Relay Chatter

[caps00us]

I agree with rollingsworth. Check the Control Xfmr.

Another possibility is some kind of induced voltages on your inputs. I have seen this recently.

We had a motor shutting off sporadically and sometimes would shut-off and come right back on.

While t-shooting we were watching the LEDs and saw one come on that didn't make sense. We followed the Input LED and found it was tied in with the Logic for the motor control.

Good Luck

 

[Kim Gold]

The output voltage going to 80 volts could be caused by the output going on and off. Because it is happening so fast your voltmeter might not actually have time to go to zero.

I would try to determine if the problem is to do with the output circuit or being caused by an input going on and off. i.e. if the output is controlled by a pressure switch or something.

By forcing the output on, you can determine if it is being caused by an input going on and off. If the chattering disappears when the output is forced on, then the output side of the circuit is ok.

Then you can start looking for an input going on and off.

 

[johnhopps]

Try this: Connect a wht 20 gage wire to the Neutral Terminal Strip in the control panel that also has the neutral wire the output card uses. then run this wire temporary however far to the Contactor Starter and connect to the Neutral terminal along with the Neut wire
already connected there. If you now measure 120 vac from the output you have identified the problem to be a faulty neutral from the original wire at the Starter. A secondary (unknown ) and resistive path is often present. T-shoot or replace that wire.

NOTE; Due to dasiy chaning neutral wires from source throughout
plant to Control Panels and equipment which are internally
tied or fixed to each panels Grounding Terminal Strip, the
alternate path THRU "GROUND" exists, and will often the
cause of the Strange low AC voltage measurments.

 

[Eric Nelson]

Does it HAVE to be a WHITE wire?... lolis

Good tip John!...
A lot of times the return path is overlooked when troubleshooting!

beerchug

-Eric

 

[peterg]

Past experience with backpane problems has seen the processor FAULT.
Have you actually replaced the connector for the card, I had a problem once very similar to what you are experiencing It turned out to be the connector.Also is there an input from a relay contact that could be affectting the output, what sort of enviroment is the equipment in ie, hydrogen sulphide , amonnia , anything that is corrosive.

 

[Mike]

I too experienced a problem like this several years ago and spent the better part of 15 hours trying to figure out what was going on.

The machine was a cutting press, which would stroke the head down on to a die. When the machine would cycle the down cylinder would chatter and strike the die several times. I check all of the PLC outputs and inputs to make sure that the signals were not cycling and they were not. I checked all of the power and commons, even to the point of going all the way back to the in coming plant power. When I first got there I asked the maintenance man if any changes had been made to the machine and he said none at all, that it just started going crazy. After 14-1/2 hours the maintenance man asked me if I thought the new relays he installed would have any thing to do with the way the machine is acting? When I asked him what relays, since there were no relays in the panel or on the machine, which I could find, he said the new ones he put in the day before.

Upon further investigation I found that the maintenance man cut an enclosure in the out feed conduit going to all of the outputs. He placed this enclosure under the machine were unless he told you about it you would have never found it. After inspecting the "relays" what I found was that he actually installed mercury relays and that every time the machine would cycle the vibration of the machine would cause the mercury to slosh around and make contact, which was causing the machine to "go crazy".

I am not by any means saying that this is what your problem could be, but make sure that the person who has been working on the machine gives you all of the information of what has been done to the machine. You may also find out that something, which seems so innocent as adding "relays" can turn out to be a real nightmare.

I wish you luck in finding your problem, but if it does turn out to be someone added mercury relays because they thought they were cool, don't worry your not the only one to go down that road and I am sure you won't be the last.

 

[johnhopps]

banghead
Good Point Mike.

ya gotta remember always to watch out for those Plant People, They've
got me many times.

They are great guys, but have to make do with what they have so often,
and it usually works, everybodys happy, etc...

Its important to include them as we t-shoot their stuff, be friends, and always ask exactly what's been done recently. Remember; they beleive they made a repair.

 

[JesperMP]

Hi "mrdegold"

You say that the system worked well for 10 years but suddenly several motorstarters connected to the same output card started to chatter.
You replaced the output card, but the problem persisted.

I think that the most likely cause is a bad rack (after 10 years, the contacts can be corroded or mechanically "tired").
Get another rack OR if you have an unused slot, move the output card in question to the unused slot (and update your program accordingly).

If this does not remedy the problem, use the PLC as a "debugging" device (it is near impossible to pinpoint infrequent errors in other ways):

Assuming that for each motorstarter you have an output to the motorstarter, and an input that provides feedback that it is activated (ON).

Test MCC start MCC
is ON
n7:0/0 o:1/0 i:2/0
--||--------||--------||--------||--------

 

[JesperMP]

Dear mrdegold,

you write that after 10 years, several motorstarters connected to the same output card started to chatter.
You replaced the output card, but the problem persisted.

I say, the most probable cause is a bad rack (after 10 years, the backplane contacts can be corroded or mechanically tired).

Replace the rack
OR
move the module in question to an unused slot (if there are any) and update the program accordingly.
If you do the latter and it does indeed stop the chatter, then get a new rack as there may be more slots that are getting tired.


