Conversion of single layer winding to double layer 2

[edison123]

I have on my hands a 40 year old, 11 KV, 12 pole slow speed hydro generator which has a single layer winding with the old fashioned hair pin type stator coils. The slots (20 mm wide) are fully open and the air gap is only 5 mm.

If I change this to a double layer winding (with diamond coils), will there be any problems ? I remember reading in a text book, that with small air gaps and wide slots, a single layer winding is better. Does any one have experience/idea/thoughts on this ?

 

[aolalde]

Edison123:
I have redesigned to diamond type windings for similar generators; we call them “Chain Type Winding” due to the typical appearance.
If you analyze the electrical position of each turn into the slots, an equivalent winding can be developed.
Due to the elevated number of turns (even with the diamond construction) I spliced the diamond coil in two loops with half the turns each, spread and then insulate each loop with two layers half lap of B-stage mica tape and hot press independently. The two halves are then matched and series connected in the knuckle (brazed). The resultant coil is ground insulated for the line to ground voltage. The winding time was reduced in a dramatic way.

 

[UKpete]

Perhaps it is a comment along the following lines that you recall, this from M G Say "Alternating Current Machines":

"In general it may be said that modern practice favours the double-layer winding except where the slot-openings would be large compared with the length of the air-gap..."
What he goes on to say is that in the exceptional cases referred to, the alternative is to use single layer hairpin coils inserted into narrowed i.e. semi-closed slots from the end of the core.

As you have fully open slots, this doesn't apply in your case. Even though you do have a small airgap relative to the slot opening, there is no reason obvious to me why you a double layer winding wouldn't do the same job, and save yourself a lot of trouble with the endwindings.

I would guess you propose to use fully-pitched coils, so that at least the two coil sides in each slot are from the same phase, to ease your insulation problems.

 

[edison123]

aolalde,

Thx. The old single layer winding has 11 turns/coil with single ckt wye. I plan to use 11 turns/coil double layer diamond coils with two ckt wye. When you say split the coil, is it along the slot depth or along or the slot width ? And why the split ?

UKpete,

Thx. I see you are also the EE Bible man. That particular sentence (very sneaky of him to slip it in as if an afterthought) is bit confusing. My understanding is with wide slot mouth and small air gap, single layer is better. Or am I wrong ? I plan to use 7/9 pitch (as in original single layer) to reduce harmonics.

Would magnetic wedges help if I go in for double layer ?

 

[UKpete]

Yes I rely on bibles! and Say is a favourite of mine. My reading of it is that at the machine design stage if the engineer is nervous about the slot opening being wide compared to the airgap, he could consider a semi-closed slot which naturally means a single layer winding with hairpin coils (lower power mush-wound machines aside).

But the person who designed your machine didn't seem that concerned and stuck with an open slot but used a single layer coil for some probably spurious reason e.g. that was what they were used to at that particular factory.

On the other matters I would ask aolalde, but for what it's worth I would consider staying with the 7/9 pitch and making sure the inter-phase insulation is up to the job. Aren't double layer coils always laid top and bottom in slots, never side by side? The conductors are always aligned wide side across the slot, to reduce eddy currents in the copper particularly near the slot opening where the leakage flux is highest.

Also, on the subject of magnetic wedges (and I'm also consulting one of my other bibles here - D G Searle, European Electric Motors), the problem with open slots is that there is obviously heavy magnetic saturation at the pole tips together with pole face losses on the rotor as a result of the flux distortion. Magnetic slot wedges reduce the problem but are subject to oscillatory forces so extra measures are required to stop them moving (particularly in vertical machines). According to Searle they are now accepted practice on large machines because they can give up to 0.5% efficiency improvement e.g. the rotor excitation can be lower with magnetic wedges fitted. There is no mention of any eddy current problems in the wedges themselves. Your machine has been working without them but they may help reduce losses.

 

[edison123]

Thx UK pete.

The reason I am bit concerned about this design change is that another identical m/c failed a few years ago and the rewind was done by ABB and they just copied the original single layer wdg. I am wondering why they didn't do a double layer, which is much more easier to produce.

I am sticking with 7/9 pitch. Yes, in a double layer, the coils in two vertical planes. Interphase insulation is never a problem since you have twice the ground insulation between the phase conductors.

I fully agree with the use of magnetic wedges since I had a bad experience doing a stator rewind without mag wedges in a m/c which originally had them. The healthy pole windings got cooked (you have to see it beleive it), burnt my fingers along with the pole coils and learnt a costly and painful lesson. I just don't want another one with this.

 

[aolalde]

Edison123:
a- I split that coil both ways radial and wide because the original coil had 42 turns. A tuff problem to insulate turn to turn at the end of each layer for 13.8 kV machine. Yours with 11 turns does not seem to have the same potential problem.
b- If the original design worked without magnetic wedges it should work if the winding you provide produces a reasonable equivalent flux distribution and intensity.
c- Magnetic wedges will improve the performance but create a weak element in the stator. Some disintegrate due to vibration. Only if the flux densities are very high and an improvement is required then take the extra challenge of designing with them.
d- Consider corona shield and gradient protections in the coil slot section to prevent corona erosion.
e- If you post the complete lamination core data and original winding I could give you my suggestion of lap winding for your evaluation. The only problem is that it could take me a few days, are you in a hurry?

