Adjacent poles vs Alternate Poles in Series 5

[Sargardani]

Hello all

For hydroelectric generator's stator winding:

I need the advantages and disadvantages of stator winding having adjacent poles as compared to winding arrangement with alternate poles. All help is highly appreciated. Thank you.


Sarg

 

[Sargardani]

I am still looking for input on this issue from the members of the forum. I appreciate all help and support.

Sarg

 

[edison123]

For two parallel ckts, though the circuits are theoretically supposed to carry halve the line current, due to air gap & permeability variations around the bore, these currents will differ slightly in reality. So, in a two ckt winding, alternate pole connection gives a better flux distibution (rather AT) than adjacent pole connection and reduces the vibrations caused by uneven airgaps. If you draw phase-pole groups around a circle, you will find the same current flows in opposite pole groups in alternate pole connection thus creating equal AT (amp-turns) force. If you use adjacent pole connection, then the currents in the first half is likely to be different from the other half leading to uneven magnetic pull and vibration.

Alternate coil jumpers are of course longer than adjacent ones.

In single series ckts, since the same current flows in all pole groups, adjacent pole conn will suffice.

btw, you could show your real appreciaion by 'starring" the posts. jus kiddin.

 

[electricpete]

I'll give you a star kumar (edison) because I always learn from you. (No kidding).

I am following on and trying to understand edison's thought process. Just thinking out loud.

If everything in the machine were perfectly symmetrical, then same exciting current in all circuits, no difference whether alternate poles from same circuit or difference circuit.

Now introduce a variation in air-gap, let's say minimum airgap at the top and max airgap at the bottom.

For groups near the minimum airgap, the reluctance goes down, magnetizing inductance goes up, would tend to make exciting current go down compared to parallel circuits, would tend to minimize the increase in unbalanced magnetic pull at that point of minimum airgap (good), but also results in unbalanced currents and as kumar says (bad).

If physically adjacent poles are in the same circuit (in series), and they both near the lowest airgap at top, they both see this effect and the effect which increases current in that circuit minimizes unbalanced magnetic pull (good) and increases current imbalance between circuits (bad) is more pronounced.

So I see a good effect and a bad effect of pairs of adjacent poles in the same circuit (in series). Now a question: Why do we have multiple circuits (parallels) in a given phase? My thought was that it was to minimize unbalanced magnetic pull (Is there any other reason?). If not multiple circuits then there is no possibility of uneven amp-turns. So, to my thinking, the objective we were seeking by providing multiple parallel circuits (minimizing ump) is better met with adjacent in series.

Let's go back to the uneven exciting amp-turns. Can you explain more why that is a problem?

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[electricpete]

I think uneven amps would only show itself as uneven heating but since exciting current is only a portion of the total current and since it is only a small change in exciting current, that part doesn't seem like a problem.

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[kh2]

I could not find any technical explaination for the preference of adjacent poles (short jumpers 1-4) over alternate poles (long jumpers 1-7)in a 2 parallel circuits 3 phase 13.8 kV lap wye connected hydro generators (18 poles and more). There are machines in-service with short jumpers and others with long jumpers. Is there a criteria used to select one connection over the other besed on cost, performance, size, number of ples, voltage, or other factors? One factor discussed is the air gap stability analysis which assesses the magnetic attraction of the rotor and the stator and their individual mechanical stiffness. It was found that once the air gap is no longer uniform there will be higher magnetic attraction between the rotor and the stator on one part of the machine than the other. To be stable at operating conditions, the mechanical stiffness of the rotor and the stator must be greater than the magnetic attraction between them. The question raised is which connection (short jumpers or long jumpers) is more stable in an event of an airgap eccentricity, say 10% and why? And is this a factor on prefering one connection over the other?

Appreciate your input on this interesting subject. Many thanks, kh2.

 

[kh2]

Appreciate very much the feedback on my July 6th question on why some machines (for example two parallel circuits per phase) are connected adjacent poles in series and others connected alternate poles in series. Is there a criteria (technical and economic reasons) on which the selection is made? Can Edison123 and electricpete and the other members of the forum help answer the question. Many thanks. kh2

 

[QCE]

I wish to give everyone a star! I hope I can understand this topic better by rereading this post over again and again. Does anyone know a text on this that is not so heavy (not in weight but in lingo)?

Electricpete,

You asked if there were other reasons:
Why do we have multiple circuits (parallels) in a given phase?

My first thought was so that if you have a coil failure you can bypass the path.

I also thought that the "parallels" would reduce the winding resistance instead of them being in series.

 

[electricpete]

Thanks QCE, that is a good point. Now that I think about it, satisfying the correct volts per turn may be a fundamental reason requiring multiple parallel circuits.

