Modicon Quantum problems

In one of our plants we are having problems with a particular rack "dropping out" in a Modicon quantum system. Forgive me if I use some incorrect terms for the Modicon stuff -- I'm an AB-guy and it's been many years since I've worked with Modicon.

The PLC is a Quantum 534. There are three racks connected via 140CRP93200's. The problem is rack 3 in a 4-rack system. Reportedly, one of the racks is dropping out -- dead... Prior to my visit to the plant, they had checked all connections, cables. Had their local SqD-guy come out to help. All have had no significant improvement. They have replaced power supplies, comm modules, cables...


When I got to the plant Thursday, I found that the terminating resistors on the RIO were not in place. So I ordered them and drop-cables, then installed them over the weekend, while the plant was down for maintenance (actually, while I had to shut them down to do another project....)


Yesterday, they started the plant back up. That system had run for less than an hour and the rack faulted again. They came and got me. Quick inspection showed no apparent comm-problems. So it appears that the problems come, then go away just as quickly.

Things I've checked:

1) Terminating resistors on RIO. Installed missing resistors.

2) Drop cables are all > 10', professionally made.

3) Unit temperatures all appear to be within reason. Most feel just slightly warmer than skin-temp. Nothing feels too hot or too cold...


4) All racks appear to be properly grounded.

5) All power supplies have the correct jumper set for 120-VAC.



Any hints for anything else to check?? I'm becoming suspicious of power problems, but am desperately needing further guidance!


Thanks!

This thread maybe worth a read....

http://www.eng-tips.com/viewthread.cfm?qid=141308

This sounds like an EMC problem, check all of the screening and earthing and see if you comms cables are running parallel to any power cables.

Any switchgear in the enclosure that may be causing a problem or a VSD or soft start nearby? Switch mode power supplies can cause interference.

Has any one played the game of swapping modules between working and nnon-working locations? I would start with the CRA first and work my way through the I/O, the rack, and the cable tees. Have you had a TDR test done on the network? these are often easier to do then checking every cable connector and tee.

JohnW said:

This sounds like an EMC problem, check all of the screening and earthing and see if you comms cables are running parallel to any power cables.

Any switchgear in the enclosure that may be causing a problem or a VSD or soft start nearby? Switch mode power supplies can cause interference.

Click to expand...

How close is too close?

The coax runs parallel to motor leads out of a large VFD - within about 8-12 inches, for approx 5-6 feet. That coax would be the connection between racks 3 and 4 -- rack 3 is "the problem rack".

I'm not entirely familiar with the system yet and I don't know that it matters, but the speed of the VFD appears to be controlled by an analog signal that is constantly changing. The VFD speed is changing rapidly between 50-65 Hz, constantly...

Jim Dungar said:

Has any one played the game of swapping modules between working and nnon-working locations? I would start with the CRA first and work my way through the I/O, the rack, and the cable tees. Have you had a TDR test done on the network? these are often easier to do then checking every cable connector and tee.

Click to expand...

I don't know that they've swapped modules between working and non-working, but they have replaced "non-working" with new. They've also replaced some of the cable taps.

Have they done a TDR test??? Not that I'm aware of -- but I honestly don't even know what a TDR test is??? They reportedly had someone from SqD come in and check the system -- found a couple of "little things", but nothing that should be causing this problem. I have no idea what he actually checked or how he checked it.

OZEE said:

How close is too close?

The coax runs parallel to motor leads out of a large VFD - within about 8-12 inches, for approx 5-6 feet. That coax would be the connection between racks 3 and 4 -- rack 3 is "the problem rack".

I'm not entirely familiar with the system yet and I don't know that it matters, but the speed of the VFD appears to be controlled by an analog signal that is constantly changing. The VFD speed is changing rapidly between 50-65 Hz, constantly...

Click to expand...

8-12 inches is too close, 12 inches absolute minimum the further away the better for inside panels the politically correct distance between VFD and analog/comms cables is 39 inches!

It is not the speed fluctuation of the VFD that causes the problem but the HF pulse frequency, that will be between 2 and 11 khz. This will definitely interfere with a comms signal. Sensitive signal cables should NEVER be run parallel with VFD cables.

