We have PT-100 RTD's installed on our Motors. These RTD's are the trip security of motors and are terminated on P+F Barriers K-System KFD2-UT2-Ex1. The output of these barriers are connected to our emergency shutdown system (Prosafe RS PLC).

For some time, when we start a motor after a month or more (For test start up), the Motor Trips showing PLC Channel Fault. No high or Low temperature trip is shown on PLC. This fault appears in a split second. Surprisingly the same doesn't happen when we start the motor again after first tripping.

The motor runs smoothly in second attempt.

We have checked & swapped RTD's circuits in the area but found nothing. All RTD's that are causing tripping are separate, terminated on separate TB's and barriers and are on separate input cards of the PLC. Grounding wires of the RTD's are well connected.

Is there a fault in our loop or there is a problem with the sensor behaviour? There is no Timer in the logic. Why does the fault only occur when we startup a motor after a month or more and why does it only occur in less than a second? How does the fault disappear when the same Motor is started after first time tripping.

Please suggest a solution.
Thank You!

 

An interesting case. I am not sure what the source of your problem but am very curious as to what it is when you find it.

A couple of suggestion to check are:

1. Power source - If the power source is a battery. The voltage level needs to be checked. It may dip too low and caused the trip. I have seen battery drained after a month of no charging.

2. Loose wiring - From the sensors to the power and communication wiring. They can cause intermittent fault.

Good Luck and please share with us the solution when you find it.

 

You might try a small time delay on the high temperature or if you have the PLC capability add a low pass filter.

 

Are the RTDs in windings or bearings?

Do you get an individual text alarm when they trip, is it always the same one?

Are the RTDs wired as 2, 3 or 4 wire?

I assume the wires are run in shielded cable grounded only at the PLC end, lift the shield off at the PLC End to verify that with an ohmmeter.

Have you considered adding a Start Delay in the PLC, after all it's hardly likely to go out on over temperature in the first 20 seconds.
Do the motors trip before getting up to speed?

Is it possible the start current is higher for the first start because the machinery is cold drawing more start current? On that note are you certain that the temperature trip has no other contact in the logic e.g. Overload?

I was once called to a site where the PLC was hanging up as big motors started, it turned out the PLC power supply came standard with the jumpers set to 220V, it operated ok until the heavy start current caused the 120V to dip a little. Check the power supply settings.