In the previous video you learned how to assign a tag or a symbolic name to addresses you're
going to use in your PLC program.
Then I wrote some simple code to energize the main contactor.
In this video I'm going to continue writing this PLC program to be able to control a 3-phase
motor using a star-delta starter.
Ok, as I mentioned in the previous lesson the way that this star-delta starter should
work is that when the start switch is pressed, both the main and star contactors are going
to be energized at the same time and then, after a few seconds, the star contactor will
be de-energized and the delta contactor is going to be energized all while the main contactor
stays energized.
So first, the main and star contactors are energized and then the star contactor de-energizes
while the delta contactor energizes.
With the current code, I'm only able to energize the main contactor.
But when the main contactor is energized and the motor starts, I need the star contactor
to be energized at the same time to control the in-rush current.
To do that, I'll click on "new network" on the tool bar to add a new network, then click
on this line and add an SR flip-flop.
With this flip-flop I'm going to energize the star contactor so I'll assign the address
of this contactor for the flip-flop.
As I said, the star contactor should be energized at the same time as the main contactor, so
I'll add an open contact in the set input of the flip-flop with the address of the main
contactor.
With this code, as soon as the start switch is pressed and the main contactor is energized,
this open contact will be closed and energize the star contactor as well.
So when I press the start switch both the main and star contactors are going to be energized.
The main contactor flows the power to the motor and the star contactor connects the
right side motor terminal together and sets the connection in star.
Now this star connection should last only for 8 seconds to reduce the inrush current
of the 3-phase electrical motor.
After that, the star contactor should automatically be de-energized and the delta contactor should
be energized at the same time.
When you need to turn on or off something automatically in your PLC program, one the
best tools that you can use is a timer.
To add a timer, in the "program elements" window, you can open up the "timers" folder.
Here as you can see, there are 5 different types of timers that you can use.
The type of timer you use in your PLC program depends on what you want to achieve in your
program.
For instance, here I need a timer that is able to de-energize the star contactor after
8 seconds.
That means I need a timer that is able to do something for me automatically after a
specific amount of time.
For that I can simply make use of an on-delay timer.
The way that this timer works is very simple.
Here when the set input is true, the output is going to be true after some delay.
The amount of time that the output delays depends on the time you enter in the time
value input.
Now there are two types of on-delay timers in STEP 7.
Normal on-delay timer which is what I currently have here and retentive on-delay timer.
The retentive on-delay timer works the same way as the normal on-delay timer but the only
difference is that for this timer, if the input goes false while the timer is timing,
the output stays true.
So for the normal on-delay timer, if the input goes false the output will go false as well
but for the retentive timer if the input goes false, the output will stay true.
I'm going to use a retentive timer here, so I'll delete the other one.
These red question marks here shows that I need to consider an address for this timer.
The addresses that you can use for timers in STEP 7 start at T0 and usually continues
up to T255.
I'll assign T0 as an address for this timer.
As I said, I need this timer to de-energize the star contactor after 8 seconds.
So I'll add an open contact in the set input of the timer with the address of the star
contactor.
By doing this, as soon as the star contactor is energized, the set input of the timer is
going to be energized as well and the timer starts timing.
I want this timer to be energized with an 8 second delay, so I should enter 8 seconds
here as the time value.
The time value should be entered in S5TIME format.
That means, in STEP 7, when you want to enter the time value for the timer, you simply need
to add S5T# before the time value.
For instance, when you need to enter 8 seconds as time value, you should enter S5T#8S.
For 10 seconds, you need to enter S5T#10S.
Okay, so with this logic, when the star contactor is energized, the set input of the timer is
going to be energized and since this is an on-delay timer, it will be activated after
8 seconds.
On the other hand, I want the star contactor to be de-energized after 8 seconds so the
timer can definitely help me with that.
How?
For that I can simply add an open contact in the reset input of this flip-flop with
the address of the timer.
Now, as soon as the timer is active, the star contactor is going to be de-energized and
that's how I can de-energize the star contactor automatically after 8 seconds.
With the current logic, in the first network, when the start switch is pressed, the main
contactor will be energized.
At the same time, in the second network, this open contact will be closed and the star contactor
will be energized.
When the star contactor is energized, in the third network, the timer starts timing and
after 8 seconds, it will be active.
Now, since I have the open contact of the timer in the reset input of this flip-flop,
as soon as the timer is active, the star contactor is going to be de-energized.
So with the PLC program that I have here, when the start switch is pressed, both the
main and star contactors will be energized and after 8 seconds, the star contactor is
going to be de-energized automatically.
Now let's go to the next lesson and continue writing this PLC program.
Many of you have expressed that this is the easiest lesson learning environment that you have come across.
We’ve heard many of you expressing how much you are enjoying this content.
If this is your position, please communicate your appreciation of our videos with a like.
The more likes we get, the more enthusiastic we are to continue to provide this content in the future.
So show us your love now and like this video.
Also don't forget to subscribe to our YouTube channel if you haven't already done so.
That way each new video will be sent to you as it's published.
In addition to the likes and subscribes, make sure that you head over to realpars.com
to find even more training material for all of your PLC Programming needs.
We offer many videos to assist you in learning PLC programming and landing that job in the high paying,
highly sought after field of Automation and Controls Engineering.
Go to realpars.com and subscribe to our highly effective training series now. Your future awaits!