Seeking help with Siemens 400PLC issues
Thread Content
The PLC400 RTD card shows normal readings before the compressor is turned on. When the compressor is turned on, the temperatures of the two channels fluctuate between 20 degrees and 100 degrees; after about a minute, the temperature reading becomes 0, the channel 7FFF, and the program displays 32767. What causes this problem? Has anyone encountered it? The resistance values of the components are normal whether the compressor is running or stopped, ranging between 115 and 125. The RTD card modules and channels have all been replaced, but the problem still persists; I’m seeking help from experts
Are your compressors driven by motors? Based on the description of the phenomenon, interference is a likely possibility. Check whether the shielding layer of the cable is properly grounded; if that doesn’t work, try connecting a filter capacitor.
It is driven by a motor, and the motor windings were recently repaired. Currently, the temperature of the windings is abnormal, but when the resistance is measured, the value is normal
What about measuring with the card disconnected at startup? Does it refer to your temperature measurement point with a transmitter?
If there is electromagnetic interference, measuring resistance is meaningless. It is recommended that you use the elimination method: disconnect the motor side from the on-site junction box and connect a resistance box to the signal line, then observe whether there are still fluctuations. Or you can measure the temperature of the stator windings; it will definitely be the U/V/W phase windings, so swap the normal ones with the abnormal ones and observe. Generally, these three signals are interlocked; the interlock must be bypassed before operation.
Check for any loose connections or short circuits; the unit is experiencing significant vibration
It comes without accessories; PT100 three-wire type, and the resistance of the switch is normal as measured
For interference issues, the terminals that need to be grounded are connected separately to the grounding bar, and after being gathered together, they are grounded using a separate grounding electrode.
Once the issue is identified, disconnect the components at the site. Are the channels and such working fine, or is it a problem with interference? The question now is how to solve this; the grounding is fine – the device itself already has proper grounding