help interpretting 12-pole generator winding configuration 3

[electricpete]

I am trying to interpret the following stator winding description for a syncronous generator stator:

KVA - 6875
Poles: 12
coils per slot: 2
Slots: 144
Connect 1 and: 4
Coils in Slot 1 and: 11
No. Circuits: 6 at 4160 Volts
Connection: 120-degrees Phase B Star: 4160
Grouping: 888 - repeated 6 times
Coils per group: 8

I know 12-poles is correct... it is a 600rpm machine (60HZ system). If I ignore the last two items, I can piece together the following:
- we have 144/(12*3) = 4 coils per pole phase group
- Connect 1 and 4: defines pole-phase group connections for first pole phase group.
- Coils is Slot 1 and 11. Reasonable fractional pitch. Full pole pitch would be 144/12=12 i.e. 1 and 13 . This gives 2 coils overlap between pole-phase groups (right?).
- With 6 circuits, that must mean that each phase has 6 parallels, two pole-phase groups in each parallel.

Now the last two items throw me a curve. Why do they say 8 coils in a group? I could understand if they said 8 coils in a circuit or parallel, but this is not normal interpretation of the word group, is it? If not then what do they mean by 8 coils in a group?

I get to look at this machine tomorrow. Any quick reply would be appreciated. Thx.

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[electricpete]

I made a comment "This would not include mmf harmonics caused by stator slotting itself". I think you have expressed disagreement.

Here is why I said it.

Energize the stator with 3-phase sinusoidal 60hz. Neglect saturation. Lock the rotor to prevent movement.

I believe the current which results must be purely sinusoidal. Why shouldn't it be? We now have a pure linear system. The phase voltages are the inputs (independent variables). The outputs (dependent variables) are fluxes and currents... these are all determined by linear operations (addition, scalar multiplication, differentian integration), and the system characteristics are not time varying. Is it agreed?

So, we have slotting, but no harmonics. My contention is that stator slotting does not cause stator current harmonics.

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[electricpete]

One more question. I have just built a spreadsheet for kd and kp. I tested results for the machine described in this post except assuming 60 degree spread.

I got results which agreed except for kp. I get the following values for kp
1.000 3.000 5.000 7.000 9.000 11.000
0.924 -0.383 -0.383 0.924 -0.924 0.383



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[electricpete]

Never mind my post on kp. I think it agrees if use coil pitch =9. Should use coil pitch=10.

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[electricpete]

I had a winder look at our generator. I asked him to explain the windings (I didn't give him any hints). He said it must be a consequent pole machine. Aolalde was 100% correct.

An interesting thing, we have 18 groups of 8. Each of those groups of 8 has both leads brough outside the machine. (36 leads attaching various points all leave the machine). The only jumpers within the machine are the series jumpers. All other interpole connections must be made external to the machine. (I don't have opportunity to look at that part of the equipment yet).

Why would they do it like that?

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[edison123]

pete,

You mentioned in your first post "connect 1 & 4". Was there such a connection between first and fourth group ? If not, then aolalde was correct.

In a 6 parallel, 120 deg spread winding like yours, all the six starts (six pole-phase groups) of one phase will go to the line terminal of that phase and all the six ends will go the neutral. It is repeated for other two phases.

 

[aolalde]

Epete & Edison123:

Please read my first post of this thread ( Jan 23) about grouping.
1-4 jumper is only a generic information that could apply if more than one group per circuit is required.

 

[electricpete]

Jumper 1 and 4 could apply equally to consequent pole or normal motor, correct? (the difference being top-to-top or bottom to bottom jumper for normal, and top-to-bottom or bottom-to-top for consequent pole).

I have inspected the stator in the shop. There are as the drawing says 18 groups of 8 coils in this 12 pole motor.

There is not a single jumper connecting any of the groups in the end-winding area. Every single one of those 18 groups has two leads brought outside of the machine through a melamine block. Quite a collection of leads circling the connection end and 36 total leads leaving the generator.

I have not had an opportunity to inspect the external device where the connections must be made.

Do you have any idea why would they bring all those leads out instead of making connections in the machine?

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[edison123]

pete,

Thx for clearing that "connect 1 to 4" mystery. While it can apply to consequent pole or normal winding, in your case given the no. of parallel ckts, that would not have been possible. All my posts about normal winding were based on this particular (mis)information.

As far the taking out all the parallel leads outside the m/c, the following are the reasons.

1. To have all the six ckts of one phase (which are distributed around the m/c periphery)in parallel would require minimum of 3 rings (for three phase leads) and 1 ring for (neutral leads)and maximum of 6 rings (i.e. 3 neutral rings) if you need differential protection. Again from these pole connecting rings, you have to run heavy cables to the terminals to carry the entire stator current. So, it is lot easier to take the pole-phase leads (which have to carry only 1/6th current and hence lot smaller in size) themselves outside the m/c, where they can be paralleled comfortably without being constrained by the machine diameter. I personally would have this arrangement only for it makes the winding inside the bore less cluttered and more easy to work with.

2. By having all the six ckts brought outside, one can test any of these individual pole-phase groups (for hipot, surge, IR, etc.) easily anytime in future and get comparative and better test results.

3. If you are keen on an elaborate differential protection, one can protect each parallel ckt, which is more sensitive to turn faults etc., than having just one CT on each end of bunched phase and neutral leads.

btw, I have worked out the harmonics for your winding for both 60 deg spread and 120 deg spread and found that other than 3rd harmonic components (which are eliminated across lines anyway), the 60 deg spread has lesser harmonics than the 120 deg spread. If you are interested, you can mail your id to eewemf@yahoo.com for my reply mail.

Anyway, I enjoyed this thread very much, thx to you and aolalde.

Kumar

 

[jbartos]

Comment on electricpete (Electrical) Jan 28, 2004 marked ///\\I understand that 3rd time harmonic currents (a different type of harmonic) are zero-sequence and cannot flow in ungrounded wye winding. That is a slightly different subject.
///The symmetrical components are resolving the fundamental waves into positive, negative and zero sequence components. The harmonics can also be resolved into positive, negative and zero sequence components. The delta winding will trap triplen harmonics. It passes non-triplen harmonics; however, no zero sequence components, fundamental or harmonics. Essentially, the zero sequence components are essentially propagated over the ground paths. The delta winding does not have any connection to the ground.\\\

 

[jbartos]

Question to electricpete (Electrical) Jan 28, 2004
Please, may I have the following clarified?
""harmonic = Dpg/Dk = [120/p] / [720/(p*k)] = k/6 = k*360/6 = k*60 degrees expressed on kth spatial harmonic basis""