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S1200 & ET200s relay problem

Thread Starter: Nikzitronik   Started: 7/18/2012 10:28 PM   Replies: 6

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  7/18/2012, 10:28 PM
Nikzitronik
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Dear all,

It probably isnt the right forum section, but I wasnt sure where to post.

I recently did a retrofit of an old ABB plc with an s7-1200 with ET200s. The ABB plc had quite a few inductive proximity sensors, oparating at 110Vac. As there arent available input modules with 8DI at 110Vac, I did the following: I wired the proximities to a relay (Relpol RM85 or RM84 if I can recall correctly), and I then wired the relay contact at the 24Vdc inputs of the ET200s. After about 2 weeks I get the following fault:

The only two IFM proximities, SOMETIMES fail to triger the relay and subsequently the input of the ET200.
I havent been to site yet to determine if the problem comes from the contact or if it comes from the coil. Replacing the relay though solves the problem.

Now Is it possible that the contact is damaged? The plc input is not a great deal of a load, nor it is an inductive load.
Is it possible that the IFM proximity sensor damages the relay coil? All relay coils have an overvoltage limitation (Relpol M93R I think) and LED.

Any help will be highly appreciated.

BR 

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  7/19/2012, 12:53 AM
huggy_d1
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What is the min/max load requirements for your inductive AC prox switches?  Did your AC relay have any type of surge suppression (MOV) to handle the relay turning off?  Otherwise, it might be less expensive to replace the prox switches with 24VDC versions, especially if it causes downtime and replacement relays twice a year.

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  7/19/2012, 6:57 PM
Nikzitronik
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What do you mean by the "min/max load requirments"? The continious current of the IFM IGS002 is 200mA.
All the relay coils have overvoltage protection.

The longterm plan is to replace the proximities with 24Vdc versions as they fail.

Is it possible that the input of the ET200s wears the relay contact?
Is it possible that the proximity messes up the relay coil?

One of the above is the case, but I dont know which one.
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  7/20/2012, 12:56 AM
huggy_d1
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Nikzitronik wrote:
What do you mean by the "min/max load requirments"? The continious current of the IFM IGS002 is 200mA.
All the relay coils have overvoltage protection.

The longterm plan is to replace the proximities with 24Vdc versions as they fail.

Is it possible that the input of the ET200s wears the relay contact?
Is it possible that the proximity messes up the relay coil?

One of the above is the case, but I dont know which one.

In legacy equipment, meaning older hardware designs prior to commonplace PLC controls, proximity switches were often designed to DIRECTLY control a load, like a solenoid, relay, valve.  The prox switches require a certain amount of leakage current to operate and allow a certain amount of load current when switched on.  The min current would be leakage requirement for when it is off, and the max would be what is the biggest load the prox can directly operate when switched on.

ET200 relays do fail after many many cycles.  If your prox is chattering off and on very fast because of hysteresis or vibrating load conditions or lose wiring, it can chatter a relay and burn it up.

If you monitor the peak and transient voltage/current response on your prox / relay coil during normal operation, you may discover the problem.

I hope this is useful information.  I suspect you already know most if not all of the above, so feel like I'm probably not really helping.
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  7/25/2012, 4:55 PM
Nikzitronik
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Ok. Yes I am aware of what your saying, but still it is not clear to me.

What do you mean by ET200 relay failing. That is not my case.

My case is that relays driven by proximities fail. As I understand it the reason of failure it that
1. Either the  ET200 kills the relay contact (I feel that this is unlikely as the DI of the ET200s is not an inductive load)

2. The proximity kills the relay coil.

I still havent find the solution. On Monday I will have the production line to my disposal for testing, so I hope that the cause of the problem will stand out.

One more weird thing is that there are a couple of proximities detecting pans travelling on a chain conveyor that causes the relay to chatter when the pan approches the proximity. These relays did not fail.

I hope I will find a solution for that.


Regards.
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  7/25/2012, 5:18 PM
huggy_d1
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Nikzitronik wrote:
Ok. Yes I am aware of what your saying, but still it is not clear to me.

What do you mean by ET200 relay failing. That is not my case.

My case is that relays driven by proximities fail. As I understand it the reason of failure it that
1. Either the  ET200 kills the relay contact (I feel that this is unlikely as the DI of the ET200s is not an inductive load)

2. The proximity kills the relay coil.

I still havent find the solution. On Monday I will have the production line to my disposal for testing, so I hope that the cause of the problem will stand out.

One more weird thing is that there are a couple of proximities detecting pans travelling on a chain conveyor that causes the relay to chatter when the pan approches the proximity. These relays did not fail.

I hope I will find a solution for that.
Regards.

For the relays that are being replaced, if they do not have surge supression on the coil, it would be a good idea to add some RC-filters across the coil to absorb the inductive kick for the relay coil's protection.  It is possible to purchase relays with internal surge suppression, but sadly, they often get replaced with pin-compatible non-surge protected relays and burn up much faster.  With external surge suppression (MOV or RC-filter), regardless of the relay installed into the base, it tends to work much longer.  If you have the part # of the last failed relay, you can find out if it has any protection built-in and possibly source a better replacement and note on the schematics stating surge protection must be installed, either externally or internally.
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  7/25/2012, 5:39 PM
Nikzitronik
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All the relays are Relpol RM84 and they all have as accesory overvoltage protection M93R installed on the relay socket as suggested by Relpol. So I guess this makes it harder for the solution to be found.

Attachment: Relpol.pdf  (8 Downloads)
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