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Communication Problem


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Hi all

I have a V1040 PLC with the following configuration.

Snap in V200-18-E6B + IO-Di8-Ro8 + 2 off IO-ATC8/AI8 + IO-AI4-AO2

The PLC is controlling a compressor with a large motor

The problem is that I keep losing communication.

I don't get any error message, just the unit will not start.

When this happens SB 90 comes on and i can only restart the unit by switching on and off the PLC.

The unit uses an inverter to control the motor speed which is located close to the controller, at first I suspected interference from this, but communication is ok when the motor is running and the loss of communication is only happening when the motor is stopped.

I also have some small 24vdc contactors in the panel.

I fitted supressors on the coils and this reduced the communication failure rate drastically, but i still get some tripping.

Is there anything else I can check to solve this problem.

Regards

Denis

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  • MVP 2023

OK, this is a new one. SB 90 doesn't have a descriptor - how did you know to look for it?

This is an energy spike noise broadcast problem. What's probably happening is a transient is getting into the I/O bus and causing the PLC's watchdog to timeout - I've had this happen to me. In my case the noise was on a Unitronics relay output module and there was enough of an inductive spike on the relay contacts to electromagnetically cross the I/O module's circuit board and get into the module's control circuit.

Since it only happens when the motor is turned off, you have some clues as to where to look.

When the motor is stopped the drive will go into regen mode and dump the energy from the motor back into the power line. Do you have a braking resistor on it? This will give the drive a another place to dissipate the energy.

First off, if you have access to an oscilloscope with a 10X probe I would get it. A scope is an invaluable tool for troubleshooting problems like this.

I would start by lifting all wiring that goes to the drive from the PLC an manually operating the drive from its local control panel. If you can start and stop the motor and the PLC doesn't lock up, then there's something coming in via those wires. You may need to add some .1 capacitors to ground from those terminals to filter the spike out. If the problem still exists, then the spike is in the power lines and is getting to the PLC through the DC power supply. In this case, you need to put a filter in the drive's supply power lines to try to catch the spike there.

Good luck. Problems like these have cost me a lot of hair.

Joe T.

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Thanks Joe

I have been using this PLC configuration for these units for a few years now (with a V280 PLC).

I think the SB 90 has was described in the help a few years ago and that's where I found it.

My only use for SB 90 is to display a message on the screen to tell the operator to restart the PLC.

During commissioning I started and stopped the drive several times and had no problems.

The only time I could duplicate the fault was in cycling the 24vdc contactors and I now have supressors on these.

I do however have some large solenoid coils on the unit and one of these also goes off when the unit stops.

maybe i will need to supress these too.

Again thanks for your help, hopefully I am looking at the right thing and will report back when the problem is solved.

Regards

Denis

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Adding my 2 cents.

I agree with Joe on the regen of a VFD. If your VFD is set to have a controlled stop ... don't.... set the VFD to coast to a stop instead of a ramped stop. I find that for many VFD models coasting to a stop isolates the VFD from the regen and can help because when it tries to control the stop the VFD will be using dynamic braking to prevent the regen and the dynamic braking can creat whole new noise front. History has shown that if regen is the cause of the problem (on going to a stop) that you will not experience the commuinications loss until your motor speed gets very low (10hz or less) and many times occurs between 1 and 3 Hz. Many VFDs will also have an overvolt condition in a controlled stop causing and over volt fault of the VFD.

You did not mention what type of compressor it was, but if it is a piston type there is a potential for a last second kick of the piston stroke to create a high regen spike when ramping to a stop. Coasting to a stop generally takes away many bad things and you get to avoid adding the braking resistors. The inertia of compressors is generally not bad unless there is a large flywheel involved like you would find in a salt water disposal pump.

What brand and model of VFD are you using?

Keith

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Thanks again Joe and Keith.

The unit is a screw compressor with a 132kw motor on an ABB ACS800 inverter.

Keith- I was thinking the opposite to you and had the inverter set to coast stop but decided to change it to a ramp stop, thinking that dropping out at a lower frequency would solve the problem. I am still getting the problem with the drive set both ways.

Today I switched the drive on and off several times and did not get any trip.

In fact the fault has never shown up when I have been present on starting or stopping the drive.

I was able to simulate the problem originally by switching on and off the small contactors, but since I fitted snubbers on these they do not cause this problem.

For this reason I am tending to look at other options than the drive.

I have spent hours watching the system starting, running and stopping with no problem, but suddenly I get the fault. Unfortunately so far I have not been there to see if the unit was running or not at the time.

The solenoids I originally suspected are 110vac driven by interface relays from the PLC, so I think these may also be unlikely.

The only other inductive loads I have on the 24vdc side are some small relays, could these be the problem?

I also have some 4-20ma analog signals between the drive and the PLC. These have not been screened, but as mentioned above the fault does not occur with the unit running.

Is it more likely to be interference on a snap in module or on a I/O expansion module?

I think this will be a case of eliminating inputs until I find the proble.

