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Discussion Starter · #1 ·
So...the stator that has been behaving suspiciously for a year or more, but I never managed to actually catch in the act (with resistance measurements when hot and cold), finally went up in smoke.

I could feel the heat from the side cover at higher revs when riding along and it was needing higher and higher revs to get a proper charging voltage as it self-destructed. Trip aborted one hour in and trundled back home slowly on battery power.

This final complete meltdown occurred since I fitted a series Reg/Rec not so long ago. My money's on the higher voltages at higher revs that occur with a series type regulator having drilled through already compromised stator insulation.

I was curious to see how/where the stator had failed and if it was reasonably viable for a DIY repair so thought I'd kick off a thread on it.

The damage: 4 cooked coils, positions 2, 14, 15 & 18 in the pic, and now the meter says it really is short-circuiting to ground.

burnt stator annotated.jpeg


inner side coil connections.jpg


I had been hoping for an obvious single point of failure where I might be able to do a localised lash-up repair, but given the more widespread signs of damage I decided to bite the bullet and strip all the windings off.

heat damage under windings annotated.jpg


After degreasing, removing the copper wire and initial cleaning up with acetone and poking flaking bits off with a scalpel: No breaks found in the coil windings but insulation burnt through with main short to ground occurring at position 15 in the pic above. Although not obvious in this picture, position 18 is also cracking and the insulation is about to fall off, probably also shorting to ground to some extent. The other 2 bad poles are just a bit cooked.

The heat damage (and insulation breakdown) is primarily around the corners/short end of the pole pieces not the long flat sides. Bending the wire round the corners stretches the insulation on the outside of the curve, the copper itself can be stressed slightly from the original bending and then in operation the process of years of slightly irregular heating and cooling means that as a straight section of wire lengthens/shortens as it heats/cools then unless the whole coil has expanded/contracted exactly proportionally at all times (fat chance) then the corners will keep getting flexed back and forth slightly. This can damage insulation and also create points of higher resistance which then become hotspots. My guess is that this is probably - the most? - common failure process. Once a coil starts to leak it unbalances the currents in the stator which may increase thermal stress on other weak spots leading to a snowballing problem, or fireballing perhaps. The above pic shows that all 3 phases ended up with cooking coils.

The quite carbonised-looking but not actually visibly broken bits of insulation showed no detectable conductivity on their surface (>4000Meg) which surprised me a bit as they looked terrible - though this was only a low-voltage resistance test.

Decided to give DIY repair a go, though obviously now it has to be a complete rewind...

Apart from limited budget, and curiosity, an additional reason for not getting an aftermarket product or getting this one rewound was that, although they look a lot smarter than this one probably will by the time I've finished with it, at least I'll have a better idea whether or not I actually have good insulation where I want it!

(Whilst looking into this, thanks to google tracking me I've had my DIY resolve tested by being bombarded with ads for stator replacements claiming to be suitable for this bike model - eg. some Chinese offering at Ali Express offer prices being about the same price as I've paid for just the materials! )

So after a bit of pencil chewing and weighing up materials options I came up with the following basic plan. The aim being to repair damage and then improve the insulation around the ends/corners of the coils and to lower the wiring resistance a little to offset some magnetic inefficiency I'll be introducing (as well as reduce resistive heating in the coils at higher current outputs).
  • Scrape off carbonised bits
  • Patch up holes with RTV silicon
  • Reinforce the end/corner areas with more silicon and possibly an overwrap of fibreglass tape
  • Possibly/alternatively make use of kapton tape as a first and/or inter-layer addition
  • Rewind with slightly larger dia copper
  • Reinforce with winding lacquer
Materials - total cost came to about £40.
 

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Discussion Starter · #4 ·
I'll try and keep the thread updated with further thrilling instalments as I go along.. soon hopefully!
 

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Discussion Starter · #6 ·
Henry, will RTV silicon caulk be tough enough to hold up under the heat and vibration? Or do you have some 'secret sauce' version of caulk that you'll use?
It's a good question..

I'm not using anything particularly exotic. I decided on Hylomar 102 which ticked the right boxes in terms of insulating properties, general ruggedness, temperature and oil resistance. (And price.)

I got fed up with trying to quickly find a convincing epoxy resin to repair with which was available in small quantities with good temperature resistance, inexpensive etc. I saw somewhere or other someone who used one of those metallised epoxies to repair a stator with (JBWeld - I have something similar in the cupboard called 'Quik Steel'). It's good stuff, they do a higher temperature resistant version, but I don't want any metallic content in there and my googling patience ran out before I found an answer about it's electrical characteristics. The heat resistance isn't much different to the silicone. ( I'm shooting for around 200ºC. )

That's not to say that deciding to try silicone was a 'second best' decision. If I was building up the stator from bare laminations I certainly wouldn't try to use RTV silicone, but for filling in a couple of pot holes I think it should be fine, and also in terms of adding some extra protection to the ends/corners I'm thinking the rubberiness is an advantage - better a bouncy cushion than a hard edge. Regarding vibration and heat expansion/contraction I think it's flexibility should be an advantage in terms of staying stuck on compared to something more rigid which could crack away.

That's the theory anyway.
 

