I want to know the details working principle of self generating vibration sensor which is used in gas turbine bearings or large generator. It's model number is 5485C and made by METRIX. Why it shows -10V dc when Turbine is in shutdown condition? and what is the output voltage of it in Turbine running condition? And some vibration sensors are showing abnormal reading in speed tronic of frame 6B mark 4 Gas Turbine. How can i maintenance it? Please Explain in details.

 

Md. Nubayeed,

Did you try typing "Metrix 5485C" into your preferred Internet search engine? This was the first (top-most) search result when I did:

http://www.metrixvibration.com/Products.aspx?ProductUid=25

(remove any spaces inserted by the control.com forum software)

The topic of velocity vibration sensors ("seismic" vibration pick-ups as they are commonly called) has been covered several times before on control.com. There is a 'Search' feature at the far right of the Menu bar of every control.com page. (It is suggested to use the 'Help' feature, as search terms are a little different than most Internet search engines.)

Seismic, or velocity, vibration pickups used with Speedtronic turbine control systems are self-powered, meaning the sensors are actually small generators which produce a voltage (usually a millivoltage) output that is proportional to vibration--when the turbine shaft is spinning.

When the turbine shaft is not spinning (when it's "at rest", zero speed) Speedtronic turbine control panels apply a voltage signal to the vibration input circuits and monitor the current flowing through the circuits. This is done to ensure the circuits are continuous, and hopefully "good", before the turbine is started.

As for unusual signals, well, that happens frequently when sensors with differing sensitivities are substituted for existing sensors which may have failed. Not all sensors have the same sensitivity, and as such they are not directly interchangeable. Some Speedtronic turbine control panels have the ability to specify the sensitivity for each vibration sensor input; some do not.

Just because the bolt holes of a new sensor match the existing holes, or just because the salesperson says, "Yeah! This is a direct substitute!" doesn't mean it is a direct substitute or that it can be used without making a change to the I/O configuration of the Speedtronic turbine control panel.

Unfortunately, there is no "calibration" that can be done in the field on these types of sensors. Simulating vibration is difficult without an expensive piece of equipment.

Another common problem is that the low level signals and wiring are not properly terminated when replaced, particularly after maintenance outages.

So, have a look at the URL above, and see if that combined with the information provided is helpful at all.

Also, when you are posting to a forum like this, you should provide as much information as possible. We don't know if you have a Mark IV, a Mark V, a Mark VI, or a Mark VIe, so we can't provide too much in the way of help by pointing you to the appropriate section in the manual for the appropriate control system. There is sometimes some very good information in the GE manuals; and sometimes there is not. The more information you provide, the more assistance and concise information we can provide. Help us to help you.

And, use the Search feature of control.com. The community of GE-design heavy duty gas turbine control users has been active here for about a decade, and a lot of questions have been asked and answered many times already. Just as when searching the Internet, you need to use different search terms and words, and even if you don't find exactly what you are looking for in your first search or two, if you read some of the results you will learn more terms and words that you can use in subsequent searches that will get you closer to your desired information. And, that's true on control.com as well--don't stop with one search if you don't get exactly what you're looking for. There is a LOT of very good information here.

 

I have some questions in this concept. How will it generate voltage? Means it will not directly contact shaft or bearings. And it will not get direct impact by shaft like Bentley Nevada proximity probes. It places on the body or cover where the bearings are placed. There is no cutting of flux.

Please explain

 

anil,

This type of vibration sensor is mounted on the bearing cap/housing/cover. It is non-contact--meaning it does not contact the rotating shaft.

The common type of construction uses a small core permanent magnet suspended on a spring inside a coil. Vibration (movement) causes the core to move up and down and generate a small voltage proportional to vibration. As such, it does not require power--it produces its own small voltage when subjected to vibration. It must be mounted vertically to operate properly. Magnetic lines of flux from the moving magnetic core cut across stationary conductors to produce voltage.

These types of vibration sensors are typically called velocity sensors or velocity pick-ups, or seismic pick-ups. They are very good for shafts with large rotating mass.

Hope this helps!

 

A seismic probe contains a relatively massive magnet supported by springs with a low spring coefficient (the springs are not very stiff and the combination has a low resonant frequency). This is surrounded by a coil fixed to the housing.

When the housing moves at a frequency higher than the natural frequency of the spring-mass combination, the magnet effectively stays still and the coil on the housing moves around it. This generates an emf in the coil proportional to the speed difference between the housing and the magnet.

The seismic probe does not directly sense the shaft position, but is fixed to the bearing housing and can only show what is happening there.