how many agree that stator failure is caused by vibration ?

[Re-Vision]

Resistance reduces current flow, whatever current does flow through a resistive connection (corrosion) will cause heat to be generated. (Power =IxE) IxE = heat which equals toasted  resistive connection.      BDC

[kiwibum]

Interesting to see the old topics are still coming up :). Heat is the major problem as has been discussed, exactly why it appears to be more of a problem on the XZ than other bikes using the simple permanent magnet 3 phase stator is unsure. The core problem is the stator produces full power (load current) all the time, the faster it spins the higher the AC voltage output. The shunt regulator converts this to DC  and the passes the required current to the battery to keep it charged, the rest of the current is dumped (shunted via a resistor) to ground.

From memory the stator produces 20Amps, so if only 5amps is required at the time to top up the battery and run the ignition the other 15 is shunted to ground which makes the regulator rectifier (R/R) very hot too. So the basic issue is the stator and R/R run at full load capacity ALL THE TIME and get very hot due to this. A fully controlled system would reduce the output voltage of the stator as the revs increase and as the load requirement decreases so both the stator and R/R only handle the minimum load required by the bike, hence running cooler. Unfortunately due to simplicity of using permanent magnets on the fly wheel round the stator we can't control the magnetic field so the fast it spins the more voltage/current we get. Alternators like in cars don't use permanent magnetics they have a field winding which produces the rotating magnetic field and a feedback system controls the voltage to this to reduce the output of the stator as required.

Note also a faulty R/R, for example one with shorted retectifier diodes will cook a stator as well because shorting one of the phase windings will cause as much current as it will produce to flow through it heating it up even more. Diodes break down due to heat so it's also possible for these shorts to occur only when above a particular temperature and when it cools down it's ok again. So it's possible to have a faulty R/R under load but fine when you test it with a multimeter. I know of auto electrical shops that have replaced a stator only to have the replacement ones go due to a apparently good R/R. So it is a good idea to replace both at the same time unless you have the right equipment to test them properly.

Very interesting to read the post about the MOSFET R/R on eviltwinsbk.com and that they run very cool. This is possibly because rather than shunting excess load to ground it disconnects from the 3 phase windings kind of switching the load on and off. There was a guy on the list some time ago looking at this but the problem was large back EMF and voltage spikes on connection/disconnect which the electronics couldn't handle. It's also possible that it's still working as a shunt system and because of using MOSFETs which are more efficient at switching don't get as hot. Think I need to read up more on how these newer R/R work.

Anyway this all comes down to needing to take temperature measurements of the stator winding. That way we can see just how hot they get and whether a little more oil, an oil cooler or in fact using a MOSFET R/R rather than the old style really does make a difference to stator temperature. I do have a stator I rewound and put a thermocouple in it with the aim of taking measurements I guess I need to find it and get the testing done :D

[George R. Young]

. . . Note also a faulty R/R, for example one with shorted retectifier diodes will cook a stator as well because shorting one of the phase windings will cause as much current as it will produce to flow through it heating it up even more. Diodes break down due to heat so it's also possible for these shorts to occur only when above a particular temperature and when it cools down it's ok again. So it's possible to have a faulty R/R under load but fine when you test it with a multimeter. I know of auto electrical shops that have replaced a stator only to have the replacement ones go due to a apparently good R/R. So it is a good idea to replace both at the same time unless you have the right equipment to test them properly. . .

I have a problem with this part.

The AC output voltage of the stator rises with RPM. The stator is essentially an inductor so it's internal impedance also goes up with RPM (i.e. frequency). The output current is therefore approximately CONSTANT, i.e. voltage divided by impedance.

The thing puts out about 20A all the time (at any engine speed useful for riding). It's just as happy delivering its 20A into a shorted diode or the normal bike load. I wouldn't change a Reg/Rect if it's producing DC voltage and clamping it around 14.5V.

[Lucky]

I have 2 engines on the bench right now and have been studying how it's all designed. I have an off the wall idea:

the center of the stator frame bolts to the case cover. The poles the wire is actually wrapped around touch nothing. Clearances around the poles are very tight and they cannot be 'bathed in oil' in the current set up. They get oil mist at the most.

The idea of cooling fins on the outside probably wouldn't hurt, but may be ineffective at cooling the poles much...

the idea of drilling the bolt to spray oil a-la Venture, would only result in the Yamaha name badge getting sprayed with oil from the inside.

Because of the magnet placement in the rotor there is only one spot to drill the rotor/ crank to route a spray of oil on the stator, and that's in line with the key, so you couldn't seal the passage, so that idea is out.

I looked at the cover, rotor and stator for a long time & really only see one way to access the poles, & everyone is going to think its nuts...

If the cover ( or a spare) is surgically drilled/cut in segments around the poles, cooling air could be introduced almost directly onto the poles, probably with a low profile scoop and a foam air filter. 

just brainstorming...


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[Cdnlouie]

The idea is great Lucky, the problem is the crankcase pressure.  It would blow out the vents you put in the case and it would be hard to filter the oil vapour out.  I like your oil-cooling idea as one of the best ways to keep crankcase temperatures down.

[Lucky]

I thought about that, I haven't done it in a while but you can remove the button over the clutch adjuster on the right case while the engine is running. There is a little pressure but not so much that you can't thread the button back on. Not bad really...

