In the previous video I wrote a PLC program that was able to energize the main and star
contactors at the same time.
I also have an on-delay timer here.
When the star contactor is energized, this timer starts timing and after 8 seconds it
will be active.
When the timer is active, this open contact here will reset the star contactor.
Ok, as you already know, the way that the star-delta starter should work is that when
the start switch is pressed, both the main and star contactors will be energized at the
same time but after a few seconds, the star contactor should be de-energized and the delta
contactor should be energized, all while the main contactor remains energized.
This logic is able to energize both the main and star contactors at the same time and after
8 seconds, it will de-energize the star contactor.
So what I should do here is to figure out a way to energize the delta contactor when
the star contactor is de-energized.
For that I will add a new network and then add an SR flip-flop with the address of the
delta contactor.
I want the delta contactor to be energized at the same time the star contactor is de-energized,
so what I'm going to do here is to add an open contact in the set input of this flip-flop
with the address of the timer.
By doing this, as soon as the timer is active, the star contactor is going to be de-energized
in network 2 and then in network 4, the delta contactor is going to be energized at the
same time.
Ok so with this PLC program so far, by pressing the start switch the main and the star contactors
will be energized and then after an 8 second delay, the star contactor is going to be de-energized
and the delta contactor is going to be energized.
The other thing that I need here is that I don't want the star and delta contactors to
be energized at the same time.
Why?
Because the whole point of using a starter like this is to be able to run the motor with
the star wiring connection and limit the inrush current when the motor starts and then switch
to the delta connection.
Now if these two contactors are energized simultaneously, they could crush the whole
system.
So to make sure that nothing like this can happen, I will simply add a closed contact
with the address of the delta contactor in the set input of this flip-flop.
And then I will add another closed contact with the address of the star contactor in
set input of this flip-flop.
With these closed contacts added, the star contactor will be energized only when the
main contactor is energized and the delta contactor is de-energized.
On the other hand, the delta contactor will be energized when the timer is active and
the star contactor is de-energized.
Ok I also have a normally closed stop switch.
As I already said, when this switch is pressed, I want the whole system to shut down.
I've already added a closed contact for this switch in the reset input of the main contactor.
So I will go ahead and add two additional closed contacts in the reset input of the
star and the delta contactors as well.
By adding these closed contacts, when the stop switch is pressed all the 3 contactors
are going to be de-energized at the same time and that's how the whole system stops.
I will also add another closed contact for the timer so that I can reset the timer when
this switch is pressed.
Now I've got a question for you.
Do you remember why the stop switch you're using in your control system should be normally
closed?
If you do, go ahead right now, yes right now,and explain it in the comments below this video.
This will help you remember this forever and also helps other electrical or automation
engineers just like you to learn the benefit of having a normally closed stop switch when
designing control systems.
And if you don't remember that, you may want to watch part 10 of this video series again.
I explained this at the end of that video.
Ok the program is now complete.
If you have any questions or comments please go ahead and put it in the comment below this
video.
Now let's move on to the next lesson and download the PLC program to the simulator to be able
to test the logic and see if work as expected.
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