Are you going to program this function in a plc?
No. I am attempting to use it an analog input to a vfd for speed reference.
Are you going to program this function in a plc?
Neither are linear, but an RTD is more linear than a thermistor. It also depends a LOT on the thermistor you chose, some have a VERY steep curve and are essentially useless as analog signal sources. Plus when you try to USE the thermistor as a potentiometer, the signal current flowing THROUGH the thermistor heats it, affecting the resistance curve and making the non-linearity worse. That's why a converter is used, it normalizes the non-linearity to be a linear analog value for the VFD input.So does it not make an issue if the signal from the thermistor is not linear, when the RTD would be linear?
Neither are linear, but an RTD is more linear than a thermistor. It also depends a LOT on the thermistor you chose, some have a VERY steep curve and are essentially useless as analog signal sources. Plus when you try to USE the thermistor as a potentiometer, the signal current flowing THROUGH the thermistor heats it, affecting the resistance curve and making the non-linearity worse. That's why a converter is used, it normalizes the non-linearity to be a linear analog value for the VFD input.
Here's an example of a particularly problematic PTC thermistor for your application, the type usually used for thermal PROTECTION of a device, looking for a threshold being crossed, not a linear analog value.
You can get ones that are not this bad, but if you don't have control of that, i.e. your customer is dictating it for you, you can get in trouble really fast.
What are you trying to do?
I am guessing that is a starndard 10K Type II or Type III thermistor that would be used in refrigeration/etc system. The curve for this is very non-linear and you can't just use analog input or RTD transmitter on it.
A thermistor is NOT an RTD. An RTD implies a specific element and nearly linear performance.So does it not make an issue if the signal from the thermistor is not linear, when the RTD would be linear?
I wholeheartedly agree. If you have the opportunity to suggest a redesign, I would do it like this.A thermistor is NOT an RTD. An RTD implies a specific element and nearly linear performance.
RTD's are widely used for process temperature measurement, as eveidenced by the wide spread availability of RTD input
temperature transmitters (4-20mA or 1-5Vdc output)
Thermistors are seldom used for process temperature measurement, hence the relative rarity of a thermistor input transmitter.
I'd suggest not using a thermistor, but using a Pt100 RTD (widely available, very common) with an RTD temperature transmitter input, ranged for the mA or Voltage input range needed at the drive input.
All this is assuming that the drive handles a temperature input as a PV to PID loop. A temperature input will be a demand signal to the drive.
Correction: A temperature input will NOT be a demand (speed setpoint) signal to the drive.A temperature input will be a demand signal to the drive.
Correction: A temperature input will NOT be a demand (speed setpoint) signal to the drive.
Thermistors are seldom used for process temperature measurement, hence the relative rarity of a thermistor input transmitter.
Neither are linear, but an RTD is more linear than a thermistor. It also depends a LOT on the thermistor you chose, some have a VERY steep curve and are essentially useless as analog signal sources. Plus when you try to USE the thermistor as a potentiometer, the signal current flowing THROUGH the thermistor heats it, affecting the resistance curve and making the non-linearity worse. That's why a converter is used, it normalizes the non-linearity to be a linear analog value for the VFD input.
Here's an example of a particularly problematic PTC thermistor for your application, the type usually used for thermal PROTECTION of a device, looking for a threshold being crossed, not a linear analog value.
You can get ones that are not this bad, but if you don't have control of that, i.e. your customer is dictating it for you, you can get in trouble really fast.
Well it could be. Depending on the application, if the drive is to be controlled in a simple proportional band type operation, the 4-20MA from a temperature transmitter could be used to directly control vfd speed.
Example:
Simple ventilation fan control.
4-20MA = 50-100F. Perhaps you want the fan to run slowly at 50f and max speed at 100F.
That would be a pretty simple control loop/system.