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?

 

[electricpete]

Regarding damper windings - they are typically on the rotor to my understanding. On the rotor, the function is simple to understand. On the stator, I have a hard time understanding how you would make a damper winding work. If you take a single group and short it, then you have shorted/circulating currents in normal steady state operation. If you connect coils in parallel with same polarity as is done here, then you do away with circulating currents during normal operation, but I don't see how any current can flow in transient either. Let's say rotor begins to speed up. The voltage induced in the parallel coils remains in phase with each other, and no current flows and no damping action occurs.

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[waross]

Okay, I understand the polarity. One group is under the north pole while the other is under the south pole. Hence the reversal of connections.
This is wye connected. The inner ring "A" is the neutral. Power is taken from "A" Phase and "B" Phase. The inner ring "B" is "C" Phase.
This is the "POOR Man's" single phase conversion of a three phase alternator. Simply take single phase from two phases and forget about phase angles. I see this most often at 120/208 Volts. In this instance the center tap is not needed so phase angles are not an issue.
The only issue would be 15% higher excitation (Flux actually) to develop the same voltage. In North America it could be described as pushing a 208 Volt winding up to 240 volts.
If saturation is a cause of the noise it should only be apparent at full excitation.
Is there noise at full voltage but no load?
Are there any shiny spots in the coupling?

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

 

[electricpete]

If saturation is a cause of the noise it should only be apparent at full excitation.

If the machine is auto-regulating the output voltage, then excitation and airgap flux density increase with load. That was the reason for my question.

This is wye connected. The inner ring "A" is the neutral. Power is taken from "A" Phase and "B" Phase. The inner ring "B" is "C" Phase.
This is the "POOR Man's" single phase conversion of a three phase alternator. Simply take single phase from two phases and forget about phase angles.

Good point. I agree if you hook up point B to a (non-existent) 3rd phase, it would be a valid 3-phase winding. That is a bizarre simplistic approach to convert to single phase!

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[electricpete]

Bill - Sorry I misunderstood your comment about saturation as response to something I was saying. But now I see you were just talking about a different angle whether this conversion approach would be expected to cause saturation.

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[rhatcher]

Wow, there has been a lot of discussion here since last night. Anyway, I am comfortable that the winding connection as drawn is correct. As I said before, it is symmetrical with 10 active poles of opposite polarity (to each other) and 10 sets of dead coils between them. The dead coils do have opposite polarity (to each other) but I cannot determine the polarity of the active poles compared to the dead sets of coils. I do not know if it matters since it is symmetrical. If it did matter, I suspect it would be a rotation specific problem (CW vs. CCW). Also, I am pretty confident that the winder did not make a mistake in taking the data that led to a drawing of such an unusual but also very symmetrical winding.

I have talked to the engineers at EASA myself when I needed help and they are a smart bunch. Their idea that the dead coils may act as a damper winding is interesting. The coils will have voltage induced in them by the rotor and will produce currents that circulate through the inner two rings. However, I have never heard of the concept of a damper winding on the stator, it is usually considered to be on the rotor. But, of course, I've never seen this type of winding before.

For edison123, the innermost ring is the wye point for the active winding. This is obviously a two wye connection. The next ring in from the center is the connection point for the dead (damper) coils. This ring and the wye ring is the path where the currents circulate between the dead (damper) coils.

For electricpete, the flux density calculations accordlx refers to are theoretical and are based on the winding design (# turns, pitch, connection, etc.), the stator core dimensions (slot depth and width, bore diameter, back iron depth, etc.), and the full load current. Of course, I am not sure how to perform those calcuations on this unusual winding.

For accordlx, here are some things that have stumped us in the past over similar problems.

