Velometer 5485C

S

Thread Starter

Sadiya

Hi Everyone,

We have a frame 5 gas turbine controlled by MARK-VI control system.

Can anyone explain the working of velocity sensors which are connected to VVIB board through TVIB terminal board.

I have read through some information about velocity probes, and it is clearly mentioned that no supply voltage is required for their operation.

But when we check the terminals at TB end (when the field sensor is not connected) we get 10V DC. and when field sensor is taken in line the voltage disappears. Can anyone explain this phenomenon.
 
Sadiya,

You didn't say if you were measuring the voltage with the unit running or with the unit at rest or on cooldown (ratchet).

No supply voltage is required when the turbine is running; they are self-powered. Magnets and coils and relative motion and induced emfs and all that weird electrical phenomenon stuff, you know.

I think the Mark VI System Guide states that when the unit is not running it impresses a "test signal" (I've never measured it; could be 10 V...?) across the circuit to test for open circuit. All vibe sensors must be good prior to a start, and the only way to test that is to check for some current flow in the circuit, and since there is no output from the sensor when there is no vibration then the Speedtronic has to apply one.

I don't have access to a Mark VI System Guide, but that's what GE have been doing with velocity ("seismic") vibration pick-ups for many years.
 
Hello CSA,

Thank you very much for the reply.

We were checking the voltage when turbine was under shutdown. As rightly said by you, the voltage across the terminals is Neg 10V DC.

Thank you.
 
Sadiya,

Thanks for the feedback!

Can you tell us something about the problem(s) you are experiencing and why you are checking the vibration sensor and the Mark VI input?

Was the Mark VI provided with the unit at original installation, or is it a retrofit of an older control system on an older unit?
 
Dear Sadiya,

The measured -10Vdc is the excitation voltage for the velocity transducer. The statement that it does not need power supply simply means that it does not require external power source connected to the sensor. The 2 cables to the Speedtronic panel and the shield cable is all is needed. Nothing to be worried of as it is a normal condition.
Do you have any problem with your vibration sensors? If yes, good way to check the condition of these vibration sensors is by using the shaker table. This way, the response and linearity of the sensor could be obtained easily.

You could find more information in GE manual or the OEM of these probes (usually Metrix Instrument).

Good Luck!
 
Hello CSA,

One of the vibration sensors bb3, when connected, shows abrupt vibration readings (exceeding alarm and trip limits). That's why we were just trying to compare TB voltages between this channel and other healthy channels, when turbine was shutdown for some other maintenance purpose. As replacing the sensor has not solved the problem.

And we could get -10VDC on all vibration channel inputs at the TB when sensors are not connected.

Thanks.
 
Sadiya,

Have you tried swapping one of the other vibration sensors with the suspect one (at the Mark VI terminal board) to see if the problem stays with the sensor or is with the terminal board, or possibly the cable(s) connecting the terminal board to the Mark VI processor rack(s), or with the VVIB board(s)?

Have you tried measuring the voltage at the bb3 input terminals when the unit is at CRANK speed, and comparing that to one of the other turbine vibration pick-up inputs at the same speed?

In my experience, one of the reasons GE used these types of vibration sensors was that they did not require a power source (other than the high temperature rating). From the Metrix website:

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

(Remove any spaces inserted by the forum software.)

It quite clearly states: "Self generating, no power required". Most of these sensors have an output measured in millivolts per thousandth-of-an-inch, so if you're going to measure voltage while the unit is cranking/accelerating/running, be sure you have your voltmeter set to the proper range. (Contrary to popular opinion, measuring voltages at Speedtronic terminal boards when the unit is cranking or running will NOT trip the turbine, presuming the proper range and input type is used and terminals aren't shorted or grounded.)
 
Dear Sadiya,

The problem is definitely not due to sensor or Speedtronics. The problem is definitely due to loose termination or damaged interconnect cables.

Please check the termination at the JB for loose connections. If all the terminations are secure, replace the interconnect cables.
The connector socket for these sensors are prone to damage when not properly installed.

Good luck and do report back!
 
G

George Robinson

We are commissioning GE Frame 9Es with Mark VI controls. We are the EPC that provide the technicians for calibrating, loop checking, etc. Separately, there are GE Technical Assistants here. They direct our technicians on whatever loops are going to be checked each day.

The GE TA told my techs yesterday there was a problem with one of 5 Velocity pickups. They are Metrix 5485C's. I asked which one was giving a fault and he said "It goes into a group alarm and I can not tell which one. Check them all". I don't believe that, but we are forced to deal with them and have to get the job done so the guys checked each one. We found the cables cut on two of them, so that's an obvious fault.

The thing of interest is that some of these measure approximately 100 ohms, which agrees with the vendor's data sheet. For these pickups the circuit has -10 volts when the Pickup is removed from the circuit and virtually no voltage when the Pickup is wired in. However, there are some pickups that measure over a megaohm and the reading climbs while being measured. Measuring a disconnected pickup shows that there is approximately .7 volts on it. These pickups act like a capacitor. When they are wired in the -10 volts signal does not go away. The GE Tech. Asst. does not tell us that these pickups are bad, but I think they are and only the pickups that show the approx. 100 ohm coil resistance and cause the -10 volts to go away when plugged in are the good pickups.

Comments on my assumption that only the 100 ohm pickups are good would be appreciated.
 
Gents, correct me if I am wrong please!

I have always had the understanding that sensors, (VEL/VIB) connected to MKVIe controllers were of the pizo electric principle.

Taking ohms readings,will not give true indication of fault; only open condition.

These are current operated devices.
Out of circuit -10v will be seen.
In circuit voltage 0v.

When in operation, sensor measured signal is not detectable, unless processed
.
 
Hello everyone,

While commissioning activities of GE-mark vi-e velocity probes 5485C (40ft) installed in turbine bearing 3, coil resistance found to be mismatched (not 100 ohms as per metric data sheet). So velometer was removed and resistance found to be 107 ohms. Again when probes was installed and bolts were tightened, the resistance value found to be 1.2 ohms.

So please can any 1 help me to resolve the issue. is my probe damaged??
 
When you removed the probe did you also measure resistance between the wires that connect to the probe? Your short circuit may be in the interconnecting wiring and not the probe.
 
Dear Otised,

The probe is integral type of. so it has fixed connector with wire, so can't be removed to measure the wires short circuit at probe.
 
Dear yaseeeen,

I guess I wasn't quite clear in what I meant. The wires that come out of the probe at some point connect to wires that travel back through conduit and one or more junction boxes to end up at the Speedtronic control panel. I was questioning whether or not you have verified that the short circuit is not in that long length of wiring that goes back to the control panel.
 
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