The VFD cable should be screened and the screen grounded at both ends. The cable should go right up to the drive chassis and the screen have a 360 degree connection to the VFD chassis. The comms cable should also be screened as should the other control cables from the VFD. A VFD will also cause an analog input to a PLC to fault if it is interfering with it. The enclosure back plate should be unpainted and grounded using braided straps or flat strip. Round cable is no good for HF grounding. The VFD should be fixed to the unpainted enclosure back plate or if it is free-standing then grounded to it with braided straps. The enclosure doors shuould also be grounded top, middle and bottom with braided straps. Steel wire armoured cable is good enough if you don't have copper braided screened cable, do not use the open steel light armoured cable that we call "SY" it does not work as a screen. Fexible or rigid steel conduit is also no good (too much gap).

One of my site electricians identified an EMC problem last week simply by switching the VFD off and the fault went away, he turned the VFD on and Hey Presto! the fault came back.

OZEE said:

I
Have they done a TDR test??? Not that I'm aware of -- but I honestly don't even know what a TDR test is??? They reportedly had someone from SqD come in and check the system -- found a couple of "little things", but nothing that should be causing this problem. I have no idea what he actually checked or how he checked it.

Click to expand...

The quick and dirty (not completely technically accurate) answer: Time Domain Reflectometer is used to verify integrity of the co-axial network by looking at the db loss.

It has been a couple of years since I worked on Quantum, but here goes. What error are you actually getting? I think someone else can tell you where to look for the error code. When racks halt I think I had Analog module issues. One of the biggies there is feeding VFD's with non-isolated analog output modules. If your module isn't isolated - can you isolate the 4-20mA signal?
George

George Graziano said:

It has been a couple of years since I worked on Quantum, but here goes. What error are you actually getting? I think someone else can tell you where to look for the error code. When racks halt I think I had Analog module issues. One of the biggies there is feeding VFD's with non-isolated analog output modules. If your module isn't isolated - can you isolate the 4-20mA signal?
George

Click to expand...

The particular rack in question is all discrete i/o.

When the rack stops communicating, it is only briefly. It "heals itself" within just a few seconds, so unless there's some way to historically capture an error code...

Unfortunately, when this happens with the process under load, it literally takes 2-3 hours of clean up to be able to start the process again.

Is this a new system or an old system ?

If its an old system then has this always worked before ?

If it has then has anything changed that may have caused this problem recently or since the start of the problem ?

Eventhough a SquareD rep checked the system, assume they did not.

Do a Google for the Modicon Remote I/O Cable System Planning and Installation Guide, its too big to upload to plctalk.net.

More than likely there is an installation violation in this one particular rack. We had a similiar problem with a Controlnet application a few years ago caused by two things:
1 - poor crimping of connectors
2 - Not enough bandwidth , we had to increase the Network Update Time

Don't know if the Quantum has similiar with its Traffic Cop or its equivalent.

Ian

504bloke said:

Is this a new system or an old system ?

If its an old system then has this always worked before ?

If it has then has anything changed that may have caused this problem recently or since the start of the problem ?

Click to expand...

I don't know for sure, but I believe this system is less than 5 years old. Reportedly, it used to work properly.

Has anything changed?? This is the first time I've seen the system, so I don't know. They don't admit to anything changing, and I'm inclined to agree -- I see no evidence of any major changes. The only minor change I can see is that the wire-tie anchors that held the coax in question have come loose and the cable was looped up over some wire-channel to hold it out of the way... In either case, it was running way to close to the VFDs.

OZEE said:

I don't know for sure, but I believe this system is less than 5 years old. Reportedly, it used to work properly.

Has anything changed?? This is the first time I've seen the system, so I don't know. They don't admit to anything changing, and I'm inclined to agree -- I see no evidence of any major changes. The only minor change I can see is that the wire-tie anchors that held the coax in question have come loose and the cable was looped up over some wire-channel to hold it out of the way... In either case, it was running way to close to the VFDs.

Click to expand...

Its a wonder then that the system operated without the terminating resistors for so long, unless someone else had been in prior and disconnected them.

A system that operated that long, does not have an install problem, rather a hardware issue.
Only item left then would be the rack as mentioned in a previous post.

Ian

curlyandshemp,

Please check your pm

I think they disconnected the lone terminating resistor as an attempt to fix the problem after it first happened. There was only one terminating resistor in place, so you're right - it's a miracle it ever worked at all!