Have you checked to be sure that none of the rotor poles are loose on the rotor? Have you checked to be sure that the stator core is not loose in the frame? Is it possible that the stator core shifted away from center of the frame in the burnout oven (core burned while standing end up)? Is it possible that the rotor shaft is broken inside of the rotor core? Are the bearing housing and journal fits good? If they are sleeve bearings, are the saddle fits good? Are the rabbit fits good for the end brackets and are the end brackets square to the frame (no angular misalignment)? Finally, of course, the coupling and the alignment are suspect until proven otherwise. Make sure that these things are good and make sure for sleeve bearings that the alignment is made to the magnetic centers of the motor and generator. As I'm sure you probably know, to determine the magnetic center of the generator simply run it as a motor and mark the shaft.

Another idea that comes to mind. Are you sure that the problem is not with the drive motor??? Once mounted on a common base and coupled together, it may be hard to distinguish the source of the noise and the vibration between the drive motor and the generator.


Of course, you should still try to post the vibration spectrum making sure to get axial and radial readings on both the generator and drive motor. I am sure that someone here could use that to help to determine whether you are actually looking at an electrical problem or a mechanical problem, a generator problem or a drive motor problem. Unfortunately, I am an amatuer at vibration analysis but I'm sure that there are others here that are better versed at it.

 

[waross]

It is becoming apparent that the unused windings are not damper windings. They are an unused third phase winding.
I have seen 120:208V three phase generators used to supply single phase loads by simply connecting the load to two phases and the neutral. The third phase is unused. That is what we have here.
My comments re saturation, if the third phase was used in a zig-zag connection less excitation would be needed.

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

 

[electricpete]

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

Based on my misunderstanding of Ray's comment...I projected my confusion about why numbering only goes to 60 (instead of 90) onto Ray. But that was not his point.

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.)

The same conclusion the rest of us reached.... much later in the thread.

My apologies Ray.

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[electricpete]

It is becoming apparent that the unused windings are not damper windings. They are an unused third phase winding.

I agree with Bill on that point.
1 - These coils won't work as damper windings since no current will flow in them even if the rotor phase angle fluctutates.
2 - The pattern matches a 3-phase machine (with one input phase unconnected).

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[electricpete]

Attached is a simple spatial harmonics analysis of the mmf created by the stator current alone (equivalent to analysis that is typically done using harmonic pitch and distribution factors). The results reveal no blatant problems, actually pretty low harmonics:
Here are the spatial harmonics expressed as fraction of the fundamental:
A3 := .5578486011e-9
A5 := .9491375323e-2
A7 := .2782589722e-1
A9 := .1241002310e-8
A11, A13, A15 and A17 are a mirror image of A9, A7, A5, A3, as expected for 18-slot fundamental pitch.

There can certainly be other tricky harmonic-related stuff going on (for example when we consider rotor as well... or low-magnitude harmonic happens to match a core modeshape which is resonant at 2*line frequency), but what is attached is the extent of my knowledge/ability for evaluation of spatial harmonics, and it doesn't suggest any obvious reason for a spatial-harmonic-related problem based on the stator winding configuration.

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[electricpete]

Actually, maybe I need to use integrals instead of sums. I'll have to come back to that. Maybe tonight... other stuff to do.

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[waross]

Any polish marks in the coupling?

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

 

[electricpete]

Attached is revised verision which corrects the summation to integration. Results still show spatial harmonics are very low:
A1 := .9727345828 [this has no significance other than as basis for normalizing the others]
A3 := .02122065950
A5 := .1552431963e-1
A7 := .2236000840e-1
A9 := .1414710605e-1

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[electricpete]

The total plot looked a little strange due to a plotting anomaly (the program decides how often to pick points for plotting...didn't pick enough). Those plots are corrected in attached rev 3 (no changes to numerical results). Now I am truly done.

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[electricpete]

There was one small error - when I changed from sum to integration, I should have changed the bounds from (0..89) to (0..90). I corrected this and all spatial harmonics remained below approx 2% of fundamental.

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[rhatcher]

waross has made an excellent point at 12:15 that posted on the thread while I was away typing my 12:59 post. The 'B' ring is definitely the point at which the third phase would connect if there was one. I should have seen that but I didn't. I even described it; "the innermost ring is the wye point for the active winding. This is obviously a two wye connection. The next ring in from the center is the connection point for the dead (damper) coils." It was right in front of my face the whole time (Duh!...smacking forehead now). Good observation waross. A Star for you (if it will let me give you another in the same thread).

