# Motor control logic

Discussion in 'Homework Help' started by Crimsonblood, Nov 18, 2013.

1. ### Crimsonblood Thread Starter New Member

Nov 18, 2013
18
1
Have a question that has my entire class stumped ;

We have two motor/motor starters, with plenty of aux's (Both normally opened and normally closed), and control relays.

We have to wire a single push button start/stop to do the following ;

On the first start press, Motor A turns on. When you hit stop, it stops.

On the second start press, Motor B turns on. When you hit stop, it stops.

On the third start press, Motors A and B both turn on. When you hit stop, both stops.

After that, it has to repeat.

We're all drawing a blank and I've been pondering it all day, and I have about 60 sketches of "****, that's not rights". Any help?

Last edited: Nov 18, 2013
2. ### MrChips Moderator

Oct 2, 2009
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This is called a finite state machine.
Create a truth table with a column for B and a column for A.
Fill in each state on a new line for every time the button is pressed.
Figure out how many states (lines in the table) are required.

3. ### Crimsonblood Thread Starter New Member

Nov 18, 2013
18
1
I'm not familiar with truth tables for wiring these. What's the best way to start?

And thank you.

4. ### MrChips Moderator

Oct 2, 2009
12,642
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You can't use the truth table for wiring.
This is the design stage.
The best place to start is with the truth table.

5. ### Crimsonblood Thread Starter New Member

Nov 18, 2013
18
1
I apologize for the misunderstanding ;

We just started designing these, and are expected to have ladder/elementary diagrams for them prior to wiring them, and truth tables have not been mentioned in class.

Is there any particular place I can look online for information on these?

Thanks.

6. ### MrChips Moderator

Oct 2, 2009
12,642
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Is this a course on PLC?

7. ### Crimsonblood Thread Starter New Member

Nov 18, 2013
18
1
No, we start PLCs next semester. This class is titled "Industrial Motor controls", and we physically wire motor starters/motors, relays and switches for operation.

Examples of previous jobs ;

Image was gigantic so here's a direct link.

http://imageshack.com/a/img513/5347/o8ko.jpg

8. ### MrChips Moderator

Oct 2, 2009
12,642
3,457
Then I am stumped. Not my area. Maybe someone else can help. Where is Strantor?

Last edited: Nov 19, 2013
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9. ### shteii01 AAC Fanatic!

Feb 19, 2010
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Electrical engineer students normally are not taught those. We normally do the truth table, then k-map to simplify it. Then we draw diagram using logic gates.

10. ### Crimsonblood Thread Starter New Member

Nov 18, 2013
18
1
Hey, thanks anyway!

And as am I, so the club is growing.

11. ### Crimsonblood Thread Starter New Member

Nov 18, 2013
18
1
I see ;

I figure there is a way to transcribe between the two, but I don't expect anyone to break their neck trying to explain it, if it's difficult.

12. ### shteii01 AAC Fanatic!

Feb 19, 2010
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Yes, there is. Please see this wiki, it shows examples of logic gates using ladder logic: http://en.wikipedia.org/wiki/Ladder_logic

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13. ### Crimsonblood Thread Starter New Member

Nov 18, 2013
18
1
I have skimmed that over previously today, looking for an answer, and while I understand the fundamentals of how ladder logic works, I can't specifically figure out the logic needed for above mentioned situation.

As far as I'm aware, series connections of switches act as "and", and parallel connections between switches act as "or", so I'd be looking at something using and/or's and nads, but I'm not exactly sure how to put it together. My limited knowledge of truth tables are showing.

14. ### inwo Well-Known Member

Nov 7, 2013
2,435
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First off, can you confirm that no higher level devices are to be used?
Timers, sequence, or alternator relays.

It is obvious to me that memory is needed.
That is the only way for circuit to "remember"the position in sequence after stop is pressed.

Is that a correct assumption?
Even if #1 is stopped, the next start pulse will either start M1 or M1-M2.

That will make it a very complex circuit.
More so than I'm willing to spend the time building with my "wire and see" approach.

The hint I believe is the most basic relay memory that takes 3 control relays.
Toggle, monostable, or flip-flop depending on how you learned logic.

I can try to draw one as a basic building block of the circuit you need.
The usual searches didn't find anything.

If there is a simple way, it escapes me. I could be misunderstanding the instructions.

I think I remember it needing three single pole relays. (or a single and a two pole)
The common of the CR1 is connected to pulse (start switch).
CR1-NC going to coil-CR1, coil-CR2, CR2-NO, and pilot lite (for testing)
CR1-NO going to CR3-coil only
Line connected to start switch and common of CR3
NC-CR3 to common of CR2

This circuit is stable with CR1/CR2 either on or off. A pulse from the start switch should toggle it.

As CR1 and CR2 coils are connected together it seems a 2 relay circuit should work. I'd have to wire it to see.

If you thing a basic flip flop will help you toward a solution I can test one, but maybe this is something you know better than me.

Jul 18, 2013
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That seems very simple logic.
Ladder is very common for PLC programming, or simply using Boolean mnemonics.
You did not state what the condition occurs if another motor is on when another is initiated?
I assume you want this in machine schematic ladder as per the drawing?
Max.

16. ### strantor AAC Fanatic!

Oct 3, 2010
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I've got a working solution on paper, but it assumes you can use a one-shot. Can you use a one-shot? You better be able to, or this is a trick question.

17. ### inwo Well-Known Member

Nov 7, 2013
2,435
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Something like this.......................untested..........

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18. ### strantor AAC Fanatic!

Oct 3, 2010
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The way I read that, you push the single button and "1" & "3" oscillate at the speed of a tattoo gun with frequency readout by strobing a pilot light, while "2" watches on feeling neglected.

19. ### inwo Well-Known Member

Nov 7, 2013
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May be wrong, I'll have to test it I guess.
That's as close as my memory serves.
There is a way to do a 3 relay flip flop. Maybe 2 relay.

20. ### strantor AAC Fanatic!

Oct 3, 2010
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Alright, well I won't be around to get the answer to my question, so here's what I've got (I hope you like blue dolphin marker):

TR1 MUST be a one-shot (a timed relay that only stays on long enough for a single relay movement, or "fleeting contact" as the Germans would say (in German)), or else this thing will cycle through states with the frequency of a low pitched fart for as long as you have your fat finger on the stop button.

I'm sure someone will point out that this could have been done with less relays; yes, I know... well then go ahead and redraw it for me. I'm going to bed.

If it turns out you can't use a one-shot, then give me your professor's email address. I'd like to see the textbook answer to this one.

P.S. Usually homework answers aren't delivered on a platter like this on AAC, but I can tell you've put a lot of work into this assinine assignment and I'm feeling a little rebellious. I'm betting the professor is expecting nobody to come back with an answer and I wish I could stick it to him myself.

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