Another thing you can do, is to use the PLC as a debugging device.
I assume that you have an output for each motorstarter, and a feedback from each motorstarter:

In the following
N7:0/0 is "TEST is ON" (set it to start evaluating)
N7:0/1 is "MCC was started"
N7:0/2 is "Output goes first"
N7:0/3 is "Input goes first"
O:1.0/0 is "startsignal to MCC"
I:2.0/0 is "feedback from MCC"

Paste these rungs into your program after you have replaced the i/o adresses with the actual ones:
SOR XIC N7:0/0 XIC O:1.0/0 XIC I:2.0/0 OTL N7:0/1 EOR
SOR XIC N7:0/0 XIC N7:0/1 XIO O:1.0/0 BST OTL N7:0/2 NXB OTL N7:0/0 NXB OTL N7:0/1 BND EOR
SOR XIC N7:0/0 XIC N7:0/1 XIO I:2.0/0 BST OTL N7:0/3 NXB OTL N7:0/0 NXB OTL N7:0/1 BND EOR
SOR XIO O:1.0/0 OTU N7:0/1 EOR

If the output goes first, the problem probably originates in the PLC or before the PLC power supply.
If the input goes first, the problem probably originates outside the PLC.

 

[BENNY]

I had a relay chattering connected to a 1756-OA16 card.I found that I had programmed the output address in 2 different sub-routines, one sub-routine had the output off and the other subroutine had the output on causing the chatter.Is it possible someone made a program change recently to cause this?

 

[mrdegold]

Relay chatter 4

Hi again first I would like to thank everyone who replied with great suggestions.
I would like to clear a couple of things up and answer the questions sent back to me.
1)I didn't say this before but my plant is currently in a crucial production period and as it stands right now I can’t do any testing or part replacement that will bring the problem system down for more than about 5 minutes at a time. In 1 to 2 days this should slow down and give me the time I need for part replacement and more in depth troubleshooting.
2)I have run the Nema starter in question thru an ice cube relay where the control voltage for the starter came from a 120Vac receptacle and the control voltage for the ice cube came from the SLC output card. Problem remained.
3)Each motor starter is on a separate output from the same card.
4)The SLC is on a plant SCADA package however there is no direct interconnection between the SCADA and this piece of logic in the SLC.
5)Also the code is designed that only one of the problem motor starters can be running ant any one time.
6)During the chattering the common supply voltage to the output card does not change only the output voltage drops. Which I think suggests that the control transformer is fine.
7)The only program changes that have been made involving any part of the problem area are that these motor starter pulse on and off (ex. On for 15 sec. Off for 5 sec.) These timers have been adjusted 4 times in the past 2-½ week 1st to 20 sec. on 5 sec. off where it ran fine for 4 or 5 days then when the problems started I change to 20 sec. On and 8 sec off. While troubleshooting I went back to 15 sec on and 5 sec off and for the weekend I took it to 21 sec on and 7 sec off.
8)No new modules have been added to this rack as long as I have worked here it has been a full 13 slot rack with 6 input cards and 6 output card.
9)I had taken the motor starter neutral directly to ground and still had chatter.
10)I have not at this time replaced the terminal connector for the output card due to time constraints. But the existing terminal seems ok.
11)All repairs that have been made by plant maintenance people have been replaced by me. And doubled check (I’m close but not quite perfect.).
12)I’m not sure that the back plane has been in place for the entire 10 years but I’m planning on replacing as part of the on going trouble shoot.
Again thanks for all the great suggestion and I try to keep everyone up to date with what I find out.
Mrdegold

 

[Tom Jenkins]

You said: " 2)I have run the Nema starter in question thru an ice cube relay where the control voltage for the starter came from a 120Vac receptacle and the control voltage for the ice cube came from the SLC output card. Problem remained." Does that mean the relay AND the starter chattered, or does that mean the relay was on solid but the starter chattered?

Also, if the answer to the above was both chattered, try putting a counter in the program for each output and each input to the PLC associated with the motor control logic. That will confirm if the output is chattering in the program logic - RSLogix can't monitor the operation fast enough. It will also confirm if there is an input card bad - I have had output chatter occur when the input associated with the logic was not stable.

 

[mrdegold]

Relay Chatter 5 End of day update

Just to bring everyone up to the minute it is the end of the day. I have swapped out the SLC 50/5 Processor with a spare loaded with the logic I saved on the 3rd of March.
Let's see what happens now????

 

[johnhopps]

response to 9) above:

Please repeat test of Neutral wire by connecting to a KNOWN GOOD NEUTRAL. as I stated in my post, you need to confirm that the low 80VAC is not present as of a result of a RETURN PATH thru GROND.

Your test has probably worked fine, if you know the Groud path was highly conductive. Try measuring Resistance between the removed Neutral wire and Ground. Should not be greater than a few OHM's.

You've got quite a few people following this t-shoot with you,
creating a resource of unmatched technical experience of support thru this Web Fourm, seeking to add to their experience when this problem is finally nailed down.

The Solution is out there!

 

[rogerhollingsworth]

Same as Tom, did the ice cube relay chatter?

Roger