In my 12:59 post I said: "The dead coils do have opposite polarity (to each other) but I cannot determine the polarity of the active poles compared to the dead sets of coils." Now, considering the 'B' ring as the missing third 'C' phase, I can say that the the polarities of the dead coils do alternate with the live coils in the normal three phase arrangement of N-S-N.

That being said, we are going to generate voltages on the dead poles (coils) because they are exposed to the rotor's magnetic field just like the live ones. But, as electricpete pointed out much earlier in the thread, there should be no circulating currents. Of course, that is obvious to the rest of us now that we correctly see the dead coils as the 'C' phase with no external lead. There will be no circulating current between them any more that there will be circulating currents between the poles of the 'A' or 'B' phases (assuming that the rotor does provide a balanced and symetrical rotating magnetic field). It is also obvious now that they provide no dampening effect any more than the other two phases do. It is great how waross's observation of the 'B' ring being the 'C' phase lead simplifies everything.

I do still stand by my comment that the EASA engineers are a smart bunch of guys but, obviously, none of us is perfect.

Electricpete, about your 13:08 post; no apologies necessary. I probably helped your confusion by being lazy and saying "every third coil" instead of the proper term of "every third coil group." To clarify this for others, 60 coil groups with 3 coils per group equals 180 coils. Two coils sides per slot equals 90 slots.

So, all that is left for me is to wonder whether accordlx has found the problem. I sympathize with the poor guy since I have also spent many weekends at my shop stumped by similar problems. I hope he finds it and I hope that he lets us know what the problem was for future reference.

 

[rhatcher]

Pete,

Do you remember seeing in the news about 6 months or a year ago where an anthropologist published some photos allegedly taken from a low flying plane that showed the settlement of a primitive amazonian tribe deep in the jungles of the rainforest? There were some natives pointlessly shooting arrows or throwing spears at the plane and there were some that were just standing there looking up with their arms at their sides and their mouths hanging open in the typical " OMG, what the hell is that" pose. Well, right now I feel like one of those natives standing there stunned and wondering "OMG, what the hell is that" upon seeing my first airplane.

I know that you are wondering what I am talking about. Well, here it is. I just got around to looking at your harmonic analysis and I am standing here stunned and wondering what the hell I am looking at like the native who just saw his first airplane. I think the last time this happened was during a discussion about odd groups in motors when you whipped up an Excel spread sheet calculating phase balance coil by coil, group by group. I did eventually get a handle on your spreadsheet but I am still clueless to how you came up with it. Now there is the harmonic analysis worksheet and I look at it and it might as well be in Chinese. I am guessing that you did include the spatial harmonic effect of the dead phase since that is one of the points we are talking about but if you did, I don't know how and I can't see it because what you did is Chinese to me.

Maybe one day you can tell me what I am looking at. Until then, just let me say good job Pete. You never cease to amaze me.

Ray

 

[electricpete]

Ray - Thanks for the kind and colorful compliment. We all come to the forum with something different/unique to contribute. I'd like to think we all learn from each other...I know for sure I learn a lot from you, Bill,Muthu and others.

If you will forgive me talking a little bit more about these spatial harmonics...

I attached here a final version that makes the final correction that I discussed before, and also adjusts the plotting range 0..90 so you can eyeball verify that the mmf waveforms are in fact periodic with a period of 90 slots.

The math I think is straightforward, but the reason it starts to look Chinese is more a result of the "Maple" programming language. That is a tool that makes it easier to do the math, but harder for someone else to figure out what the heck you did. But I think the graphs tell the story.

