Control.com - Nerds in Control

We experience problem with VFD el.motor -1500HP, ABB multidrive 600VAC. RTDs occasionally cause motor shut down. Their value sometimes shoots up to 90C or even higher, sometimes the value changes for a very short period, sometimes it steady. All three phases have the same problem. RTD's cable is shielded.

My question is, what is wrong with RTD and if they got faulty, is there any way to replace them?

Thanks and best regards
ET

It sounds like noise on the RTDs due to the fact they are in the windings and your control system is at a different ground potential;
You may need to use a signal isolator so that the RTDs can be referenced to the motors ground.

Post a sketch of the circuit diagram showing where the inputs etc are grounded.

Thanks a lot for your promt respons, but how can I post the sketch here?

Best regards
ET

Are your RTDs wired to a protection relay such as multilin or back to a PLC input card.

From memory they sometimes ground the shields at the motor end, contrary to normal instrument practice, check the manual for the protection relay for their recommendation.

Another thing, what type of RTDs are we talking about, 10 Ohm copper, 100 Ohm Platinum, Nickel or something else?
What is the resistance of your RTD cables?
Are the RTD cables run seperate from the motor cable?

Roy

Hi Roy

Our RTD (100 Ohm Platinum) connected to Bartec Profibus IS unit, 8x4-20 MA. After Bartec Profibus unit the signal goes to controller via Profibus

The resistance of RTD's cables is 132 Ohm by ambient temperature 20° C = 68° F

High high Temperature Trip setpoint = 190° C = 374° F
RTD's cables run from the same opening in motor frame and in one connection box, along with the incoming power cable connection. We have 3 such motors, working for one Drawwork and connected together through a gear. And only by the Motor#3 VFD's tripping has been occurred when drawwork works in tripping mode, i.e. lowering and where braking is being involved. Though I noticed there have been temperature alarms too on another two VFD, but they never tripped by this issue.

Below the link to the sketch you asked:
http://www.file-upload.net/download-6656340/Sketch.docx.html

ET

You only 2 wires between the RTDs and instrument so your readings will be higher than actual but that's probably not the reason for the spurious trips. It's ok to wire with 2 wires if the cable is short but not good practice.

Short one pair of wires together at one end and measure the resistance in Ohms then you can look up the error on a set of RTD cables (add the value to 100)

The non connected white wires at the RTDs should connect to the 3rd terminal at the instrument (jumper removed) that it can correct for the length of the leads.

Sometimes in motors they ground the RTD shields at the motor end, you could try that also, just lift the shields off the right hand side and run a ground down from the terminals to the motor frame.

Hi ET,

you may add a thermal resistance isolator for solving this problem.