4 - 20 mA problem

Hello,

I have a strange problem. We have 4 gensets on a oil rig. We use a 4-20 mA signal from a Woodward 2301D speed controller to send the load signal back to a Siemens s7 plc. I connected one of the signals, and it worked out great. 2 wires, powered from the speedcontroller. Set the Siemens analog input card to C on the dip switch. Everything worked, signal was acurate. I am using FC90 analog input in the program.

The I came to the next one, connected it the same way. this time the signal went straight to 32767, when connected to the speed controller. Even when I removed one of the wires it was still 32767. When I removed both, it went to -32768.
I removed the signal and used my loop calibrator. No problem, showed 0 at 4 mA.
The cable between them should be about 20-25 meters.

It seems to be overpressuring the whole signal or something, but I am reading 4 mA in the loop. It is really strange...

Are any of you specialists on 4-20 and knows what this might be??

reg
Andreas

Andreas,

It could be that you need Signal Isolators in between the Woodward Speed Controllers and the Siemens PLC. Many PLC analog inputs are not isolated (one wire is internally Common to all Inputs), so that the signals may interfere with each other, if you do not use an add-on isolator board for each signal.

This would explain why one signal works, but the second one does not, but that you still measure a good signal AFTER you disconnect it from the Siemens Input module.

Of course, from this remote distance, it is only a guess!

Another common solution is to provide the same power source to all speed controllers, so that one side of each circuit is common to all. Sometimes that is easier, sometimes using signal isolators is easier.

This is a classic ground loop problem. I suspect that the AI on the PLC are not isolated, very likely "single ended".

Follow Lancie1's advice and put an 4-30mA isolators in-between the genset and the PLC.

Dan