simple ladder logic required

S

Thread Starter

sean

Could someone please help. Im very new to plc ladder logic and I need a design to prevent motors from starting at the same time to reduce overloading of the supply.

Specs: 4 (or more) motors which could start at any time need to be interlocked so that if two or more try to start at same time (within a
say 5 second period of the first motor start) then the second, third, ... motors are sequenced in order of priority. If only one motor is
trying to start then it is started instantly.
I guess it is sequencing on demand logic that I require.

Thanks to anyone who can provide an efficient solution.

Sean
 
Sean:
> Specs: 4 (or more) motors which could start at any time need to be
> interlocked so that if two or more try to start at same time (within a
> say 5 second period of the first motor start) then the second, third,
> ...

You'll need one timer, T1.

Algorithm:
if motor 1 to be started and timer T1 expired
then start motor 1, reset timer T1
if motor 2 to be started and timer T1 expired
then start motor 2, reset timer T1
if motor 3 to be started and timer T1 expired
then start motor 3, reset timer T1
etc

If necessary, add the appropriate rising-edge logic to make sure the reset only happens once for each motor starting. If your PLC doesn't provide timers with multiple resets and multiple contacts, you'll need to put in a couple of auxiliary coils.

Check that a timer reset takes place instantly rather than waiting for the end of scan - the manual should tell you, but if it doesn't you'll have to do some experimenting. If you're making it from a single-reset timer, make sure that a timer reset also resets the timer-expired aux-coil.


How it works: the timer is used to indicate that a motor has been started within the last 5 seconds. Priority is handled by the order in which the logic is listed - typically timers only expire at the top of scan, so this is guaranteed.

HTH - HAND

Jiri
--
Jiri Baum <[email protected]> http://www.csse.monash.edu.au/~jirib
MAT LinuxPLC project --- http://mat.sf.net --- Machine Automation Tools
 
If they are always activated in sequence, it's fairly simple

Simply enable subsequent motor turn-on signal with a status bit that the prior motor has been running for at least 5 seconds.

You can do a similar thing with the "off" cycling, if desired.

And disable starting the first motor with an "or" of all the motors run statuses (and possibly a timer).

If they are started in an arbitrary order it gets dicier.
I might have a simple "no motors have changed status in the last 5 seconds" and then select which motor will start now.

Rufus
 
J

Johan Bengtsson

<P>I would probably use one timer for each motor and start it at the same time the motor is started. The condition for each motor to be started would then be</P>

<PRE>
motor 1 motor 2 motor 3 motor 4 motor 1 motor 1
start running running running stop running
-| |--------*--|/|----*--*--|/|----*--*--|/|----*--*--|/|-------( )---
| | | | | | |
| motor 2 | | motor 3 | | motor 4 | |
| timer | | timer | | timer | |
motor 1 *--| |----* *--| |----* *--| |----* |
running |
-| |-----------------------------------------------*

motor 1 motor 1
running timer
-| |-----------------------------------------------( )---
</PRE>

<P>And similar for all the others with suitable substitutions this would stop any motor from starting when any other is starting up, if implemented correctly this would be true even if their start condition happen to be turned on at the same scan.</P>

<P>This can of course be varied depending on if you actually have start/stop signals or just a "run enable" signal.</P>

<P>Btw, how does this end up in the PLC program archives?</P>


<P>/Johan Bengtsson</P>

<P>Do you need education in the area of automation?<BR>
----------------------------------------<BR>
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Box 252, S-281 23 H{ssleholm SWEDEN<BR>
Tel: +46 451 49 460, Fax: +46 451 89 833<BR>
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----------------------------------------</P>
 
M

Marc Le Douarain

<P>A lot of solutions possibles, why not this one : </P>

<PRE>
M1 run
-[^]----*--------[MONOSTABLE 5s]
|
M2 run |
-[^]----*
|
M3 run |
-[^]----*
|
M4 run |
-[^]----*

M1 start Monostable running M1 stop M1
-[ ]--------[/]---------------*--[/]-------( )-
|
M1 run |
---[ ]------------------------*

M2 start Monostable running M1start M2stop M2
-[ ]--------[/]----------------[/]---*-[/]---( )-
|
M2 run |
--[ ]--------------------------------*

and so on...
</PRE>
 
M

Michael Griffin

If you look on the "www.control.com" web site, you will see a link to the PLC Archive ( "http://www.control.com/PLCArchive":http://www.control.com/PLCArchive ). At the top of the PLC Archive page, you will find instructions on who to send it to. Joe Jansen has been kind enough to manage the archive, and I am sure he would be delighted to receive something from you.

Basically, you turn your submission into a PDF file and e-mail it in. Some sort of explanation of what it is would be a good idea of course. You might look at some of the existing submissions if you want some ideas.
If you don't have any software to turn it into a PDF file, let me know and I can tell you how to do that with free software. If that is too much trouble, I suppose I could convert it for you.
 
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