The approach that I used takes advantage of a unique aspect of single-phase winding (compared to 3-phase).....the single-phase winding creates a "standing wave" flux pattern around the airgap... it doesn't rotate or travel like a 3-phase winding. So all we have to do for a single-phase winding is look at the mmf at a snapshot in time, and we know the shape for all time (which is the basis for the spatial harmonic analysis). In contrast, we couldn't do it like this for a 3-phase winding where the shape changes over time (the slot boundaries are stationary, but the fundamental moves).

Each conductor creates a step in the mmf.

If you look the graph of mmf_profile1, you can recognize the pattern 6 slots stepping up, 3 steps constant and 6 steps down. The flat spot on top/bottom of the sinusoid is the result of those 3 unused coils.

mmf_profile1 only represents one half of each coil (let's say the half that falls in bottom of a slot). To get the other half, we create mmf_profile2 which is the same as mmf_profile1, but inverted (current flows in opposite direction in the other half of the coil), and shifted by 7 slots (coil pitch 1-8).

We add them together to get the total mmf – I personally was expecting that it would be very choppy with spatial harmonics at 3rd, 5th 7th which might have contributed to noise (a cause/effect which would have been worthy of its own discussion). But in fact the short coil pitch did a great job of smoothing out the flat spot and creating a sinusoidal mmf (look how much more sinusoidal the graph of the total looks than the two graphs before it).

The final results is a fourier transform of the total stator mmf waveform (with position around airgap circumference as the independent variable)
A1 := .9945006841
A3 := .4868941122 e-9
A5 := .008331740666
A7 := .02140225834
A9 := .1118970159 e-9

An interesting thing I would pause to observe is that the 3rd and 9th harmonics are virtually zero. I don't know why but I have to think that is not just a coincidence but instead somehow a result of the symmetries present in the winding (the number 3 shows up in many ways in this winding).

While it's not exactly the same thing, it reminds me of a vaguely similar mysterious thumbule about eliminating 3rd harmonic based on slot size.

Here, we apply the well-known finding that if two-thirds of the rotor surface are slotted and one-third is left slotless, not only the third harmonic component but any of its multiple harmonics called triplen harmonics are eliminated in the air-gap magnetic flux density, and also the low-order odd harmonics (fifth, seventh) are suppressed

http://books.google.com/books?id=_y...onics (fifth, seventh) are suppressed&f=false

I have the feeling maybe I have beat this aspect of spatial harmonics to death. I'll move on unless someone else has comments.

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[edison123]

pete - I have had to speak to you earlier about your math problem, IIRR.

Reg that winding diagram, let me see if I can get my mind around it today, Ray. I am convinced it's an electrical problem and not a mechanical one like misalignment, loose poles etc. But loose stator core is another probability.

Muthu

http://www.edison.co.in

 

[waross]

Pete;
I can't follow your math. That is my problem not yours. Short term memory is shot to "What were we just talking about??"
There is a point that concerns me when you mention single phase windings.
This is a combination of two of three phase windings. If each winding has a flat spot, those flat spots may manifest themselves as hard to spot deviations from a true sine wave centered at about 90 deg and 270 deg of the resultant wave form.
Put another way, we are taking a line to line voltage similar to taking 208 Volts from a 120:208 Volt system. This would be a 139:240 Volt system.
Question, does the noise sound like a 60 Hz hum or is it lower pitched?
It is hard to imagine (but if I'm wrong I expect a quick education) any electrical noise at less than the well known 120 Hz. or a multiple. I would first look for a mechanical issue if the noise is pitched lower than 120 Hz.
Electrical noise from the alternator would be expected to be at 200 Hz.
Here's me cringing in case I have made a mistake here.


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

 

[electricpete]

Bill – It was surprising to me, but what my "analysis" suggests is that this configuration actually does a very good job at approximating a sinusoidal mmf distribution around the airgap, with only small deviations (and it is of course a standing wave rather than rotating wave). The phase angles (in time) of the original 3-phase winding are left behind.... all that remains is the position and connection of the coils which is what was analyzed.

I agree with you the lowest frequency for consideration of an electrical problem would be 2*electrical frequency. (In this case, that would be 2*100=200hz for the generator).

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