Hi and welcome to part 10 of the star-delta starter PLC program and wiring which is proudly
brought to you by RealPars, the one and only online education platform dedicated to helping
you learn practical PLC programming skills, which will enable you to break into one of
the hottest and highest paid jobs in the marketplace as an automation or controls engineer.
In the previous videos, you’ve learned how to configure the hardware and then defined
the addresses for each and every input and output that you have in the system.
In this and next video you're going to learn how to assign a tag or a symbolic name to
each address and then start writing the PLC program.
After configuring the hardware, to get started writing the PLC program, I'll open the "S7
Program" folder and then open the "blocks" folder here in the SIMATIC Manager.
As you can see, the "blocks" folder contains the "system data" and another block named
"OB1".
OB1 is where I can write my PLC program.
I'll double click on this block to get into the programming environment.
Here is the programming environment for STEP 7 version 5.5.
As you can see, the programming environment looks very simple.
This section is where you can write your PLC program and this section is where you can
have access to different PLC programming instructions.
As I mentioned before, when you have all the input and output addresses defined for the
system, you can assign a tag or a symbolic name to each address.
This makes both writing and troubleshooting the PLC program much easier.
To do that, from the option menu, I'll open the "symbol table".
This is the table that I can use to assign a relevant tag to each address that I'm going
to use in the software.
As you can see, there is already a tag considered for OB1 in this table.
Here is the list of the inputs and outputs that I'm going to use in my PLC program.
Based on this list I'll start creating a few tags or symbolic names for each address.
The first tag or symbol I'll insert here is the start tag, then in the address column
I'll insert the address which is I 0.0 and then hit enter.
As you can see, the data type for this address will be automatically set as Boolean which
indicates that this is a bit or a digital address.
I'll enter the tag stop for the stop switch and then hit enter.
As you can see, this time when I hit enter, both address and data type will be set automatically.
For the main contactor, I'll consider the tag name main.
I'll revise the address to Q 4.0 for this tag and hit enter again.
For the star contactor, I'll enter star and for the delta contactor I'll consider the
tag delta.
So here are the tags that I have considered for each address in software.
Now when I use each one of these addresses in my PLC program the relevant tag appears
next to the address and as I said this makes both writing and troubleshooting the program
much easier.
Now I'm going to hit save, close this window and get back to the programming environment.
Now considering the 3 phase motor and the star-delta starter I have here, I need to
write a PLC program with the following conditions: When the start switch is pressed, both main
and star contactors are going to be energized at the same time.
• After 8 seconds, the star contactor is going to be de-energized automatically and
at the same time the delta contactor is going to be energized while the main contactor remains
energized.
• When the stop switch is pressed, all the contactors are going to be de-energized at
the same time.
• The star and delta contactor should not be energized at the same time under any situation.
Now let's get into writing the logic.
As the first step, let's write a simple piece of code to energize the main contactor when
the start switch is pressed.
To do this I'll open the bit logic folder here, select an SR flip-flop and then drag
that and place the instruction on the first network.
This instruction has two inputs of set and reset and also one output.
When the set input is true, the output is going to be energized and when the reset input
is true the output is going to be de-energized.
That's it, very simple.
Now I'm going to use this flip-flop to control the main contactor so what I'm going to do
is to assign the address of this output for the instruction.
As you can see, since I've already defined a tag for this address, the tag will appear
under the address.
Now if I enter any address that has not been associated with any tag, I will only see the
address here.
As I said, with this flip-flop instruction, when the set input is true, the output is
going to be energized.
This means, if I make this input true, I can energize the main contractor.
So what I'm going to do here is to add an open contact with the address of the start
switch.
Now with this code, when I press the start switch, the main contractor is going to be
energized.
As I said when the stop switch is pressed the contactor should be de-energized.
The stop switch that I have here is a normally closed switch.
So I'll add a closed contact in the reset input of the flip-flop with the address of
the stop switch.
With this code, whenever I press the stop switch, the main contractor is going to be
de-energized.
Now you may ask why I used a normally closed switch for the stop command instead of a normally
open switch.
Well the reason is when you use a normally open switch instead of a normally closed switch,
if the wire that connects the switch to the PLC is ever disconnected, whether intentionally
or because of a broken wire, you'll be notified and this helps to ensure a more reliable system.
If you want to dig more into why you should use a normally closed switch for the stop
switch, we have some videos on this exact topic in our main library.
To watch these videos, if you're already a subscriber to our library, you can go to realpars.com
and then in the search menu simply type in the word "stop" and you'll see the relevant
videos pop up in there.
We will also put a link for this video in the description below.
Okay, so far I wrote a simple line of code to energize the main contractor, now let's
move on to the next lesson and complete this logic.
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