 

[UKpete]

Possibly there were a couple of reasons for them replacing like-for-like. Firstly the double layer winding requires extra insulation in the slot i.e. a separator between coil sides, occupying space otherwise available for copper. Secondly, there is a view that the end-turns on the double layer winding are closer together than single layer and therefore need better insulation. With modern thinner insulation materials I don't suppose it is a problem. Plus of course you do have twice as many coils to tape. That's three reasons.

Anyway, there are others out there who have direct experience who can help. We would of course be interested to know how it turns out.

 

[edison123]

Thx aolalde. The m/c details are

Voltage - 11 KV Poles - 12 Speed - 500 RPM Frequency - 50 Hz

Slots - 108 Stator core ID /Length - 1450 / 800 mm Rotor OD - 1440 mm Air gap - 5 mm Slot width - 20.1 mm Teeth width - 22 mm app Slot depth - 71 mm below edge

No. of turns per coil - 11 Winding type - Single layer chain Winding connection - Single Circuit Star Winding pitch - 1-8 Airgap flux density - 0.54 Tesla


Conversion to double layer diamond type lap winding - No. of turns per coil - 11 Winding connection - 2 circuit Star Winding Pitch - 1-8

I will appreciate your opinions.



Thx UKpete. I agree with your points. Still, making of single layer coils is a challenge.

 

[aolalde]

Edison123:

You did not provided; kVA rating, full load current, temperature rise, back iron depth,
Width and number of vent ducts, Total Slot depth, Conductors wire size, strand insulation and array. Is this generator around 2000 kVA?

I made some preliminary calculations with the data provided and I have the following comments:

The original winding with 54 coils should have 3 coils per group for 12 poles. You reported span 1-8 for all the coils, but 1-8 spans for the inner coil and 1-12 for the outer coil makes sense in order to layout the coils for the three phases. Am I interpreting something wrong?
With the above array assumed I calculated the AIRGAP flux density at 0.7785 T (50.225kL/sqin).

An equivalent lap winding (108 coils) with 11 turns and span 1-9 will require 51 kL/sqin in the air gap.
An equivalent lap winding (108 coils) with 12 turns and span 1-8 will require 49 kL/sqin.

If the insulation and current densities allow it, I will use 12 turns 1-8 span, with 97.6% of the original flux densities and no magnetic wedges.

Remember to provide a corona suppression system with gradient tape or paint at the straight section ends.

 

[edison123]

aolalde,

KVA - 3055 FLC - 161 A PF - 0.9 Temp Rise - Class B 120 deg C Back iron depth - Don't know yet Ventilation ducts - none Slot depth - 74/80 Conductor - 2 of 12 x 2 mm Turn insulation - Don't know yet

The original wdg has 54 coils (all with a pitch of 1-8) with coil groups of 2,1,2,1,2,1,2,1,2,1 per phase. All these coils are identical in shape.

Interestingly, I got flux density as 0.54 T. Where am I going wrong ?

Anti corona system (tapes) will be used.

And thanks again for your time.

 

[aolalde]

Edison123:

I reviewed my numbers base in your new information.

The air-gap flux density for the original winding with 54 coils, 11 turns, span 1-8, 1 wye, 11kV, 50 Hz is 51.314 kL/sqin which are equal to 0.795 Tesla.

The lap winding closer to be equivalent is:
108 coils, 3 coils per group, 2 circuits wye, 11 turns per coil, span 1-8, (2) two wires 2mm x 6mm side by side Double glass over film insulation.
The new air-gap flux density is 53.5 kL/sqin (0.829 T).

I hope this will serve as a reference for your rewinding project.

 

[mc5w]

With modern insulation systems you might also be able to use 1 wire size larger than original which would make the generator run cooler leading to increased life of the insulation between the laminations.

You might want to do a nominal check of the insulation between the laminations. I had an instance of a 10 HP induction motor that had been rewound a few times and seemed to draw excess starting current possibly because of higher eddy currents. Some of your rewinding issues are not any different than what I experienced with industrial motors.

Using higher temperature rated wire insulation will help to some extent but you cannot run the laminations any hotter than before unless you pull them apart, clean them, and stick in modern varnish.

 

[aolalde]

Hello mc5w:

High voltage windings ( coils, conductors, insulation, transpositions, corona protection, etc) are quite different as compared with low voltage random windings.

 

[mc5w]

I am just trying to say that the generator should be examined for hidden damage. It would be a shame to rewind this thing and have the core overheat because the lamination insulation had seen better days.

 

[aolalde]

mc5w:

Your concern is very important. Quality repair shops perform core loss tests before and after stripping (removing) the damaged windings. The core lamination losses at operating or standard flux density are evaluated and shorted laminations or hot spots are scanned.
These tests can be achieved forming a toroid with the back iron and an insulated cable coil around it. AC current is applied until the required flux density is reached, the power loss is measured and the watt/kg loss is evaluated. Test based on this principle are “ELCID” (Electromagnetic Core Imperfection Detection) and “Loop Core Test”.