There is induced voltage in the temperature-sensing resistors of motor ABC’s three windings! What to do?
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There is a 1100-kilowatt three-phase motor. The temperature-sensing resistors of its A, B, and C windings (in a three-wire system) often reach their maximum values and fluctuate wildly, causing the motor to shut down due to overheating. Upon inspection, it was found that the shielding layers at both ends of the signal wires were not connected; moreover, there was 27 volts of alternating current between the three signal wires and ground, while there was 17 volts of alternating current between the shielding layers and ground. This is likely induced electricity. Could it be that this induced electricity is causing the shutdown of the motor? There are also three sets of spare thermistors, and there is a 37-volt induced voltage when measuring against ground... Good heavens, I hope an expert can analyze this for me. Thank you! ...Can the shield wire be connected directly to the motor housing?
Leaving aside the issue of whether there is induced voltage or not, first measure the resistance at the cabinet end where the thermistor cable emerges, and check whether its value is within the normal range. Of course, interference of this kind is often not detectable by a regular multimeter as well. So, first connect the shielding layer to ground, ensuring that the grounding resistance is less than 4 ohms. If that still doesn’t work, try adding a barrier.
The shielded cable is grounded at one end, usually to the shield ground between cabinets. If that still doesn’t work, try adding a barrier.
This is related to shielding; such a high voltage applied will definitely affect the measurements. It is recommended not to connect it directly to the casing, and it’s best to use a ground wire dedicated for instruments
I connected the shielding layer to the motor, and sparks were generated during the connection... The conditions required for operating the instrument alone aren’t met either, so I’ll wait to see how it performs after the motor starts... Now the induced voltage in the signal cables in the PLC room has been eliminated; it should be no problem anymore!
There is electrical grounding on the grounding bar inside the cabinet; if I also connect the shielding layer of the instrument to it, it might have an impact...
This potentially possible electrical ground resistance may not meet the requirements of the instruments
So I had to connect the motor first... The moment the motor started, the temperature of phase A’s winding reached its upper limit before returning to normal levels. Thankfully, there was a delay before the machine shut down; otherwise things would have been ruined again... It seems that the resistance was affected and caused an open circuit at the moment of startup... I’m not sure if it was due to voltage applied to the resistor, sigh
It seems to be a grounding issue; the PLC room is grounded at only one end