PLC to AC Drive Via Modbus RTU
PLC to AC Drive Via Modbus RTU
(OP)
I've been using Modbus RTU for some time know communicating from a PLC to various devices, including AC Drives. My biggest complaint is the delay between sending a run command and getting a response at the drive. This delay seems to be random in length and is always a problem whether I am using one drive or 10. I am using 19200 baud rate and I have experimented with changing all sorts of settings with no luck.
Any ideas as to why this occurs? Am I just using a slow network? The drives I use only have Modbus RTU networking available.
Any ideas as to why this occurs? Am I just using a slow network? The drives I use only have Modbus RTU networking available.





RE: PLC to AC Drive Via Modbus RTU
Alternatively if you are communicating with multiple drives you may have to wait until the PLC has finished with the other drives befor it can process you request.
What type of PLC and Drive are you using?
RE: PLC to AC Drive Via Modbus RTU
RE: PLC to AC Drive Via Modbus RTU
Do they always vary in time?
How long are these delays you speak of?
Do you ever get cases where nothing actually occurs at all?
Keith Cress
kcress - http://www.flaminsystems.com
RE: PLC to AC Drive Via Modbus RTU
Yes, the time varies every time and is not consistant.
The delay time varies from 0 to 2.0 seconds.
This happens all the time, on every system, regardless of drive quantity.
For example, if I have 4 drives and send a run command to all of them at once, one might start in 1/2 a second, then another in 1 second then another may start in 2 seconds. The stop commands works the same way. The next time it would be completely different.
RE: PLC to AC Drive Via Modbus RTU
A VFD has a somewhat complex microprocessor based control system that must make constant updates to the operating parameters of the motor firing circuit, so that always gets priority. Modbus communications, besides being relatively slow, usually requires processing power in the mP of the VFD to convert the commands to ts machine code, but it is not given very high interrupt priority. That is why most VFD manufacturers have their own protocol for communication between their VFDs and their PLCs; it is optimized to keep the overhead as low as possible. You have chosen to use a PLC from a vendor who does not make VFDs, and a VFD from a vendor who has no reason to optimize Modbus comms because it allows the use of other vendor's equipment. As long as you do that your comm rate will be less than optimal.
That said, one of the few VFDs on the market that does do a decent job of open communications is the Altivar 71 drive, because they went with a dual mP topology. One processor does nothing but the firing control, the other does all the other peripheral stuff such as communications, protection etc. But that is only the ATV71, none of the rest of the Schneider drives have that feature.
RE: PLC to AC Drive Via Modbus RTU
then it sounds like a noise issue, then upgrade the cable that you use to better quality cable. If you have a shield is it grounded on one end only. Is the sheild carried through to the other drives?
Typical comm problems, shielding and quality of cable.
RE: PLC to AC Drive Via Modbus RTU
RE: PLC to AC Drive Via Modbus RTU
Thanx
RE: PLC to AC Drive Via Modbus RTU
RE: PLC to AC Drive Via Modbus RTU
Do you only get the problem when the VFD's are running? (meaning, when you are not running, can you monitor the data from the VFD with no problem?).
If yes, then it is highly likely the problem is HF noise. A very common problem with connecting VFD's on an RS485 based network. Chances are the body of the VFD's are not bonded too well and the HF noise is getting into the comms network and causing the disruption to your communication.
How far apart are the VFD's? if quite a distance, then (as a test) try running a large ground cable between VFD's and to good grounds to ensure they are all of the same ground potential.
Try things like reducing the PWM switching frequency in the drives, this will reduce the level of HF noise in the network.
RE: PLC to AC Drive Via Modbus RTU
RE: PLC to AC Drive Via Modbus RTU
Roy