PTC Thermistor Relay (Siemens 3RN, Telemech LT3, ABB CM-MSE) - Can PLC analog input be used instead?

[123MB]

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/getfil...f16e-41e0-a862-cf1b46151ccc&file=Snapshot.jpg

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

 

[ScottyUK]

For what purpose - monitoring, protection, hazardous area requirements?

 

[123MB]

Protection against motor overheating

 

[jraef]

The issue is, the thermistor trip circuit is sometimes calibrated for a specific resistance value that takes place at a specific temperature. Without knowing those data points, what will you do with the information in the PLC? The resistance change in a thermistor is unlike an RTD or even a thermocouple. It has a very steep curve and a very rapid knee point at which it changes value. It's almost impossible to use it effectively as an analog input. I've seen people try, I have not seen it work very well.

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[123MB]

Hi Guys

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.

 

[123MB]

No ideas guys? cheers

 

[123MB]

 

[triden55]

I've always used either trip relays or the VFD thermistor inputs for this purpose, so I cannot verify that a PLC input will work. It does look quite possible though. A PLC voltage input card should have a very high impedance (in the megaohm range), so it shouldn't have any unwanted loading effects on your circuit. Can you set it up in the lab on a bench with some PTCs?

 

[123MB]

Thanks for your reply mate

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.