DeltaV Input Card Issue. Help needed

[bigbb]

Control loop for motor with VFD . One cable Start, running indication, + another for 4-20MA speed control. Running indication is Dry contact (Delta V 24VDC input card) taken out of VFD (Toshiba).
When everything ready to go ,channel status LED on the input card is blinking. Checked voltages and found out some AC voltage (30VAC) coming from the VFD. Checked the cable(it is long 5 stores up) megger is OK. When power is down for this MCC everything is fine. Another strange thing, we have almost the same package next to this and do not have any problem. Only difference is different VFD (Danfoss). I think this is a grounding issue, because there is no separate ground for power and instruments at the plant. Have to figure out what to do.
Thanks
BB

 

[controlsdude]

Never said was this an existing vfd installed or you just installed this vfd?

If it was installed i would check the parameter list to make sure no one fat fingered a default parameter into an unknown state.

What is channel status mean? Is this on the vfd or on the plc deltav?

Usually there is a status word code in the vfd that tells u why its in this state. Go to this word.

Check the last fault, usually holds 16 to 32 past vfd faults

 

[bigbb]

We are in the process of replacing Bailey DCS to Deltav.
everything was working on Bailey same VFD same cable just different input card for dry contact 24VDC instead of 48VDC. Stadus LED on the Deltav Input card. No fault on VFD. I guess this AC "background" was OK for 48VDC on Bailey but critical for 24VDC nominal and 15VDC actual on DeltaV. Thanks for your suggestion to check the VFD parameters.

 

[roydm]

I think you are telling us the discrete input is at fault (picking up some AC in the cables.
Have you tried loading the input down e.g. something like a 10K resistor to ground on the running feedback wire. Or perhaps a small capacitor to ground.
Regards
Roy

 

[bigbb]

Tnanks guys for your ideas, we tried capacitor, did not work. Finally made some changes to avvoid using 120VAC in this paricular starter for control and do not have anymore this issue.
Now just curious, Is it common in the US do not have a separate ground structure for power and instrumentation(control) If yes it should be a lot of similar issues.

 

[roydm]

Personnaly I dont believe in a seperate ground system. The instrument ground should connect to the plant ground at one point, usually just where it comes out of the ground and connects to the electrical system.
Having a seperate ground I equate to standing in a Faraday cage with your arm stiking out through the bars.
That's my 2c worth
Roy

 

[jeezlouise]

DeltaV guidelines recommend that you isolate the signal ground from AC ground in control enclosures, etc., and only connect them together at the grounding electrode by using separate grounding electrode conductors.

Having said that, the conductor sizes that DeltaV calls out for their signal grounding conductors are absolutely, ridiculously huge. My opinion is that if their equipment is that sensitive to noise then I don't want to use it.

 

[bigbb]

Maybe I am just an old fasshioned PLC person, but Delta V is so not "user friendly" for trobleshooting, like the situation when you cannot measure 24DC voltage in the loop for dry contact when contact is close, and you just have a LED ON on the card that indicates that that this contact is closed. I was told by Emerson guys that they use current drven input cards and not voltage deiven cards. But this does not help when you are trying to troubleshoot some flow switch in the field and have to go to look at the input card to see the status change.
Sorry for off topic. I do not like DeltaV, but have to deal with it.