# Reverse relay and travel limit switch - PLC

1. Feb 9, 2016

Hi, I'm currently studding a module on PLC and have a question on "what will happen if you do this ....". As I don't have a problem with explaining operation of each rungs of the ladder diagram I need some help on explaining the function of the reverse relay and travel limit switches. Can anyone help please?

Last edited: Feb 9, 2016
2. Feb 9, 2016

### jim hardy

I talk my way through them.

R1 clearly means "powered on"
R2 clearly means "running forward"

so i'd start by talking left to right across the rung
"Reverse actuates when:
powered on AND NOT running forward AND P4 pushed AND NOT past travel limit switch 2 . "

So - presumably if you're in between the travel limit switches you can run either direction
if you're past either limit switch you cannot run farther in that direction

so pay attention when wiring up those limit switches . I think maybe you could wire it so you have to run all the way to a limit switch before reversing directions. What is this thing ?

old jim

3. Feb 9, 2016

Thanks Jim for your reply. I was looking for more of a explanation what the travel limit switch and reverse relay does, what's the purpose of it is.

4. Feb 9, 2016

### jim hardy

A picture of the machine would help .

It's obvious P3 is a forward pushbutton
and P4 is a reverse pushbutton
(you'd think they would have given the button same name as the relay wouldn't you ? Mnemonic is the term for good labeling.)

i assume the travel limit switches prevent this thing from running off the end of something when somebody falls asleep at the switch.
Observe there's one for each direction.

https://en.wikipedia.org/wiki/Montparnasse_derailment

Got pictures ?

5. Feb 9, 2016

Jim's Picture aside - if you think of a bridge crane - the bridge moves in two directions, Forward and Reverse, when the bridge reaches the end of it's range it hits a limit switch and de-energizes the relay.

When testing somthing like this - you typically MANUALLY operate the limit switches long before the bridge ( or whatever) reached the switch to ensure the correct switch is in the correct ladder rung.

Also note the R3/2 and R2/2 contact that prevent BOTH the FWD and REV relays from being energized at the same time.

6. Feb 9, 2016

### jim hardy

Mr sponsoraw : Even a humble garage door opener has similar travel limit switches. That's what they do - limit travel.

7. Feb 10, 2016

Thanks guys for you answer. So when looking at this from the ladder diagram... both travel limit switches are N/C and they will activate (open contacts) when the travel distance is reached what will cause the R2 or R3 to de-energise. Apart of activating, the switches P3 and P4 also work us manual, where you can manually de-energise the R2 and R3. The forward and reverse direction of the relays don't matter for the purpose of the ladder diagram and its operation. It's only an indication which way the connected equipment operates. Is my thinking correct?

PS. I like your picture Jim.

8. Feb 10, 2016

### jim hardy

That's how i would read it. Mechanical engineers are really clever, but if we assume the machine is something straightforward like a crane or garage door opener or easy chair activator - that's what it does.

Glad to see the picture clarifying for you !...
de-energise ?
or energize?

Look closely at P3 and P4. See the small gap between the dots and the bar ? And pushbutton atop bar?
Pressing that pushbutton moves the bar down to connect the dots meaning they're N/O (normally open) contacts.
So you'd push and hold P3 to energize R2 and go forward.... and if you accidentally set your lunchbox on P3 the machine will stop when it gets to limit switch.
A NC contact would push the bar away from the dots- bar would be underneath and touching not above the dots. Look at P2.

Half of the art of reading schematics is learning the draftsman's conventions. Before CAD draftsmen took pride in making their drawings aesthetically pleasant and intuitive to read. In 1950's I knew a weatherman whose hand drawn weather maps were always embellished with zephyrs and sailing ships..

old jim

Last edited: Feb 10, 2016