Relay Chatter

Hi
I hope someone can point me in the right direction.
I have a SLC processor that has been running a bulk material transfer system for about 10 years.
About 5 days ago I started to get sporadic relay chatter on 120 Vac nema size 1 motor starters causing the motor overload to trip.
The 120vac control voltage to the motor starter is run thur some aux. contacts and then returned to the SLC as a motor running input.
I've replaced one of the old nema style motor starters with a new iec type with no change. If I watch the output card durning the chatter I see that the 120Vac card supply remains unchanged however the output voltage itself drops to what my analog meter shows as 80Vac.
I've also relaced the output card and attempted to run the motorstarters from a different output card altogether. with the same result. I've also move the netural to a ground inorder to eliminate a floating netural as the cause.
I'm quite frankly out of ideas of what to try next in order to solve this problem. I would be thankful for any ideas of where to look next.
thanks for the help

Did you rule out a wiring problem? Tighten the terminations or replace the wires or terminal strips if they are the problem. Judging by everything else you have done, that's got to be it. If I think of anything else, I'll reply again.

Vetteboy

Relay chatter 2

I'm sorry I sould have mentioned that we are talking about at least 2 motor starter maybe as many as 5.
Also when I used the 2nd output card I connected the wires on the same side of the terminal block.
Thanks for the help

Have you put counters on all the inputs for your motor control to "catch" which one might be loose (if any)? Loose input wiring is hard to catch in RSLogix without using counters. I don't know. I'm playing pin the tail on the donkey here 'cause I don't have the whole picture. If you are super explicit on the control programming and physical wiring I can try steer you in the right direction.

Vetteboy

Here are some guesses.It would help to be specific as to model of PLC and type of output card, i.e., relay type or solid state (triac).

Check each 120 vac fuse by pulling them out of the clips, look for funny erosion marks. Loose clips can cause this. The PLC power supply might be suspect. Inputs running on an external power supply such as counters, encoders, limit switches, etc. might be shorting or grounding intermittently.

There are lots of possibilities, these are a few hints where to start looking.

Relay chatter 3

The plc is a SLC 5/05 with a P4 power supply(Which I replaced earlier today) the output card is a 16 bit relay output. and I believe that the 120Vac supply to this relay output card is ok. I saw no change in output card supply voltage durning the chatter. I only saw the voltage change on the outputs terminals.
again thanks for the great information.

I want to make sure I have this all straight
1. You have changed contactors/starters
2. changed Output card
3. Checked voltage to output card and it stays at 120vac
4. Output voltage drops to 80vac at the time its chattering.

Are these contactor/starters paralleled from one output? Or is the chattering contactors driven by different outputs on the same card? Or are the different outputs on different cards?

Actually if the incoming voltage to one or more output cards is 120vac but the output isnt maintaining at 120vac then you could have a backplane problem or a program glitch...I tend to lean on the backplane theory, something along the lines of a bad connection powering the relay output cards. 80vac could maintain the led's but there could be more happening, an oscilloscope connected to the outputs would give you a better look at what the output is actually doing. If its not maintaining a clean output then look at the backplane aspect.

Another option just for gp is to clear and reload the program if you can, programs can get corrupted and funny things happen.

Let us know what you find. Another thought is to disconnect all but one solenoid and test individually, a bad coil on one starter could create problems with others activated at same time.

If I think of more I will post it.

It's starting to sound like something in the program may have changed. Is it networked or have a HMI connected that could have edited anything or changed the values in a data table? If so, maybe you could download an old backup for a few days to see if that's the problem. I'm still guessing.

Sid

You may want to try to energize the starters individually from a seperate power source, maybe even a cord plugged in to an outlet. Just be sure everything is isolated. This will will tell you if the problem is at the PLC or at the starters.

I would try forcing the outputs on with the live cable going to the relay disconnected and see if the problem still occurs,then maybe try connecting some other apparatus to the output card to see if it has the same affect.

Find out why the operating voltage is dropping to 80Vac and you have found your problem.

The motor starter contactors will need more than 80vac to overcome the contact spring tension.
Try running the motor starter relays from a proven independent source, if all is ok then you have eliminated the starters as being a problem.
If the starter relays still chatter then check to see if the shaded pole is still in place around the laminations, some relay laminations have these, they are intended to stop ac contactors from chattering.
Monitor the voltage as close to the source as possible, straight off the transformer, see if it drops to 80vac, if not then you have ruled out the transformer. If the voltage drops to 80vac at the transformer then you have too much load for that transformer, that could mean a weak transformer or a load problem is pulling the transformer down. Another point to check is the wire size and the length of the run.

Some output cards need the power terminals at the top of the card jumpered together to share the load.
As stated before, we are shooting in the dark here, a lot depends the particulars of your system.

Good luck
Roger

I agree with RH above

The transformer for your sys is sounding weak. If your VAC is below about 87 VAC you will begin seeing probs. Surging in your sys (say 80 to 90 VAC) and you get Chatter. Good luck and post YOUR fix for all of us.
Little Bob

If the voltage into the output card stays at 120 volts during the chatter, the transformer and supply are ok. I would suspect something in the program is turning off an interlock for a scan, then allowing it to come on again. If the starter does not completely drop out, the auxiliary contacts will not open and the output will energize again. Has anyone changed anything in the program about the time this started? Also, an input interlock could have a loose connection that could effect more than one starter.

If it is safe to do this, briefly force the starter output on while the chattering is occurring. If the chattering stops, you know the output is being turned off by the PLC. If the SLC has a histogram function, the reason the output is being turned off can be found very quickly.

Vic

Inductive kick

mrdegold

You may have damage to the output cards if the high voltage spikes, from the discharge of these contactor coils, exceed the PIV of output card components. Chattering relays can cause havoc in the PLC world, and if they are in close proximity to a PLC or remote adapter they can shut communications down. (A-B)

Then again maybe not but it is something to keep in mind just in case you find a damaged output card. Could be one of those cases of which came first.......

Let us know
Roger

You say you've already changed the power supply. Have you recently added any new modules to the rack that might be overloading the power supply?

I have encountered a similar problem in the past. When enough outputs were on simultaneously, the load got too large for the power supply, the backplane voltage dipped below threshold level, and the PLC dropped out of run mode, turning the outputs off. When the outputs turned off, the backplane voltage level returned to normal and the PLC went back into run mode, turning on the outputs and once again dropping the backplane voltage below spec and dropping the PLC out of run mode, etc, etc, etc.

Check for a good connection between the output module and the rack. Try replacing the rack. Recheck your power usage calculation to make sure you're not close to the limit of the P4's capacity.