PTC Thermistor Relay (Siemens 3RN, Telemech LT3, ABB CM-MSE) - Can PLC analog input be used instead?
PTC Thermistor Relay (Siemens 3RN, Telemech LT3, ABB CM-MSE) - Can PLC analog input be used instead?
(OP)
Hi All
We are designing a distributed control system and we would like to find some way of wiring our PTC thermistors into our field remote I/O panels (utilising B&R X20 I/O).
To clarify when I say 'PTC thermistors' I am referring to a thermally sensitive resistor that is NOT intended to measure temperature. It has an extreme resistance characteristic which is ~ 1500 ohm when cold and ~3600 ohm when hot.
The characteristic becomes very steep when the trip temperature is reached and the resistance becomes very high very fast - indicating the winding(s) are overheated.
Many vendors sell relays which will evaluate the state of the PTC thermistors and produce a relay output which can be wired to a PLC.
However many VFDs include a dedicated input for a PTC thermistor which just utilities a 0-10V analog input with a voltage divider - a resistor is added which is chosen to specify a trip point voltage. See attachment http://files.engineering.com/getfile.aspx?folder=c...
My question is: is there a reason I can't do the same but with a standard 0-10V PLC analog input? I have never seen this done.
Thanks
Michael
We are designing a distributed control system and we would like to find some way of wiring our PTC thermistors into our field remote I/O panels (utilising B&R X20 I/O).
To clarify when I say 'PTC thermistors' I am referring to a thermally sensitive resistor that is NOT intended to measure temperature. It has an extreme resistance characteristic which is ~ 1500 ohm when cold and ~3600 ohm when hot.
The characteristic becomes very steep when the trip temperature is reached and the resistance becomes very high very fast - indicating the winding(s) are overheated.
Many vendors sell relays which will evaluate the state of the PTC thermistors and produce a relay output which can be wired to a PLC.
However many VFDs include a dedicated input for a PTC thermistor which just utilities a 0-10V analog input with a voltage divider - a resistor is added which is chosen to specify a trip point voltage. See attachment http://files.engineering.com/getfile.aspx?folder=c...
My question is: is there a reason I can't do the same but with a standard 0-10V PLC analog input? I have never seen this done.
Thanks
Michael





RE: PTC Thermistor Relay (Siemens 3RN, Telemech LT3, ABB CM-MSE) - Can PLC analog input be used instead?
RE: PTC Thermistor Relay (Siemens 3RN, Telemech LT3, ABB CM-MSE) - Can PLC analog input be used instead?
RE: PTC Thermistor Relay (Siemens 3RN, Telemech LT3, ABB CM-MSE) - Can PLC analog input be used instead?
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: PTC Thermistor Relay (Siemens 3RN, Telemech LT3, ABB CM-MSE) - Can PLC analog input be used instead?
Sorry for the late reply and thanks for your responses.
Jraef: IEC34-11-2/DIN 44092 (I have just become aware) requires the trip point to be less than 2.5VDC
So my understanding is that if a voltage divider network is provided on the analog input then the appropriate branch resistance needs to be chosen in order to provide a ~2.5VDC input when the PTC resistance goes hot (~3600 ohm for 3 PTC thermistors complying with IEC34-11-2)
So we program our PLC to look at the analog input and if the voltage increases above 2.5V we trip the motor.
As long as the voltage divider resistances are correct then the analog input voltage above 2.5V should indicate tripping?
Is this not exactly how the PTC input is implemented on a VSD (i.e. powerflex, yasagawa, etc)
All too simple?
Cheers guys.
RE: PTC Thermistor Relay (Siemens 3RN, Telemech LT3, ABB CM-MSE) - Can PLC analog input be used instead?
RE: PTC Thermistor Relay (Siemens 3RN, Telemech LT3, ABB CM-MSE) - Can PLC analog input be used instead?
RE: PTC Thermistor Relay (Siemens 3RN, Telemech LT3, ABB CM-MSE) - Can PLC analog input be used instead?
RE: PTC Thermistor Relay (Siemens 3RN, Telemech LT3, ABB CM-MSE) - Can PLC analog input be used instead?
I have also done the same, however I am now working on a project with alot of distributed IO in the field and alot of motors with thermistors hence it would be a significant advantage on this project if we could wire the thermistors back to the local remote I/O box instead of all the way back to the MCC. Also we would prefer not to locate trip relays in the field cabinets (although, if they are self-resetting, it's probably not a bad idea)
We dont really have the budget to test this on the bench for this project but I will look into it.
Thanks again for your reply.