Humming Sound from Generator

[sethar02]

Steam Turbine Generator has the following specifications:
OUTPUT 88,500 kVA
VOLTAGE 11,000 V
CURRENT 4645 A
Ex. VOLTAGE 320 V
Ex. CURRENT 820 A
INSULATION CLASS F
STANDARD IEC 60034-3 (2005)
RATING CONT.
PHASES 3
POWER FACTOR 0.8
FREQUENCY 50 Hz
SPEED 3,000 rpm
MOMENT OF INERTIA(GD2) (10.0 ton-m2 ) Design Value
POLES 2
COOLANT TEMPERATURE 48 ?
TEMPERATURE RISE STATOR 77 K
ROTOR 72 K

The specifications of exciter are as follows:
BRUSHLESS EXCITER
OUTPUT 290 kW
VOLTAGE DC 320 V
CURRENT DC 906 A
EX.VOLTAGE 125 V
EX. CURRENT 15.3 A
STANDARD IEC60034-1 (2004)
INSUL. CLASS F
COOLANT TEMP. 48C
TEMP. RI SE ST 72 K
TEMP. RISE RT 72 K
PROTECTION IP54

The problem is that when the generator reaches at 3000 rpm and excitation is applied to its field winding, a high humming sound comes from the middle of the generator body. This sound matches exactly as the humming sound coming from power transformer. But the generator vibration and temperature (both stator and rotor) do not exceed beyond the specified limits (remains normal).
It should be noted that the generator is newly installed (8 months ago) and this sound was not coming initially. But during first 4 months it was running quietly.
What may be the cause of this high humming sound and is this dangerous or not for the continuous and long-time running of the generator?

 

[Skogsgurra]

If it is the same humming sound as you get from your transformers, then it is a 100 Hz sound. That is twice the normal frequency of a 3000 RPM generator and I would not sit there wondering what it is about. That could be quite costly.

Bring the supplier there and make him do a thorough check. That can save him (and you) a fat bundle.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

A 50 Hzmagnetic field, probably the stator field is causing something to vibrate. It may be a stator lamination problem but it could be anything. I think that you will do well to follow Gunnars advice.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[sethar02]

while searching this problem, I found this thread:

http://www.eng-tips.com/viewthread.cfm?qid=262378&page=3

 

[electricpete]

Since the other thread was mentioned, there we were talking about high-pitched screaching presumed of electromagnetic origin which is usually slot harmonics and sidebands around them.

Low-pitched humming is more often twice line frequency with perhaps a few harmonics. And as Gunnar said if it matches transformer pitch, that strongly suggests it is twice line frequency.

Possible causes for 120hz vibration which appears when the excitation is applied at a magnitude higher than used to occur upon applying excitation:
* change in excitation voltage
* change in adjacent noises (maybe you just didn't notice it before)
* looseness of anything within or attached to the machine (even a nameplate) can cause harmonics of 2*LF in response to the 2*LF force (just like looseness excited by 1x creates harmonics of 1x in vibration spectrum). These harmonics make the sound more noticeable.
* Change in foot conditions
* change in airgap.
* change in axial position of rotor
* mechanical changes associated with base and suport that might affect amplification of this frequency (such as tuning a resonance)
Some things perhaps we can rule out:
* Unbalanced load is in general a source of 2*LF on a generator but wouldn't show up until apply a load.
* shorted turn would generally show up at running speed, rather than 2*LF (confirm your frequencies)

It certainly may not be a problem since there are many benign causes of 2*LF. For a machine this size I agree for a machine this size it deserves more attention such as vibration spectral analysis to confirm the frequencies and harmonic content. Even though sound appears before applying load, you might consider checking load dependence and excitation voltage dependence for clues as to the source.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[edison123]

Electro-magneticstriction. Quite normal as long as it doesn't excite any vibration.

Muthu

http://www.edison.co.in

 

[Yuma]

Muthu,

If the problem is that the thru-bolts don't hold the core laminations together as they should, then you wouldn't get any vibrations and still you would have a problem.

Sethar,

Did this noise started after some event, i.e. a mechanical problem in the steam turbine?
Can you tell us the who the manufacturer of this generator is?

 

[sethar02]

This sound started suddenly after about four months normal operation of the generator and there did not happened any event, especially on turbine side. Turbine is made of a well known international electrical company. Steam turbine is a part of combined cycle power plant.