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EHELP: Using the TRY or RBX to Get Temperature Measurements of Generator Windings

EHELP: Using the TRY or RBX to Get Temperature Measurements of Generator Windings

Q: We have been using your SPA series temperature transmitters in our hydroelectric power plants with excellent results. These devices have high protection against electromagnetic interference. We would like to purchase temperature transmitters for head mounting and rail mounting that have the same high level of isolation and protection against electromagnetic interference. The sensors are 10 Ohm Cu three wire RTDs and they represent the temperature of generator windings in the hydroelectric power plant.               

A:  Model SPA/TPRG/2PRG/U/-AO [DIN]  provides 0.25 mA RTD excitation current.  If it is performing well then our PC configurable temperature transmitters, which have the same excitation current, should also perform well. In that case we recommend the TRY series. The head mount style is model TRY/PRG/4-20MA/10-42DC [LH2NS]

If your SPA has the SP2 option instead (Model SPA/TPRG/2PRG/U/-AO-SP2 [DIN] ), that option provides 5mA of excitation current to the 10 ohm RTD instead of the usual 0.25mA. This ensures that a robust input signal from the RTD to the SPA resists interference from the generator’s very strong electro-magnetic fields.

The choices available for a head mounted transmitter with similarly high current RTD excitation are limited. Bear in mind that the objective when designing RTD temperature transmitters is to reduce the excitation current as much as possible to minimize self-heating of the RTD, thereby maximizing the measurement accuracy.

Also due to the reduced power budget of loop powered transmitters there isn’t sufficient power available for the 5mA excitation current of the SP2 option.  

A possible solution is one of our older analog loop powered transmitters, the RBX series, which has a relatively high excitation current of 1mA.  The RBX, however, is a strictly analog design and must be specified according to the intended resistance range instead of temperature span. For example, for 10 ohm Cu and 0-200 deg C, the resistance is 9.035  to 16.776 ohms. The span is 7.741 ohms. The 7.741 span falls within the 3W5-10 input range. 

The head mounted model number is: RBX/3W5-10/4-20MA/12-42DC/-EZ9 [BH2NS].

Published: February 8, 2016