Thermistor Protection 2

[rajendrarohra1978]

I have Thermistor protection relay Siemens 3UN 2100-0AF7, please can you tell me how it works. During testing it activates once Sensor is disconnected. During testing when we simulated temperature by increasing resistance we could not activate this device.
Please let me know in detail how it works. There is no setting on this device.


Thanks in advance.

 

[RRaghunath]

Rajendra, Wikipedia writes as below about the thermistors used as temperature protection in motors and dry type transformers:
"Many electric motors and dry type power transformers incorporate PTC thermistors in their windings. When used in conjunction with a monitoring relay they provide overtemperature protection to prevent insulation damage. The equipment manufacturer selects a thermistor with a highly non-linear response curve where resistance increases dramatically at the maximum allowable winding temperature, causing the relay to operate." The thermistor label could be telling at what temperature it acts like open circuit (almost).
So, in your case too, the thermistor resistance increases dramatically at the tripping point amounting to almost open circuit and thus causing the relay to operate. This is essentially the difference between PTC Thermistor and RTD I suppose.

 

[che12345]

Dear Mrrajendrarohra1978

Q. " I have Thermistor protection relay Siemens 3UN 2100-0AF7, please can you tell me how it works...."
A. Google for the [Siemens 3UN 2100-0AF7 manual], it is (free) on the net. The manual is very comprehensive.
Che Kuan Yau (Singapore)

 

[jraef]

A thermistor relay puts a voltage on the circuit that should be going to the thermistor and looks for a specific about of current flow based on the resistance of the thermistor, which increases with temperature. When the current flow drops below the threshold, meaning the temperature is high, the relay changes state. That's why, when you don't connect to a thermistor, the relay trips; open circuit = no current flow.

But thermistors are not linear devices, in fact most that are used in motor protection have a very steep "knee" point where the resistance changes fast. Here is atypical curve.

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