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[edison123]

The main reason for having parallel ckts is to split the large currents (in big generators and motors) so that winding copper does not become unwieldy.

Other reason in motors - using a standard machine frame for different voltage/power/speed ratings may result in non-integer turns per coil. e.g 2.5 turns/coil in single ckt. So a two ckt, 5 turns/coil will be used here.

Bypassing one ckt is not an option available in all cases. In a 2 ckt winding, it is impossible. In multi-pole, multi-ckt winding, it may be possible but is done only in rare circumstances. Definitely, no winding designer designs multi-ckt windings with the idea of bypassing them in future.


kh2,

As mentioned in my earlier post, alternate pole connection in a two ckt winding will result in better amp-turns distribution around the machine with lesser ump (uneven magnetic pull) and vibrations connected with it. Good designers will always use alternate pole connection in 2 ckt winding. As I said, try to draw the pole-phase groups in a circle and you can see clearly the AT distribution (especially in the 180 deg opposite pole groups)

Of course all this discussion is about lap wound machines. Wave wound machines (very common in non-US generators) are self connecting and need normally only three jumpers regardless of no. of poles.

 

[QCE]

We are talking about a hydro unit here so we probably have hundreds of coils and many paralleel pathes (not necessarily). I have seen many units with bypassed coils. I'm sure that is not the reason for the pathes but it is sure handy.

Maybe we should ask Sargardani how many circuits he is talking about? It maybe dif for 2 circuits and 8 circuits?

 

[QCE]

I don't want to highjack this thread but:

I have a question and it might be a dumb one but doesn't the jumper just depend on the sequence of the winding?

 

[kh2]

QCE,
I am answering your question in a general way. The jumper is the connection between the groups from the terminal or line point to the neutral point of each phase. Consider the circuit ring as a jumper too. For example, a 3-phase 22 poles machine has 3 x 22 = 66 groups. In case the stator has 264 slots, therefore 264/66 = 4 slots/group. In a 3-phase connection, group 1 is phase A, group 2 is phase B, and group C is phase C, and group 4 is phase A again and so on. Connection of group 1 to gruop 4 by a jumper is connecting phase A to phase A and connection 2 to 5 is phase B to phase B and so on. Also, if group 1 is North pole, group 2 is South pole, group 3 is North, and group 4 is South. Therfore, connection of 1 to 4 is North to South, and connection of 1 to 7 is North to North. That is why 1 to 4 is called adjacent pole in series (N to S) and 1 to 7 is called alternate poles in series ( N to N). Also, 1 to 4 is a short jumper and 1 to 7 is a long jumper. Hope this will help.

edison 123,

Thank you for the valuable information on this topic. Another advantage atributed to the alternate poles in series for two parallel circuits is that it will reduce the unbalance causing high mechanical forces if a fault occurs. Also, will eliminate the need for long neutral circuit rings.
You mentioned the magnetic pull (stiffness) and as you know, the number of parallel circuits is inversley proporional to the per unit magnetic pull. The fewer parallel circuits per phase the higher per unit the magnetic pull. Less magnetic pull is better because it results in less magnetic attraction. For example, six circuits configuration is lower in magnetic pull that four circuits and one circuit is the highest. Some designers claim that adjacent series pole stator winding has better magnetic stiffness in an event of uneven airgap. The explaination is parallel winding concentration (1/2 of stator in two parallel circuits, 1/3 if three parallel circuits, and so on). Take the case of two parallel circuits, if the airgap gets short on one side of the stator, the magnetic flux density in the air gap will be increased on this side. That will increase the generated voltage in that circuit. But since the other circuit is in parallel, its voltage will also be increased, which will increase the magnetic flux density on the other side of the stator and this will have a centering effect for the rotor in the stator. This effect is not found in the alternate pole where the two parallel circuits are blended together around the stator unlike the adjacent poles where half the stator is one circuit and the other half is the other circuit.

The question still remains if one connection is better than the other why we still manufacure both. Is there a criteria used to select one connection over the other?

Thanks again to all participants specially to Sarg who initiated the discussion and to electricpete for his sence of humor and helpful feedback.

 

[kh2]

I will try to answer my question based on the pieces of information gathered from this forum and other sources. The choice between adjacent pole or alternate pole in series connection on parallel circuits on a lap wound machines is a matter of industry preference due to cost and experience. This is similar to other choices such as between lap and wave, Roebel bars and multi-turn coils. For example, installtion of 1-4 jumpers requires less effort and make the access to the stator winding easier during maintenance and repair work. Some prefer 1-7 jumpers because of the reasons that edison123 has indicated on his comments.

Please comment on the accuracy of the above statement and of your experience on the subject. Many thanks