what can makes a generator vibrate more as load decreases 2

[ManTicora]

I have a 45MWh generator that's showing a 3 mills of vibration ampliture when operating below 8MWh. The vibration will reduce to 1.2 Mills as soon as power production exceeds 8MWh.
It's clean to my mind that torque produce by the Prime mover(steam turbine) will increase the load angle and subsequently the Power being produced in the Generator Stator.
Since the frequency on the generator is constant at 60Hz(3600RPM) why the more vibration is produced at lower load (lower power production)?
Thank you all for your contributions.
M.D.

 

[byrdj]

how about, the field is "thermal sensitive" and it has been balance for running fiels current
20% is getting fairly low for a steam turbine so is it possible the increase is coming from turbine crosseffects

Is this phenomina something new, then maybe a field short has developed (making the rotor thermally sensitive) and just by luck the thermal vector is opossite the the unbalance vector

 

[ManTicora]

@BYRDJ
Thank you for your kind answer. I don't understand the full extend of your response... It's seems like you pointing out a thermal sensitivity of a field? (Magnetic? Electric?).
Temperatures in the generator bearing are very low (below 60C). Do you still believe that "thermal vector" will have such much effect?
Once again the question is:
Since the frequency on the generator is constant at 60Hz(3600RPM) why the more vibration is produced at lower load (lower power production)?

Kind regards,
M.D.

 

[ScottyUK]

A shorted turn on the field can result in different heat dissipation on the shorted side of the rotor winding compared to the un-shorted side. The outcome is that the hotter side expands more than the cooler side, and the rotor develops a thermal bend. A bent rotor will be out of balance and has a specific a vibration signature. It is normally more dependent on reactive power than power output, although both will influence it. Can you hold power output constant and flex the reactive output? You will have to wait a little between changes in excitation to see any effect because of the thermal lag in the rotor.

 

[electricpete]

If the horizonital and vertical stiffness supporting the bearing are unequal, this provides a means for coupling between torsional system and radial system. Change in torsional reaction load on stationary turbine or generator components can cause then to move which can change the alignment state. In that case, one would presume if aligned under no load, that machine would be closest to alignment at no torque load. But contrary to intuition, well aligned sleeve machines can sometimes vibrate more than misaligned machines… the radial load associated with the misalignment can reduce vibration.

That's one possibility. I think most machines are supported well enough that change in torque load don't affect alignment, but you never know.

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(2B)+(2B)' ?

 

[electricpete]

I should add that I have seen changes in torque load during factory loaded test run of a large motor cause change in vibration which the OEM attributed to flexing of the frame under load.

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[ManTicora]

Hi Guys,
This is a real life situation involving persons of different backgrounds (operators, managers, engineers, acountants, technicians..).
Thank you for you kind responses... But a clear and insightfull explanation is still missing.
Here again is the question:
Since the frequency on the generator is constant at 60Hz(3600RPM) why the more vibration is produced at lower load (lower power production)?
This type of question will require to explain fundamental notion such as why Generator output (MWh) increase while the shaft (rotor) spins at constant synchronous speed (3600RPM).
So keep your contribution clear, simple and descriptive.
M.D.

 

[electricpete]

This type of question will require to explain fundamental notion such as why Generator output (MWh) increase while the shaft (rotor) spins at constant synchronous speed (3600RPM).

I don't think many people have a problem grasping that piece (that's why they call it a synchronous generator).

My scenario simplified is that the transmitted torque can make the stationary components move which can affect alignment, bearing loading, and vibration.

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[djs]

Some misunderstanding of the terminology. The "field" no most generators is located on the rotating shaft of the generator. This field assembly usually consists of several distinct structures located around the shaft. They are connected mechanically to the shaft so that they rotate along with the shaft. Each of these structures, called poles, has a coil of wire through which an electrical current flows. If the coil of wire in one the poles is shorted, that pole will have a different temperature rise than the others. This temperature difference between the various poles could result in vibration.

The vibration amplitude (mills) is somewhat independent of the vibration frequency (60 Hz.). So in this case the speed is constant, but the amplitude of the vibration varies. The rotational speed of the machine in this has no case or effect with the change in the vibration.

