Using RTD's for Motor Protection

[ilan7]

I got a new motor from a respectable manufacturer with RTD's for measure and protect the windings from Over-temperature. I was surprised to see that the RTD’s are three wires, with the compensation wire connected to the ground and does not have any terminal.
Wondered: is it normal to measure the motor temperature with only two wires RTD? My feeling is that the error might be higher then the set point. Thanks to all.

 

[hacksaw]

two wire rtd's are normal. the ground is to drain off common mode noise.

two/three/four wire use has more to do with lead compensation. you can always zero out(calibration) the lead resistance even with two wire setups.

 

[ilan7]

I ran few wires calculations and have figures of 6 deg. error for 150 ft of AWG 18 wires, and another 4 deg. error for 30 °C amb. temp. change. Total is 10°C (18°F) error. When using smaller wire gage (22 AWG) error is double, about 20°C (36°F). That's the different between good motor at a hot day and a bad one ....

 

[randi]

Check out the current issue( Sept, 2003 ) of ISA magzine, page 61, regarding measurement with RTD's and thermocouples and send your query to : President, Mr.H.M. Hashemian, Analysis & Measurement services, Knoxville Tennesse, because your measurement tecniques appear to be way off the mak.

Also read up a little on measurement with RTD's under "OMEGA" i.e., Omega Engineeering, it provides lot of valuable technical information, free of charge.

 

[tiffy023]

It sounds to me that the rtd is 2 wire which means there is no compensation lead. The way to verify this would be to measure the resistance between cores.
Only 3/4 wire setups have compensating leads.
Where i work we use rtd's and thermocouples and input these to SMART temp.transmitters, this way we don't need to worry about cold junction or about compensating leads as we mount the transmitters as close to the process measurement as possible.