VFD to PLC Comm Loss Intermittent Issue
VFD to PLC Comm Loss Intermittent Issue
(OP)
I'm dealing with a natural gas compressor that's controlled by a Schneider ATV71 VFD and a M238 PLC that's been intermittently shutting down due to comm loss over the CANOpen bus. It's been going on since a lightning strike some time ago, according to our customer. After the strike, the drive and PLC were both replaced, but the compressor would occasionally go down due to comm loss. I'm fairly new to VFDs and PLCs, and new to my company, and I've inherited this problem. Some things that have been tried are replacing the shielded CANOpen cable, replacing DC power supplies, cleaning and verifying grounds, slowing the baud rate, and checking/replacing Line Terminating resistor on CANOpen terminals. I also checked for a Time Out parameter, but it seems this drive only has an adjustable Time Out for Modbus, not CANOpen. We're still getting a comm loss fault 2-5 times a week on that machine, and I'm hoping for some suggestions.
I'll be visiting the compressor again this week and have plans to replace the PLC, replace most of the wiring inside the enclosure(which houses both the VFD and PLC), and connect a direct ground to the CANOpen shielding to try to further eliminate any noise from the VFD. Are there other measures I can take to try to eliminate the issue?
I'll be visiting the compressor again this week and have plans to replace the PLC, replace most of the wiring inside the enclosure(which houses both the VFD and PLC), and connect a direct ground to the CANOpen shielding to try to further eliminate any noise from the VFD. Are there other measures I can take to try to eliminate the issue?





RE: VFD to PLC Comm Loss Intermittent Issue
and now you're saying you're going to:
.
Is that twice??
At this point you need to put a scope on the comm line to see if there are anomalies and if so what they are. Bad rise times? Noise? Large common-mode voltage (caused by a blown lightning arrestor or something similar).
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD to PLC Comm Loss Intermittent Issue
RE: VFD to PLC Comm Loss Intermittent Issue
CANOpen has its own heartbeat arrangement to detect loss of communications, exactly how its implemented may vary though. If you've replaced all the hardware and still have the problem, then the issue is more likely to be the configuration, which one would expect would have been copied over when the original equipment has been replaced. Its possible to buy CAN sniffers in the same manner as the RS485 equipment, but its not as easy to hook up. Here is a USB-CAN interface device, I've seen (and used) some diagnostic software to monitor the CAN stuff before, but can't remember what it is unfortunately. The penalty for using most of the protocol monitoring equipment (like the USB-CAN) above, is that it won't pick up any hardware level issues, which is where itsmoked's recommendation of a scope comes in.
CAN as a communications medium is generally quite reliable, I'd expect to have far more problems with the equipment than 2-3 times a week if it was to do with the cables or terminations (which you've probably concluded also, having replaced it all). Its also possible (although I'd expect its less likely) that the faults may coincide with other things in the plant (some large equipment switching or similar), you'd likely need timestamps of the faults to further investigate that possibility.
Having said all that, I'd expect that for fully operational equipment prior to the lightning strike, that they would have been experiencing the issues prior to that, or other issues would also be experienced after the lightning strike, but not noticeable compared to a loss of comms fault.
I like the Tektronix scopes personally, although they're not what I would expect to be 'light on the wallet', particularly not the battery powered ones.
EDMS Australia
RE: VFD to PLC Comm Loss Intermittent Issue
The lightning strike may have damaged something other than the VFD and the PLC.
I would put a scope on the power lines and on the output of the control power transformer. I would be looking for a small persistent arc somewhere in your system.
Is your power system grounded, impedance grounded or floating?
Anecdote. I forget the details but I do remember going around in circles for a couple of days because the diagnostics were reporting a fault that was not in any way related to the actual fault.
The point is that some other issue may be falsely reported as a comm loss.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: VFD to PLC Comm Loss Intermittent Issue
If you want to hunt some down and provide us links we can give you yea-nae opinions on them.
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD to PLC Comm Loss Intermittent Issue
All of the components are grounded to the cabinet, which is Grounded to Earth
What would you see on the O-Scope in this case?
RE: VFD to PLC Comm Loss Intermittent Issue
https://www.tek.com/oscilloscope/tbs1000-digital-s...
https://www.circuitspecialists.com/hantek-200mhz-d...
Also, I'm curious if springing for one that's capable of 2 GS/s would be necessary for CAN signals. Also, with CAN signals being non repetetive, is a scope with "digital storage" good enough or should I be looking into storage capacities as well?
RE: VFD to PLC Comm Loss Intermittent Issue
RE: VFD to PLC Comm Loss Intermittent Issue
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: VFD to PLC Comm Loss Intermittent Issue
If no comm loss anymore, very probable is a problem in supply and / or grounding.
Check if CAN_GND was to ground connected.
RE: VFD to PLC Comm Loss Intermittent Issue
I'd look for scopes that specifically state that you can hook the probe to the hundreds of volts one might see in controls. I'd probably look for them stating CAT II, CAT III, or CAT IV input and battery powered. Lots of field work can be very hard to get plug-power to.
A 10MHz scope would be adequate.
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD to PLC Comm Loss Intermittent Issue
It is solidly grounded.
Unfortunately my PLC is powered by 120VAC, but as of yesterday it's being fed from a new transformer, and the CAN_GND is directly connected to a ground.
RE: VFD to PLC Comm Loss Intermittent Issue
VFD have big caps in input filter and in transient conditions may "move" localy ground potential in respect to CAN_HI and CAN_LOW, for short time, enough to alter CAN voltages and report comm loss.
If both, VFD have communication GND isolated from their chassis, may try to keep both GND together but isolated from ground (chassis).
RE: VFD to PLC Comm Loss Intermittent Issue
RE: VFD to PLC Comm Loss Intermittent Issue
I think is not too complicate to remove 120Vac wires and connect 2 batteries.
RE: VFD to PLC Comm Loss Intermittent Issue
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD to PLC Comm Loss Intermittent Issue
The transistor output model is powered by 24VDC, but we use the relay output model, which is powered by 120VAC. The suggestion is much appreciated though.
Really? Even with just a single CANOpen line directly between the VFD and PLC and only a few feet between them? Also, my VFD connects with an RJ45 port. Is it possible to disconnect the shield in this case short of stripping the end and reterminating with a new RJ45?
RE: VFD to PLC Comm Loss Intermittent Issue
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD to PLC Comm Loss Intermittent Issue
RE: VFD to PLC Comm Loss Intermittent Issue
Keith Cress
kcress - http://www.flaminsystems.com