Hi guys,

Anyone of u guys expert in vibration frequencies of synchonous generators? Any tips for this one... not critical though.

Can't seem to in point actual problem that is causing sidebands in this 1000RPM generator. Line Frequency here is at 50Hz. There r 2 sets of 1x sidebands centering at 300Hz (18x) and 1333Hz (80x). Both amplitude modulation very apparent in timewaveform

Its super old... way b4 anyone one of us were born. Running only a half to a quarter of its rate load (couple hundred kW only). So no more manuals to tell how many rotor/stator slots.

3 hydrodynamic Bearings - means Drive is Overhung... but sideband only in Generator bearings.

The 18x could be the comutator carbon brush (unlikely - smooth, no spark)OR even a 6 poles x 50Hz. Should be 6 poles right? 2 x line Hz /poles = motor Hz.

The 80x??? u guess is as good as mine. My guess ...rotor dynamic eccentricy? which means rotor modulating stator slot freq.

Worth to mention: there is dynamic unbalance & re-aligned several times b4. Can see both on spectrum. Possible or not re-aligment distorted generator housing causing modulation freq.?

I am lost finding actual cause to the prob.

Doc1.doc (160 Kb, 28 downloads) GenSideBands

Would you have any known status of the exciter system and possible rectification involved? Just a comparison on what is found on DC drives at 300Hz from 3x100Hz from fullwawe rectification. Just a farout guess, may be wrong. Question in the same direction, are the peaks coming or going when going on and of the grid or with load or temperature? Just to verify or negate the possible electrical connection. Olov

This message has been edited. Last edited by: OLI,

I think u r right OLI... 300 Hz is probabaly something from the excitor & commutator. 300Hz only present at near bearing 3 which is beside the excitor.

This is mini-hydro station running continuous load... low temp all the time. Bearing temperature also normal.

Still... what is modulating the frequencies at 1x turning speed? especially the one around 80x. Amplitude rather high.

Definitely electro-magnet + mechnical related.

Likely you are right, on big hydro gen´s you may need to do a off grid balancing and on grid balancing as the stator is tricky to place well centered, if this is a oldie, the same maybe apply if the windings are a bit uneven? Is it a big diameter axially "thin" device? Normally they spin like cats and go forever. Just worked on a 1922 vintage but on the wet end, no problem in the sparky end. Olov

Gentlemen:

If this is a salient pole generator, the 300Hz with running speed sidebands will most likely be a fault in the rotating fields. These tend to fail from the inside out over time.

The same type of signature appears in electrical signature analysis (however, it shows as line frequency sidebands around the number of poles times the line frequency in Voltage Signature Analysis) when issues appear in the field coils as they develop shorts or mechanical looseness.

Check the coils for grounds or looseness. One way is to perform an insulation to ground test, another is to perform an infrared test on the exciter. If it is overtemperature, then the coils are failing (or have failed).

Sincerely,
Howard

The magnitudes do seem relatively low.

I don't recognize that vib signature.

If you do suspect rotor problems as Howard suggested and have an opportunity for off-line test, I would recommend also doing an ac pole voltage drop test to check for shorted rotor turns. Unlike ac coils which generally self-destruct rapidly in presence of shorted turn due to auto-transformer effect, dc coils such as generator rotors only have a slight increase in current with shorted turn and can continue to operate for long periods of time with shorted turns.

thanks a lot guys. So its rotor fault. Right MotorDoc?

I can do the thermography, but I not really sure how to check the coils for grounds. Can it be done while m/c running? and coil looseness... unless opening it up, I dun see how else.

M/c runs all the time. Its not thin turbine... its a Horizontal Francis turbine. Dynamic Unbalance is due to the failure of one of its water gate.

electricpete... u mentioned that 4 DC coils, slight current short is no prob. Means... no need to fix this m/c? it can still run ok in years to come? What predictive symptoms am I looking for this m/c when its gonna fail?

Err... this may sound stupid but can u pls elaborate on the AC pole votage drop test??? I thought the voltage is constant... can't drop right? otherwise generator would be motor the turbine.

As mech. eng., I'm not so good wif electrical subjects... bare wif me, ok. Means...I have to get some other electric eng. do some test? on-line test possible?

c.w.m.
It looks like Pete might be off for the holidays so maybe I can help you with the question about AC drop test. This is an offline test and you apply AC voltage to the rotor leads and measure the voltage drop across each pole. i.e. if you apply 240 VAC you should get 40 VAC across each pole; 240 VAC / 6 poles = 40 VAC. In our shop, we also apply the rated DC voltage and check the drop across each pole. With the AC test, the amperage will be quite a bit lower than rated amps even though the AC voltage is higher. This test will tell you if you have shorted turns in any of the coils, a coil with shorted turns will have a lower votage reading across it than the other coils.

Hope that helps,
Ronnie

I wasn't suggesting shorted turn would not be a problem. But when it comes to ac windings some folks are skeptical of any finding of "shorted turn" during any predictive maintenance since the machine most likely would have tripped off-line already for shorted turn in ac winding. That doesn't apply to dc windings such as generator field winding.

We use a percent of variation of +-5% for the AC drop test. This would suggest that the if the amount of shorts in any given coil does not cause greater than +-5% variance in the drop test then that coil would still be in acceptable condition. With the large number of turns in a synchronous pole, it is possible for a few turns to be shorted and not exceed the +-5% tolerance. This is for DC poles. With AC windings, we use a surge comparison test and no shorting is acceptable.

Ronnie