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     Timebandit |
Here are close up photos of one of the dead stators I've received. The first 12 photos are of the motor side, using the pole numbering system described above. I'm going to post these one at a time, please hold your commentary & analysis until I'm done uploading. There will be 24 photos in all. Thanks! | ||
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[URL=http://imageshack.us/photo/my-images/715/stator1mo torside04.jpg/]  [ | ||
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The next 12 photos will be of the cover side of the stator: | ||
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I'd be interested in hearing peoples' analysis regarding failure modes. In this example I see two. | ||
| Timebandit |
Reference photo:  | ||
| Hildstrom |
It looks similar to my failed stator. I see cracking and shrinkage of the epoxy in various places, even where it is tan instead of black. I also see severe overheating (blackening/carbonization) of the four poles centered around the three gaps where the delta pairs pass from the engine side to the case side. These poles are also backed by the insulated and siliconed stator leads. They have the least surface area for convection cooling, little to no oil dribble from the bearings, and the least air flow between them and the case. I suspect that softening and failure of the epoxy and insulation eventually led to a short in the darkest area. On my failed stator, you can see where the insulation flaked right off the copper wire in numerous places during disassembly. One thing I considered while rewinding is that the 2008 model has 18 poles and the 2009 has 12 poles. I understand the maximum power capacity is different, but they both have sufficient power and voltage generating capacity for a stock bike. The 2008 achieves voltage with fewer turns per pole and more poles. The 2009 achieves voltage with more turns per pole and fewer poles. So, the 2008 generates less heat per pole and has more surface area for convection cooling. The 2009 generates more heat per pole and has less surface area for convection cooling. The 2009 needs less heat (series regulator, shorter trips, etc) and/or more cooling (modified rotor). When you get into disassembly, notice how thick the plastic safety coating is on the laminated core. It is there to prevent shorts to ground, but I think it also provides a lot of thermal insulation between the windings and the core. If we removed the plastic layer, carefully smoothed the laminated core, and minimally coated the laminated core, I think more of the stator heat could be conducted through the core to the case. | ||
| Timebandit |
as i took those photos i thought about the puddle of black goop that had formed at the base of the hot poles, and had coalesced to form a droplet that was hanging off of the stator core. i didn't see that on your stator, or any of the other stators i've looked at. i've only seen that in one stator. i think it represents the extreme case. i'm not sure whether that puddle of re-flowed/charred material represents thermoplasic flow of the epoxy that was used to encapsulate the stator coils or whether that represents thermoplastic flow of the kapton itself, or both. if you look closely at the photos, both the epoxy and the Kapton are charred. charred! and the wire is bare/naked! i talked to someone in industry who supplies high temperature sensors for stators and rocket engines as his main job. he tells me that the thermal failure rating for the epoxy has to be in the range of 450*F to 500*F, and that the thermoplastic flow temperature for Kapton is somewhere in the range of 800*F. these numbers are in agreement with the numbers you mentioned on your web page. even if we look at the lowest numbers that have been mentioned so far, even the 500*F number is scary, as the best synthetic oils are commonly rated to only 400*F. i noticed that on your web page you mentioned that you thought your stator had to spend some time in the range of 518*F / 270*C for the polyimide coating to fail. i looked up the temperature ratings for Essex's "Allex" brand of polyimide wire: Allex.pdf. They rate it to 300*C/572*F for thermal shock, and 500*C/932*F for cut-through/thermoplastic flow. if we are indeed seeing epoxy failures, that puts us in the range of 450*F and up. if we are seeing kapton failures via thermal shock, that puts us in the range of 500*F and up. if we are seeing kapton failures via thermoplastic flow, that puts us in the range of to 800*F to 900*F. yikes. |