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Discussion Starter · #9 ·
Is this a cooling issue? Would be interesting to measure temperature behind the cover. Using engine oil to cool a value-engineered alternator rings alarm bells.
Did you mean in general, or my 'value engineered' repair experiment, or value engineered aftermarket stator products?

The temperature under the cover shouldn't normally be too extreme. The coolant temperature hangs around 85-105ºC-ish range and the side cover is air cooled on the outside. Stator heating can then add to that of course, especially if it's failing..

I don't believe there's any significant engine oil flow through the stator location acting as a cooling system - though it does get a little oily in there.
 

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Discussion Starter · #11 ·
Ali Express ~£44 "For Triumph Sprint RS 955 2000-2004" they evidently have a different idea of the models/years than me, though amazingly they specify the right stator dimensions for an '02 (although core thickness not specified).
Many list one as suitable but have the wrong dimensions.
Carmo aftermarket off the shelf for "Triumph Sprint 955i (1998-2004)" (?!) about £140+ but wrong dimensions.
Carmo rewind with 3yr warranty ~£200
ELectrexworld (don't give them any money) G75 (out of stock) £120+
Fowlers, World of Triumph etc.etc. ~£350+

Several anecdotes floating around about the cheap ones failing quickly.
 

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Discussion Starter · #13 ·
Should be ok but, did some quick additional checking for incompatibilities:

Material property Font Electric blue Plastic wrap Comfort food


The black RTV sticks the fibreglass tape down nicely. The generic polyimide ("Kapqon"!) tape sticks nicely to everything except the silicone. The whole lot was varnished over with the xylene-based winding lacquer. Nothing batted an eyelid. The lacquer didn't really stick to the silicone - no surprise - could just pull it off as a crinkly sheet once cured, but the silicone didn't seem to be affected by the xylene during the drying time.

The polyimide tape didn't flinch under blasting with hot air jet at 200ºC, started crinkling at 300ºC but largely uncrinkled again when the heat was removed, and remained intact.
 

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Did you mean in general, or my 'value engineered' repair experiment, or value engineered aftermarket stator products?

The temperature under the cover shouldn't normally be too extreme. The coolant temperature hangs around 85-105ºC-ish range and the side cover is air cooled on the outside. Stator heating can then add to that of course, especially if it's failing..

I don't believe there's any significant engine oil flow through the stator location acting as a cooling system - though it does get a little oily in there.
The Triumph components will be value engineered. Given the reported failure rate, the aftermarket components are likely to be value engineered. Given the potential consequences of a flat battery for a 955i (sprag clutch failure) then bespoke should be performance engineered, but that should be your decision,

Isn't the stator designed to be cooled by engine oil?
 

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Discussion Starter · #16 ·
The Triumph components will be value engineered. Given the reported failure rate, the aftermarket components are likely to be value engineered. Given the potential consequences of a flat battery for a 955i (sprag clutch failure) then bespoke should be performance engineered, but that should be your decision,
Right. The values may be slightly different between a distant aftermarket source just trying to knock out units on a low margin compared to a Brand with some reputation to maintain, warranty periods etc. but of course I agree - pretty much everything is a value engineering compromise.

'Performance Engineered' sounds expensive! :oops:
My budget is rather limited and there are other things on this 18yr old bike that will me more important to have funds for. :)

But actually I'd argue that what I'm trying to do here IS bespoke performance engineering - I'm actually aiming to improve the performance in several respects: increased insulation resistance around key failure zones, increased physical resilience around those key failure zones, reduced winding resistance to lower heat generation in the windings...
 

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Discussion Starter · #17 ·
The very last update on the 1050 engine was an extra oil gallery to improve cooling of the stator. My 2020 Tiger Sport has it,. My 2017 Speed Triple and Tiger Sport did not
(and @Dibnah)

That's interesting!

I didn't dig into this aspect for this particular exercise as it didn't change what I was doing - there's oil in there so the stator has to be suitably oil-resistant anyway.

Sounds like a later idea then.

On the 955 there is no oil way in the cover that the stator mounts on, and the stator is surrounded by the rotor so the only way oil would be deliberately getting onto the stator is out through the centre of the crankshaft, and I didn't notice anything like that when I was dismantling it. Though perhaps I should take a closer look..
 

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On 2011+ bikes there's an oil drilling in the end of the crankshaft and the rotor retaining bolt is hollow to provide an oil path out onto the stator. There was also an update around 2007/8 where an extra hump was added to the stator cover which was supposedly to do with another oil path for cooling the stator but I don't know the details.
 

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Discussion Starter · #19 ·
After my time :)
Intriguing - unless the oil fairly squirted out of the rotor retaining bolt it would all just centrifuge out to the edge of the rotor and fly off... ah no perhaps not if the bolt head extends into the hole in the middle of the stator then it would fly off and hit the stator.
I guess the extra hump is what Tritigex was highlighting in second photo. Feeding oil round the cover to the stator sounds straightforward enough at first, but then not so obvious how you get it to get sufficiently even coverage of the stator.
 

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Discussion Starter · #20 ·
So here it is with newly rubberised bobbins after a couple of passes with the silicone, mainly around the ends. Looks hideous of course - but I'm hoping never to have to look at it again.

Automotive tire Tire Wheel Rim Bicycle part


oops - missed a bit, a couple more touch-ups needed...
 
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