Line Frequency or rotational Speed?

[Sofistioelevib]

Hello,
making vibration on 2 pole generator (3000RPM) often the 1X can be confused with line frequency.
Keeping apart both are linked togheter, sometimes there are necessity to understand if 50 Hz vibrations coming from mechanical 3000 RPM of generator or phase unbalance (or any other electrical source problem).

What type of test you suggest in order to identify the source of the problem? it is mechanical or electric?

Regards

 

[Strong]

You could:
Measure vibration spectrum (FFT) with high resolution for accurate vibration frequency. It should be less that 50-Hz for induction motor with load.

Measure vibrations and shut power off. Mechanical vibrations will remain momentarily until speed drops.

Usually electrically induced vibrations are at 2x line frequency, and 1x line frequency vibration is rare.

Walt

 

[Sofistioelevib]

THank you strong but i think all you suggested is useful for a asynchronous motor and not for 40MW generator

Effectively:

-Measure vibration spectrum (FFT) with high resolution for accurate vibration frequency. It should be less that 50-Hz for induction motor with load.

Generator have to be connected with line frequency so have to be exactly as the power line frequency

-Measure vibrations and shut power off. Mechanical vibrations will remain momentarily until speed drops.

This should be usefull but it is not so simple disconnect Generator from line and restart again more complicated than a normal asynchronous motor

Anyway thank you for your support

 

[Strong]

You did not say synchronous generator or 40-MW size in your OP.
You could run to full speed no-load for mechanical vibrations
Excitation on for electrical fault (I did find a rotor fault this way)
Load versus to time for thermal issues

Yes, these are more complicated than for asynchronous motor, so you need a good reason for doing the tests!

Walt

 

[Eng-Ahmed]

1. Using current signature in this case is so useful
2. 50 HZ is rare for electrically problems (personally i never get it in such a case 20 years)
3. finding phase angle of 1X will be so useful (H-V)
4. measure vibration at many locations and match it with Infra-Red to find stator faults

 

[Sofistioelevib]

Strong,
exactly what i needed.
Furthermore as you said, these test are invasive so is not so simple to ask to production and your reasons have to be correlated with very strong indication.

Thank you

Eng_Ahmed,
When you are talking about phase of 1X what really you meaans?
What phase you are talking about? difference between electrical and mechanical 1X? ho do you made this?

regards

 

[electricpete]

I think the scenario of shorted rotor turns deserves mention because it doesn't follow the rule about showing up at 2*LF even though most would agree it's an electrical fault. It shows up at 1x running speed (for 2-pole sync machine this is 1x line frequency). For shorted rotor turn you should see a dramatic step change when you open or close the field breaker (during shutdown or startup).

You should also look at traditional causes of 1x vibration which includes unbalance, bow (possibly thermal), misalignment.
Watching the vibration over time and during changes in conditions may give a clue.

You asked about 1x phase H-V mentioned by the other gentleman. I think that is phase difference between horizontal and vib at a given bearing. When combined with the magnitude that may give you an idea of the filtered orbit shape, which might steer you in a particular direction. Unbalance would typically cause 90 degree difference H vs V (lagging in the direction of rotation). Misalignment is sometimes mentioned to have a 0 degree phase difference (straight line orbit). Others say misalignment may show up with a figure 8 shape of unfiltered orbit (includes some 1x and some 2x components). At any rate a more detailed analysis than just looking at the 1x magnitudes may (if you're lucky) steer you a little bit towards a particular cause.

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(2B)+(2B)' ?