P
Hello list members, I recently replaced the 11 single-loop temperature controllers on a plastic pipe extrusion machine with a PLC based system. It has worked flawlessly, however I'm a bit worried because the t/c (thermocouple) inputs (double ended) in the PLC are non-isolated, and the module's specs call for a maximum common mode voltage of 5 volts between inputs.
I've measured common mode voltage between all inputs and it never exceeds a few millivolts, which is to be expected since the t/c's are in direct contact with their respective barrel and die zones, and all the zones are mechanically (and therefore electrically) linked.
However, this is what worries me, since I guess that the heating resistor in particular certain zone could short out and/or make contact with the barrel or die, possibly affecting my t/c input modules either through a voltage spike at a t/c input, or a common mode voltage mismatch between zones.
Has anybody got any experience with this, or possible problems/precautions that I should take into account? is this normal practice in extruders? By the way, I'm using an Automation direct d4-450 cpu with two 8-input t/c modules. The machine is supposedly protected against resistors shorting out, shutting down inmediately.
Also, the manual recommends connecting the t/c's negative terminal to the module's common in case of grounded thermocouples.. should I do that in this particular case?
Regards, Pat
I've measured common mode voltage between all inputs and it never exceeds a few millivolts, which is to be expected since the t/c's are in direct contact with their respective barrel and die zones, and all the zones are mechanically (and therefore electrically) linked.
However, this is what worries me, since I guess that the heating resistor in particular certain zone could short out and/or make contact with the barrel or die, possibly affecting my t/c input modules either through a voltage spike at a t/c input, or a common mode voltage mismatch between zones.
Has anybody got any experience with this, or possible problems/precautions that I should take into account? is this normal practice in extruders? By the way, I'm using an Automation direct d4-450 cpu with two 8-input t/c modules. The machine is supposedly protected against resistors shorting out, shutting down inmediately.
Also, the manual recommends connecting the t/c's negative terminal to the module's common in case of grounded thermocouples.. should I do that in this particular case?
Regards, Pat