Hello,

Anybody know what is the maximum cable length of RTD cable from the field motor to the MCC in the electrical room? I hope it can be over 250 meter.
This issue may be quite common in the MCC application.

Thanks!

 

An RTD signal is pretty low level so I wouldn't run it 250 meters. Use the same two or three wires and put a 4-20ms transmitter at the motor end so that the RTD run can be kept to a minimum. Less than 5 meters will be fine. You can get transmitters that are small enough to fit in a junction box, so it there is no control encloxure at the motor location, that's no problem. Or if you have other I/O in the area, you could run some kind of remote I/O at the location, such as Modbus or Profibus.
--
Steve Myres, PE
Automation Solutions
(480) 232-6862 Mobile
(480) 813-1145 Office

 

The issue with conductor length for RTDs is that typically a small fixed current is injected into the RTD and the voltage drop is measured to determine the resistance of the RTD.

If you have long cables, you will get voltage drop in the cable itself. So in cases where longer cables exist, 4 wire RTDs are generally used. They have separate conductors for the current to go out to the RTD and for the sense lines that detect the voltage drop across the RTD. There is no VD on the sense lines so no loss of accuracy. There will be almost 20 Ohms just in the cabling for your 250 meter run if you use 20 AWG conductors.

There is also the issue that long runs of cabling tend to attract electrical noise that is not easily shielded.

 

The higher quality instruments will provide a spec for their analog input.

The Honeywell HC-900 that I'm working on at the moment says:

RTD, PT100 3 wire, 40 ohms balanced maximum

Bud

 

Thanks, Friends!