Motor RTDs

M

Thread Starter

mikebro

Is it possible to wire 2 x 3-wire RTDs in series? How would the 3rd wire be connected?
 
Is it possible, no, 2 RTD's in series is not the way commercial RTD analog inputs function.

One RTD per analog input. Each RTD is isolated from any other RTD or ground.
 
Thanks for the reply.

I have not used RTDs before. For the project I am working on I have been given a set of drawings from an 'equivalent' project; which has a note "Winding thermistors connected in series" (but no detail)

Are RTDs simply a 'known' resistance/temperature profile? If you connect two in series would the resistance be the 'sum' of the two? (i.e. if you halve the reading you get the 'average' temperature?)
 
> Are RTDs simply a 'known' resistance/temperature profile?

Yes, an RTD is an element that has a very well defined temperature/resistance profile. The main reason RTDs are used rather than thermocouples is inherent accuracy. But to achieve that accuracy, the resistance of the lead wire must be taken into account, because a 2 wire RTD temp measurement includes an error due to the resistance of the lead wire and the RTD element. A 3 wire RTD temp measurement uses a circuit (in the analog input) that compensates for the lead wire resistance.

Some motor thermistors are wired in series. Refer to http://www.usmotors.com/Service-Support/FAQ.aspx, search for thermistors, near the bottom of the page.

I am unaware of any commercial RTD circuits that run RTD's in series, but that doesn't mean that none exist. Conventional lead wire compensation would go out the window. RTD's are the most expensive of the sensor elements and I'm not clear on why one would wire RTD's in series, it would be cheaper to use T/Cs to get an average.

> If you connect two in series would the resistance be the 'sum' of the two?

It would be the sum of the two plus the lead wire resistance. Without looking up resistance values on a chart, I don't know much error the non-linearity contributes. Lead wire resistance, as low as it is, can add considerable error.
 
Mikebro... 3 RTDs will provide an average temperature if output is divided by 3 or if instrument is calibrated accordingly.

Unfortunately, reliability is severely compromised. If one of the three fails open, there is no temperature measurement.

Thus, the question to ask is how critical is the motor in which the RTDs are installed?

Regards, Phil Corso
 
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