270KW KATO Generator problem 6

[accordlx]

We rewound the stator on a KATO 270 KW/300 KVA 460v 625amp 100HZ single phase 10 pole synchronous generator that is driven by a six pole 60HZ synchronous motor. The generator puts out the correct voltage and frequency and for all purposes, it functions as it should. The problem is that it now emits a noise that resembles a very loud, high speed jackhammer. Our vibration specialist insist that the noise is electrical and not being caused due to a mechanical issue.
There are 90 slots, 90 coils (30 groups of 3),7 turns per coil, 1-8 coil span, 5 wires in multiple used to make coils-4#15 AWG & 1#14 AWG. I have attached a copy of the internal connection drawing.
KATO will not release any information as it is proprietary. Does anybody have any idea as to what would be the cause?

 

[waross]

Have you checked the coupling?
The angular alignment may be critical to ensure a best fit between the torque pulses produced by the motor and the torque pulses demanded by the alternator. Also check that all mounting bolts are tight.

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

 

[edison123]

One way to rule out electrical is to run the generator unexcited and see if the noise stays or goes away.

Muthu

http://www.edison.co.in

 

[accordlx]

I should have mentioned that the noise goes away when it is not excited and gets progressively louder as the load is increased.

 

[edison123]

Sounds like rotor winding (turn shorts in pole coils) or uneven airgap. Do a AC drop test on the rotor winding and measure the airgap across all the ten poles.

Muthu

http://www.edison.co.in

 

[accordlx]

WE did do a drop test on the rotor (AC) and the results are within acceptable levels. Also air gaps were checked and are uniform around the stator. I am particularly interseted to see what every body thinks about the internal connection configuration.

 

[catserveng]

Why was the unit rewound? Did it have a fault, a low megger reading or an operational problem.

Have you looked at the output with a scope? Are the wavsforms clean, the output voltage balanced and the proper spacing between the phases?

Do you have any excitation data, field volts and amps from before the repair in "normal" conditions, and compared that data to what you are seeing now?

Along with the noise is the measurable vibration? If so, what order and magnitude.

I had a similar problem with a 400 Hz unit several years ago, turned out the core iron had been damaged during the burnout.

Mike L.

 

[Strong]

The answer will be revealed with a better diagnosis of the vibrations/sound. Can you post the vibration and/or sound pressure spectra Vs load?

Walt

 

[rhatcher]

I've seen a lot of winding connections before but never one like that. It would appear that a three phase stator is connected for single phase use with every third coil being a dead coil (ie. coil 7-8, 1-2, 55-56, etc.). Also, the coils that are connected do not have alternating polarity individually but do have alternating polarity in pairs to give the 10 poles. I'll need some time to figure this out and will show it to my rewinder on Monday to see what he thinks.

Otherwise, the winding looks symmetrical so I do not think that is the problem unless you have some shorted coils or unless the actual connection is different than the drawing due to a mistake. If you wanted to check for shorted coils you could do a surge test comparing one circuit to the other. To find a mistake in the connection you would have disassemble the unit and physically check it. One possibility is that the L1 and L2 are swapped on one circuit. You could check that with the unit disassembled by applying DC to the circuit with each L1 as positive and using a compass or flux meter in the stator bore to determine that the poles do alternate as you pass from one circuit to the next. Otherwise, I would guess that if the voltage and frequency are correct that the winding is correct.

Air gap or rotor problems as suggested by Edison123 would be a likely candidate but it would appear that you have ruled that out. Also, catserveng may have a good point about core damage but I have never seen that cause this type of problem myself. A core loss test using a thermal camera or temperature probe would reveal this problem if it exists.

waross has an excellent point with the suggestion of checking the coupling and the alignment. My vote is for that being the problem. Of course, we are assuming that this is a separate motor and generator instead of a single unit with both being in the same frame.

Finally, Strong is correct in suggesting that posting of the vibration frequencies may get you a second opinion of what vibration analysis indicates the problem actually is.

 

[accordlx]

I'm going to try to get a copy of the vibration report to post. Also, I think we will try the drop on the rotor with a higher voltage ( the initial drop was done with 100 vac). The motor and generator are separate components joined by a coupling. We did a polarity check with a gauss meter with 200 vdc applied to the winding as well as a surge test comparing one half of the winding to the other. The stator passed the core test with and without the winding in it. We excited the core to 85 Kilo lines. No indication of "hot spots" with increased excitation.
I've been working in this field for 35 years and I have never seen this type of issue with a generator.

