Intermittent Communication failure of Modbus RTU connectivity when connected to VFD

[Austyno]

I have a modbus protocol which uses a TCPto Serial gateway for connection to slave devices. Each Device has its own seperate address/Slave ID and there are a total of 29 slave devices connected to this gateway. Labview is my master and queries these slave devices throuch TCP connection to the Gateway.

I have 4 power meters that are connected to the load side of the VFDs (variable Frequency Drive) to acquire Power, Voltage, current and frequency values and store them in a database. Also acquired through some other slave devices include Pressure, temperature, speed. These acquired values are stored in my database. The problem however is that if the VFD is not running (in which case I am just acquring data directly from across the stater (Direct-On-Line) Power meter ), everything runs fine but once the VFD is turned on and connected its Power meter querried to read values, the connection becomes intermittent causing the values to show zero values sometimes. Also voltage values obtained are in the 500s as against an input voltage of 460VAC.

Does anyone know what can be done to get these issues resolved?

[RavensFan]

VFD's can be very electrically noisy, perhaps that is disrupting the communication.  Can you put an oscilloscope on the communication lines and see how it looks before and after turning the VFD on?  You may need shielded cables.  Make sure your grounds are all good.

[Austyno]

Ok. will do that and get back with you via this forum

[Austyno]

I dont have Oscilloscope to look at the signal but I used a fluke meter to measure the voltage. So when the VFD is not on, the voltage reads about 2.3VAC and is a bit steady (Does not fluctuates alot between values) but immediately I switched on the VFD, the voltage drop to about 0.5VAC or less and this ready is very unstable (Fluctuates alot between values)

[arteitle]

You can't tell much about the quality or content of serial communication lines using only a voltmeter, unfortunately. The reading you're getting may be fluctuating just because the data being transmitted is changing with the VFD running. If you don't have a way to test whether there's interference on the lines, you might have to just try repositioning them or replacing them with shielded cable and see if that helps. One thing I'd suggest is to modify your Modbus master software to properly handle communications errors, so that if there is interference that causes the Modbus RTU checksum not to match, then rather than just returning zero and the rest of your software treating that as a valid reading, I'd use the error line to prevent the rest of your software from accepting the invalid result.

[ZKami]

I couldn't find anything about the ChirpX2 either. However, arteitle's earlier post seems like a good place to start. 

[RavensFan]

That post sounds like Spam to me.

[Austyno]

Researched and could not find any documentation on this.

[RavensFan]

Be careful.  Knacko seems to be a spammer.  The links he provides go to a very incomplete, suspicious website.

He revives a nearly year old thread to just get a link out to his website.