Regards

Denis

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  • MVP 2023

Don't discount the interface relays - if two or more of them are turned off at the same time the spike voltages will add up. This spike can be a lot larger than you would guess. What is the part number and manufacturer on them?

My experience with the problem is on the I/O expansion modules. Try putting some reverse-biased diodes across the coils of the 24VDC relays. Use a higher voltage diode like a 1N4006.

Joe T.

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Thanks Joe

As I had tested each output and input separately and not found a problem I was thinking it was a combination of two or more devices.

Many times if the unit trips on another fault I also get a comms fault. This would make sense as the motor and many of the coils will then go off at the same time.

It will be a few days before I get back to this site but I will definitely need to supress the other coils.

The relays are SCHNEIDER ELECTRIC RXM4AB2BD 24vdc relays.

Again thanks for the help and advice.

It's assistance like this that sets Unitronics service miles ahead of everyone else.

Regards

Denis

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  • MVP 2023

Those are very similar to the Omron MY series and have a "real" coil in them. I'd definitely put the diodes on to suppress the flyback pulse since you're experiencing intermittent mystery problems.

Here's a good writeup on what's happening:

http://en.wikipedia.org/wiki/Flyback_diode

I encourage everyone not familiar with this phenomena to read this. It will make you a more knowledgeable control engineer.

The flyback pulse is a good thing if you're trying to generate a high voltage from a low one, such as a DC/DC converter or a high voltage supply for the CRT in an old television.

They have scope captures of the pulse, which reaches -300V for about 0.4 ms. As the article suggests, the diode clamps it to it's forward bias voltage, typically about a volt.

Joe T.

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At first blush this may sound a little off the wall, but in a VFD system you need cooling. If you have fan(s) that are going through a thermostat there is a much more intermittent issue that comes up where the bounce accross the thermostat contacts will cause an electrical noise problem, but it will be extremely intermittent.

We sell a very large quantity of VFDs (mostly FUJI but also a fair number of ABB ACS800s and for years we experienced no particular issue until our main fan supplier changed a very small aspect of their fan design and simultaneously the mechanical thermostat we use also had a change that we were unaware of.

The manifestation of the issue was that the VFD would spontaneously fault (say one out of 50 times) when the cooling fan(s) were activated. We investigated the problem believing that the fan was the issue (because the fan manufacturer had a publicly stated change). Communications was also affected at the same time when the fans activated. We went round and round the problem and then under careful testing we were seeing instances where any fan design could be made to exhibit the issue and narrowed the problem to the thermostat. Since the fans are 120VAC we had not taken their affect too seriously.

We have since added a spark arrester product made by OKAYA (specifically we buy the XAB1201) and our instances of noise caused by cooling fans has been reduced to zero (in the last 2 years)

http://www.alldatasheet.com/datasheet-pdf/pdf/336742/OKAYA/XAB1201.html

The parts are purchasable from a large number of online sellers and they are very inexpensive it is at least possible that you need to adress this type of issue.

Keith

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Keith

This could quite possibly be the source of the problem.

The panel does contain two 230vac cooling fans controlled by a thermostat.

I have seen the system start and stop several times without giving a communication fault.

The fan will always continue to run on after the system stops until the panel cools.

It is possible that when the fan stops that the communication is interrupted, or at least a combination of the fan and another input.

Should the 1 spark arrester be fitted to each fan or just 1 between the feed to the thermostat and the neutral?

Details of fans and thermostat attached.

Many thanks

Denis

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  • 2 weeks later...

Keith / Joe

I have fitted snubbers on the 230vac cooling fans and on 5 x 230vac solenoid coils.

The unit has been running for a number of days without any communication problems.

I did not fit diodes onto the 24vdc relays as I wanted to narrow down the source of the problem.

I suspect that the problem was the cooling fans, as Keith suggested, but I don't want to interfere with the setup anymore in case the problem returns.

Again thanks for your assistance on this problem.

Regards

Denis

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  • MVP 2023

Glad to hear it's behaving. I would put 30 days of non-faulting operation as a 99% success of solving the problem. I never say 100%.

Overall, this post has been a very good overall conversation as to the intermittent mystery problems and solutions that are often encountered with a real installation. If you've solved the problem, then it wasn't large frequency drives or three phase motors but seemingly trivial cooling fans and solenoids. The devil really is in the details.

For the record, the "Spark Quencher" is an old-school RC snubber. We put them on every inductive device as a matter of practice and I buy them 100 at a time. You don't want to run out.

I'm sure I'll be referencing this post in the future.

Joe T.

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  • 10 months later...
Good morning, tell them started using unitrinics bypassing the Modicom. Buy two, one test and another one V120 / V280. The first I have already put in a bottle machine and working properly, I'm starting with the programming of another machine with the V280 but I can not communicate in any way with the pc plc. For this I use a converter ADAM-4561 module usb to 232 and from there to the plc. When trying to communicate something Comunication stops at PCOM Request Protocol Modem Output and shoot me an error sign. The truth is that two days ago I'm with this and I can not solve. Sorry for my English, and I hope your answers.

Thanks Francisco from Argentina.

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