the device that controls contact B1 (7,8) [3] is on page 6 in column 2 [4].
From page 6 we read that device B1 [4] is a phase monitoring sensor CKF-316. This sensor disconnects contact 7,8 when it detects a loss in at least one phase (L1, L2 or L3) or voltage asymmetry between phases. The purpose of this device is to protect the pump motors from starting with inadequate voltage.
Therefore it can be concluded that the operation of the relay K1 [2] (and its contacts [1]) symbolizes the correctness of the 400V supply. The first condition for switching on the pumps in both manual and automatic control will be correctness of 400V power supply (that is activation of relay K1).
Next, to switch on contactors ST1 and ST2 you need contacts of relay K5 [1] or pushbutton with self-return S3 [4] &#; see Figure 5. Analogously as for relay K1 you read what information relay K5 transmits. At the K5 [1] contact, there is a reference 7.6 under the symbol, so again turn to page 7 and find the coil of the K5 [2] relay in column 6. On page 7 you can see that K5 is switched by float P3 [3], which is signed &#;Dry-running level&#;.
(In a typical sewage pumping station there are 3 floats. 1 &#; dry float, 2 &#; pump start float, 3 &#; overflow alarm float. Dry-running float informs about low sewage level and is designed to turn off sewage pumps. The start float informs about the level suitable to run the pumps. The alarm float informs about high sewage level and switches on the second pump when the first one can&#;t pump out the sewage (e.g. due to high sewage inflow).
Thus, contact K5 11.14 [1] will close when the waste water has reached this level until the dry float is up.
The button S3 [4] is a service button and is used to force the pumps to run at low effluent level when the dry float P3 is lowered. The S3 button is a bridge on the K5 (parallel connection) on contact K5. Running the water and wastewater pumps dry (no liquid) can damage the pumps!
The next contact on the way to switching on pump P1 is switch S1 [5]. It must be in position R (Manual) for the pump to work in manual mode. Switching the switch to R position will short-circuit contacts S1 23,24.
Figure 6 shows the last contact on the way to starting pump P1, namely contacts 11,14 of relay K2 [1]. The K2 relay provides alarm information, and if it is activated, it means that pump P1 is ready for operation (no pump failure). Relay K2[2] will be activated if:
Thus, the failure of relay K2 [2] to energize informs the control system of the failure of pump P1, which includes:
For better contrast, below is a description of the conditions that must be met for pump 2 to turn on, refer to Figure 7:
I recommend that you download this diagram and practice on other sites. You will get better with every new schematic. An experienced automation engineer reads schematics like nursery rhymes &#;
This article is unlike any of the articles we&#;ve done before. We&#;re going to delve into the world of panel drawings! This is an exceptionally useful weapon to have in your armory, as it not only helps you to build panels and understand the inner workings of them, but they help to troubleshoot problems if they occur later. If you can follow a panel wiring diagram, you will be able to find the root cause of any problem in a panel! Learn how to follow an electrical panel wiring diagram below.
Let&#;s go back and have a look at the control panel, and try and figure out some of the connections by following a wiring diagram.
As I&#;ve mentioned in the previous articles, this is a control panel that is used for a system that turns wastewater into clean water.
It is a 2-door control panel on the front of which we have some switches that are connected to the PLC inputs and outputs.
We are going to look at these switches and try and figure out the wiring behind them, as you may be curious as to how these switches are wired to the PLC!
Let us first identify our push buttons; we have the Mute Buzzer push button, the ESD Reset push button and the Emergency Stop push button.
Try to remember these from the previous RealPars article about Reviewing the basics of an electrical control panel, and see if we can find any of these items in the electrical drawings.
All the wiring that you see in the panel is done based on the wiring diagram.
This is what we draw using AutoCAD Electrical.
Each page of this wiring diagram shows the exact wiring for different sections of this control panel.
If you want to learn more, please visit our website Qinfong.
Featured content:For instance, for our Emergency Stop push button, it shows the wiring for this switch. You see that there are four wires that are connected to this switch.
These are wires on the back of the Emergency Stop push button, on the rear of the door.
Each of these wires has a tag number. The tags for these two wires are 1 and on the other end is 2.
For the upper wire, it shows that there is a wire that comes from page 200 section 1.
We have the page numbers at the bottom of each page. For example, it shows that we are on page 311.
The wire comes from page 200 section 1.
So I&#;ll go to page 200 and then section 1 and this is where this wire comes from.
As you can see it says this wire goes to page 311 section 1, which is the page and the section that we were looking at. So if I go to page 311 and section 1, I can see this wire.
Now if I go back to page 200, you see that this wire comes from page 150 section 9.
So if I go to page 150 and section 9, I can see where this wire comes from.
You can see that it says the wire goes to page 200 section 0 which is where we just came from.
So this wire goes to page 200 section 0.
Then it goes to page 311 section 1.
This is page 311 section 1 and this is the wire that is connected to the Emergency Stop push button.
The same goes for the other wires as well. For example, this wire comes from page 310 section 9.
This one comes from page 200 section 1 again.
And this one connects to our PLC digital input. And the tag for the PLC input is 300U2.1.
In the back of the Emergency Stop push button, you see that we have four wires, just as what we have on the wiring diagram.
Two wires are tagged as &#;1&#; and two wires are tagged as &#;2&#;.
Based on the diagram, one of these wires with the tag 2 goes to the PLC digital input.
Let&#;s see if we can find this wire.
In the wiring diagram, it says the tag for the PLC input that the push button is connected to is 300U2.1.
These are the tags for the PLC inputs and outputs.
Now based on the diagram I need to look for a wire that is tagged as 2. So in the image below, you&#;ll see the wire that I am looking for.
One end of this diagram is connected to the push button and the other end is connected to the PLC input. So this is how easy it is to read the wiring diagram for a control panel.
It goes exactly the same for the other switches that we have here as well.
So to sum it all up, here is what we learned in this article:
&#; All the wiring that you see in the panel is done based on the wiring diagram. This is what we draw using AutoCAD Electrical.
&#; Each page of the wiring diagram shows the exact wiring for different sections of the control panel.
&#; Each of the wires in the wiring diagram has a tag number. These tags can be found in the panel as well.
&#; Using the page numbers and the sections, in the wiring diagram, you can easily follow the wires and see where each wire is coming from.
That&#;s it for this article!
There was a lot of back and forth in that article, hopefully, you didn&#;t get lost!
If you didn&#;t, you&#;re well on your way to becoming an AutoCAD designer, or panel wireman, or maybe you want to work out in the field?! Well, you&#;re on track for that too.
This article was brought to you by RealPars in partnership with Pro-control in the Netherlands.
They are experts at control system design and industrial automation. They have a team of world-class automation engineers and have been designing and implementing industrial control systems in different industries for many years.
If you want to get in contact with them, you can check out their website at pro-control.nl.
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