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sorin.dumitriu@rdslink.ro

3g3rv-p10st8 Communication problem

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Hi! I am trying to solve a simple problem: how to eliminate the noise produced by a CIMR-F7 inverter on the serial connections of the plc option board 3g3rv-p10st8-e. On the RS232 port I am using a NT2S-SF121 with a custom serial cable (RS232) and on the peripheral/RS422 port there is a RS422 connection with a SCADA program. The SCADA transmision type is RS422 and converted back to RS232 using a serial convetor Omron K3SC-10. When the inverter (22kW) is stoped, everything is working just fine. When the inverter is started the comm. with the NT2S is stopped and the scada is giving time-out messages and then restore the connection. All the components are earthned properly, all the cables are screened and connected at one end to PE. The NT2S cable is 1m long and the scada boudrate is 9600bps. The RS422 link is terminated with 120Ohm/1/2W resistors and the distance is 50m (aprox. 150feet). The only one thing that I didn't do is that I did not use input and output filters at all. What else should I do? Ferite breads, filters............... change the solution? Please help me!

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Call it a simple problem.... I asume you have a standard 3 phase input filter installed ? You might want to reduce the carrier frequeny of the inverter. It can create acoustic noise in the motor but it will help reduce noise interference. Be sure to physically separate data and motor cables. Don't let the motor cable enter the cable duct but connect it straight on to the inverter. Leave the screening of the motor cable around the cable wires as far as the inverter, don't cut it and ground it at the glands what I see a lot. Earth the screen at the inverter. You might want to apply AC and perhaps also DC reactors on the inverters. These help a lot I can tell. Combined the effect is the best. Omron has special reactors for every inverter type. Just some suggestions that pop up. Perhaps someone else has more tips.

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The aplication is a pump sequencer and the inverter output is separated from the motors by mean of magnetic contactors. I have no ideea how to screen these wires. Reducing the carier frequency is a good ideea... Regarding the AC/DC reactors, I am sorry but they are not available at Omron in Romania. Sorry... Anyway, thanks for the advices. It will help me a lot!

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Well, you could replace the wire sections between inverter output and contactors by sections of screened cables, that won't hurt. I checked and a DC reactor is built in from 22 kW on. An AC reactor doesn't necesarily have to be Omron, you can use any AC reactor with the right specifications. Check here for instance. Perhaps you can just do a test by connecting the screened cable directly to see if it helps.

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Is it the 'magnetic contactors' that are coming on at startup giving you the 'splat' of noise?? could well be just the coils on these that need a snubber? As previous keep the 'big stuff' away from the little stuff. You may need to use fibre if the RS485 is getting hit on the way to the screen but I expect its coming from a 'common source' - may be the power to the PLC needs to be hooked up via a UPS to keep it away from the inverter power if its 22Kw and has no 'reactor - filter' (which it ought to have). Also could you not set the inverter output to a very soft start to reduce any noise fro the motor / drive.

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I don't think you've understood what type of PLC we are discussing. The 3G3RV-P10ST8 is a PLC option board for the E7/F7/L7 inverters. It is mounted in just the same way as all other option boards - inside the inverter (nope, not a very nice environment from the RFI point of view though). Power is supplied internally, from the inverter. Nothing possible to change there. I've used the PLC board with an F7 inverter (2,2 kW) and had the same problems. Also, they show up not only when accelerating or breaking, they show up all the time the run signal is supplied. I didn't have an optimal setup (I used ready-made cables that were too long - a couple of meters of extra cable looped together right beside the inverter in a smallish enclosure is very unoptimal!) Anyway, the only tips I've seen so far in this thread that would help are 1) keep your cabling tidy and 2) decrease the switching frequency. I'll be looking out for more tips, and closely follow this thread. :)

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Aha!, anyways- the magnetic contactors twixt the inverter and motor would be my first port of call. After all the drive should be well shielded against itself and as long as he's kept the cables apart and it's grounded correctly should not be the source of the noise. I used to always have 'loop contactors' on drives and they can give no end of problems if they do not come in before the 'enable' signal is on (therefore giving a good splat as they make contact) and also can be tricky to 'snub' the coils of as they can be big beasties and give ugly back EMF's. And if you cant get rid of the noise then fibre is the way to go with the serial link if it's likely the 'entry' point is not at one end - used to do loads of DNC links to plasma cutters and the like - fibre was the most reliable connection in that very noisey enviroment.

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