Negative Voltage of Bently Nevada

Re: What is the basic principle of vibration measurement by BENTLY NEVADA vibration measurement system? and Why it gives the -ve voltage output? Answer #1

This is a very good, accurate writeup done by someone else for a similar question earlier on. Hope it helps. The vibration sensors used in turbine, generator etc are all same type i.e proximity type of sensors. The sensor are mounted at the proximity of the Turbine and generator rotor. From the sensors one coaxial cable runs to the proximater, which is mounted in a separate JB nearby. From TSI Monitors, this Vibration probes gets +24 Volts DC. The proximater gives a signal 0 to -22.4 Volts DC depending upon the vibration measured by the sensor. Generally the distance between the probe tip and the rotor is maintained such that it gives -10 Volts DC when the turbine is not running. The sensor works on eddy current principle. As the rotor vibration increases, the eddy current generation remains constant, but the absorption of power in terms of eddy current increases. This loss of power is sensed by the proximater and DC voltage signal is transmitter to TSI monitor for Measurement, Controlling & Indication purpose. Good luck Sam

Re: What is the basic principle of vibration measurement by BENTLY NEVADA vibration measurement system? and Why it gives the -ve voltage output? Answer #2

As a 20-year Bently Nevada employee, and as someone responsible for answering such questions over the years within our company as well as externally, allow me to explain the rather interesting circumstances behind the use of negative voltages (-24 vdc) for powering eddy current vibration oscillator/demodulator devices. I will then proceed to explain the operating principles behind the eddy current proximity probe and how it converts physical gap between the probe and its target into a voltage. First, the question of negative voltage. The short answer is that When Don Bently worked on making solid-state versions of the eddy-current measurement system (it was actually originally designed in the 1930s by GE engineers using vacuum tubes), he had a choice between using N-P-N transistors or P-N-P transistors. At the time, transistors were quite expensive, so he chose the least expensive of the two: P-N-P (apparently, PNP transistors they were less expensive to manufacture 50 years ago than their NPN counterparts). Because the circuits used PNP transistors, a negative bias voltage was required rather than a positive bias voltage. Don chose -18V. This was later changed to -24V to allow more linear range from the transducer. At that time, the industrial instrumentation community had not yet standardized on +24 vdc, and by the time they did, there were so many Bently Nevada eddy current vibration sensors installed that changing to +24V rather than -24V was not greeting with enthusiasm by users. Hence, it has remained -24V to this day. This was not a deliberate effort to "be different" or "nonconformist" on the part of Bently Nevada. It was quite

literally based on which components were the least expensive when the technology was originally introduced 50plus years ago.

Now, the question of how an eddy-current proximity probe works: An eddy-current probe works by passing an alternating current through a coil of wire and measuring the coil’s impedance. This impedance changes when the probe is brought near an electrically conductive material and the impedance change is proportional to the physical gap between the coil and the conductive target. The sensing electronics in turn convert this impedance change to a voltage, providing an electrical output directly proportional to physical gap. The above is an excerpt from a recent article in our ORBIT technical journal. You can read the full-length article here: http://www.gepower.com/prod_serv/products/oc/en/orbit/downlo ads/1Q09_Prox_Probes.pdf Here are some other articles that may also prove helpful in understanding how proximity probes work in actual machinery vibration/position measurement applications: http://www.gepower.com/prod_serv/products/oc/en/orbit/downlo ads/1q01sabin3.pdf http://www.gepower.com/prod_serv/products/oc/en/orbit/downlo ads/1293jordan.pdf Finally, I am including a link to a datasheet for our 8mm proximity probe system. Although it does not go into a lot of detail on principles of operation, you may find some useful information therein. Should you have additional questions, please contact the undersigned. Regards, Steve Sabin Marketing Communications Manager Bently Nevada Asset Condition Monitoring GE Energy Services