 

[electricpete]

I haven't worked with single phase windings, but I tried to figure it out. I have the same question as Ray / rhatcher.... there are 90 slots, but only identification of positions 1 thru 60? Does it really mean every 3rd slot is empty? I assume it does and that each box represents two coils.

We have two rings running around the outside at potential L1 and L2.

We have two rings running around the inside, let's call them A and B. A is the inner-most of the two inner rings, and B is the outermost of the two inner rings.

But what is connected to B? Only boxes (2 coils per box) that have their other end connected to A! So the current flows from A through the boxes to B and then where? It has no other path to flow but through another coil back to A , which is nonsensical. Either this winding is seriously screwed up, or I am seriously missing something.

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[edison123]

I can't figure put that winding diagram either. And I call myself a winder ? Sheesh...

Muthu

http://www.edison.co.in

 

[electricpete]

I see that the boxes connected between A and B are every 3rd box. 3 boxes corresponds to 1 pole pitch. So the induced voltages in these parallel coils would alternate. I would think this would be a recipe for serious circulating currents. Again, it is seriously messed up or I am seriously missing something. Could be the latter.

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[electricpete]

Muthu - I'm glad I'm not the only one...I'm in good company.

I was wrong about the circulating currents... comparing those boxes connected between A and B, they do have alternating flux polarity, but they also have have alternating connection polarities which cancels out to create in-phase induced voltages among the parallel boxes with the result that no current flows.

But still these boxes (2 coils per box) connected between A and B seem to have no useful purpose if there is no current flow in them.

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[waross]

Hello Muthu and Pete;
Does this make sense?
This is a three phase wye connected winding, the inner ring is the neutral which is not brought out. The next ring is L3 and is not brought out. The coils connected to L1 are reversed. Then every second group of three is reversed.
Still guessing, but the vector diagram may be a wye with the polarity of one phase reversed.
Now, looking at the group labeled 12,11,10,9,8,7. If #10 were connected to #8 instead of to #11 we would have a zig-zag winding to produce single phase from three phase.
Then every second group of three would have to be reversed.
As it is the machine appears to be shorting itself out.
Another guess, the machine may not be connected as per the possibly faulty diagram particularly if it is putting out normal voltage.

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

 

[electricpete]

There is no circulating current, but there is also no path for normal load current through these coils between A and B. So those coils would seem to be deadwood accomplishing nothing. Considering the size of the machine, that would be a lot of money spent on coils which accomplish absolutely nothing, and therefore suggests to me an error in design (would certainly like to talk to the person that designed it to ask for explanation).






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[accordlx]

The boxes on the drawing each represent a group of three coils in series having the same polarity. The numbers on the right and left sides of the boxes (1-60) represent grouping ends. I asked the engineers at the trade association to which we belong, EASA, to review the drawing and they seem to think it could possibly be a "damping" winding of some sort. All the density calcs are within acceptable limits and given the fact that the generator puts out what it is supposed to, we have to assume at this point that the connection diagram is correct.
We have the generator back in our shop now and the connections were lifted and verified to the print. Our initial thought was that the technician made an error when he connected the stator. This was not the case unfortunately. At least if an error was found we would have some hope of coming to a resolution.

 

[edison123]

What is the purpose of two inner shorting rings ?

Muthu

http://www.edison.co.in

 

[electricpete]

The boxes on the drawing each represent a group of three coils in series having the same polarity.

OK, that answers Ray's question. No unused slots.

We are left with 1/3 of coils that have no purpose other than possible damper winding.

You mentioned flux density was verified... was that at full load? And was excitation consistent with actual field circuit measurements at full load? Was just wondering whether the core might be getting pushed deep into saturation at full load in order to keep terminal voltage.

The other scenario for load related noise/vibration that comes to mind is of course spatial harmonic effects associated with stator slotting. Those would be load dependent. I guess someone could run through the equations to see if there is a low-spatial order harmonic that stands out.

fwiw - attached is marked up diagram to show coil polarities in red. It does form a symmetric 10-pole pattern with 6 coils per pole and 3 "unused" (or damper?) coils between poles.

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[electricpete]

Fwiw - the 2 inner shorting rings are what I referred to as A and B. Attached I have added labels for A and B.

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