 

[squeeky]

I only run smaller units up to 20MW and am no expert. I would try to note the excitation current under "normal" conditions where you get the vibration. Then raise or lower the load and then under or overexcite you generator to the field current that you had when the vibration was present. This may iliminate the magnetic coupling and heating debate.

Look at oil lubrication pressures. The pressure area on the bearings will move with load. Are the bearing having a temperatuer change when the vibration is present? Make a temporary grounding rod with soft conductive material. you can either ground the shaft and see if it is decreased or measure or "look" for disturbances with an occiliscope of PQM. The aim would be to illiminate electrostatic influences.

 

[ManTicora]

@electricpete (Electrical) Thank you for such a clear statement! Under your scenario, if transmitted torque were to move stationary parts then Vibration will increase with shaft torque. But in this case vibration are higher (2.5MILLS) at 8MWh.
@djs (Electrical) In my case the vibration amplitude is inversely proportional to torque.
@ squeeky (Electrical) I'm glad you mentioned the Lubrication Pressure. I was thinking about the same idea... I will check it today.
Any other thought?
Sincere thank you to all contributor to this exchange.
M.D.

 

[electricpete]

@electricpete (Electrical) Thank you for such a clear statement! Under your scenario, if transmitted torque were to move stationary parts then Vibration will increase with shaft torque. But in this case vibration are higher (2.5MILLS) at 8MWh.[LOW LOAD]

It can happen that way. As MW load increases, torque increases, stationary components move, misalignment increases, radial loading on the bearings increases, shaft movement can DECREASE. When you measure "mils" with prox probe it is movement of shaft with respect to housing…. typically interpretted as movement of the shaft journal within the bearing clearance. A simplistic way to think about it: a shaft journal within a bearing clearance with no radial load can easily be pushed around within the clearance by a small unbalance. … but if you put a heavy load (let's say downward) on the shaft then it forces the shaft to the bottom and it can't move around (vibrate) as much. The actual dynamics can be a lot more complicated and difficult to predict.

We have a family of 6 turbine driven pumps that all act that way. They also tend to exhibit large changes in alignment with ambient temperature (driver and driven are oddly enough on separate support structures and see different thermal growth). When the bearing temperature goes up (indicating increase in bearing loading due to misalignment), the relative shaft displacement (mils) tends to go down.

The less likely part of my proposed scenario imo is the change in misalignment under load. Most (not all) well constructed machines and foundations/supports won't allow significant relative movement of movement of the two machines under torque load. It is more likely if driver/driven are independently supported or if the support structure from bearing to frame to foundation is weak or degraded.

New subject - Can you observe vibration response during a rapid change of MW loading? Rapid change of vibration would tend to rule out thermal-related factors while slow change would focus you back toward thermal factors.


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[electricpete]

Do you happen to have shaft centerline traces? In my original scenario, when load increases and misalignment increases, you would see shaft centerline go further off-center.

For that matter, any further vib data (orbit plots etc)?


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[byrdj]

ScottyUK's expaination is what I was refering to as "thermally sensitive" rotor. While you are currently correlating the vibration to load, you should be aware that changing load means you also change the excitation current being aplied to the field. excitation is not my expertise, but ScottyUK mentioned, and I have heard recommended, you should make note of the field current as you reduce load and the vibration just begins to increase. you then increase and hold a load with minimun vibration, and reduce tthe excitation (which will have a direct effect on vars and must be limited to not cause problem) to the current noted and see if vibration is field current dependant.


the most common effect of thermally sensitive rotor is that the unbalance vector caused by the unequal heating casues the vibration to increase, but, if your unit was balanced to negate a thermal bow at rated load, reducing field current will result in the balance weight correction actually cause unbalance at low load

 

[xj25]

I know about a case somehow similar to the mechanical issue described by Pete, this is about a low load índuced vibration in a transmission. The system is inverter+170kw motor+gear, at idle load the vibration increased notably. After much investigation was found to be connected to problems with a pair of cone bearings working without axial load.

I was not directly involved so not know the details, bit i.e. when the drive was used in applications with comparable long idle times the gear was junk in 30% of its usual life.


My raccomendation here is more physcological, it is nice to explain the possible problems to everybody involved in understandable forms, but take care because managment will easily think that the solution is equally simple...


good luck!