My thought about this situation is that as long as the scoop is designed properly it might work. It would have to be sealed well to the case, there would need to be some small drain back holes inside, and a foam filter and baffling used for oil deprecation. The filter would need to be set back into the scoop a bit.

I'm half tempted to cut up some cases to experiment but I think a few of you would freak....


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[Rikugun]

I thought about that, I haven't done it in a while but you can remove the button over the clutch adjuster on the right case while the engine is running. There is a little pressure but not so much that you can't thread the button back on. Not bad really...

Is that at idle or under load at cruise RPM?

It is interesting but I'd be fearfull of riding in the rain with standing water or just going through puddles for that matter. The chances of foriegn matter being scooped into the engine just seems like a real pitfall. At the very least it seems the engine would soon become an oily mess. External fins seems like a better compromise. Just my 2 cents...  :D

[Lucky]

Yep that would be a problem. There might be a better way to introduce cooling air (or oil?) such as a snorkel. I'm just presenting possibilities. It would be physically possible to access the poles on the stator. Direct access introduces the potential for an outside the box idea...
Maybe a closed loop where the oil mist is forced up a tube by crankcase pressure* and dumps back in at the oil fill. If I could sketch this out on paper it might make a little sense. The tube, being in clean air might cool the air/oil mist some...

I had the plug off at idle, at speed it might blow out with some force. Maybe we simply need to build a crankcase air cooler lol

Again, just brainstorming...


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[Rick G]

Lucky, you will  notice in the first page, I suggested this. It was the result of a conversation with Don Minor in 2008.
If we introduce air into the  filler , from a hose connected to a small scoop, located above the radiator and  a second hose running from the rear of the alternator cover , ending in a catch bottle, located up under the seat, it would cool the stator. The high location of the intake and exhaust , along with the catch bottle "should " prevent oil migration and loss.   Any one want to try it?

[QBS]

Cooling jacket fashioned onto the stator housing.  Small electric pump moves coolent through jacket to small, in air flow, radiator.

[Lucky]

Rick that's where I got the idea from the first time you mentioned it although I didn't go back & reread your post. The stator is so isolated inside the rotor that I'm thinking the air may not be much cooler by the time it reaches the poles. 

I do see the advantage of the intake & drain back being up high. I think for this to work along the lines I'm thinking it would be pretty ugly huh?  :)

QBS, I'm thinking along the lines of cooling the poles more directly only because I'd imagine it to be more efficient. Honestly though I think a higher quality stator with more heat tolerant insulation along with better connectors, the newer style r/r  and a cleaned up electrical system are really
the way to go.


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[treedragon]

There is a heavier duty stator from one of the early XV's that fits. Can't recall the exact model just now but I have offered one up and it does fit and they are available new.......

Opinion has it that the XV unit will last due to meatier construction and is rated at higher amps.

[kiwibum]

. . . Note also a faulty R/R, for example one with shorted retectifier diodes will cook a stator as well because shorting one of the phase windings will cause as much current as it will produce to flow through it heating it up even more. Diodes break down due to heat so it's also possible for these shorts to occur only when above a particular temperature and when it cools down it's ok again. So it's possible to have a faulty R/R under load but fine when you test it with a multimeter. I know of auto electrical shops that have replaced a stator only to have the replacement ones go due to a apparently good R/R. So it is a good idea to replace both at the same time unless you have the right equipment to test them properly. . .

I have a problem with this part.

The AC output voltage of the stator rises with RPM. The stator is essentially an inductor so it's internal impedance also goes up with RPM (i.e. frequency). The output current is therefore approximately CONSTANT, i.e. voltage divided by impedance.

The thing puts out about 20A all the time (at any engine speed useful for riding). It's just as happy delivering its 20A into a shorted diode or the normal bike load. I wouldn't change a Reg/Rect if it's producing DC voltage and clamping it around 14.5V.

Yes, the impedance goes up with frequency, therefore the voltage must be increasing as well for the current to remain constant otherwise the current would drop off. Here is the output graph of the AC generator in the service manual.


This is with a load of 280watts. If a diode shorts this will decrease the load resistance which will increase the current drawn from the phase the short is on. Extending this further if one phase has a complete short across it the impedance of the coil is the only limit on the current so more than 20amps will flow I think. I think the 20amps is average and the peaks are higher because the phases are being shorted to ground to regulate the output voltage. The stator will get hotter the higher the RPM due to the longer periods the phases are shorted to ground as the AC voltage is higher.

Because it's a 3 phase shunted system it will still produce DC voltage regulated to 14.5V with a faulty diode or shunt transistor. Remember the phases are being clamped to ground to regulate the output voltage, if one phase is shorted to ground permanently the other two phases will be clamped less to keep the output voltage at the correct voltage. The average DC output will still be 14.5V but the AC ripple on it will be from two phases rather than three, so you wont see it on a multimeter only measuring with an oscilloscope. Mean while the stator is cooking because the peak current is flowing in the shorted phase.

The stators I've pulled apart have all been black and charred like this one below.



On this one the final problem was cased by vibration with the insulation breaking down and the wire rubbing on the stator poll. However this was allowed to happen because the encapsulating glue holding the coils on the polls breaking down from excessive heat allowing everything to move round and rub together. This is really a harsh environment for an electrical component to operate, cycling heat loads from cold to very hot (from the engine as well as the electrical load), constant varying high vibration and oil.



Reading more about the MOSFET R/R they are still shunt circuits operating the same as the original SCR based ones so the stator will still have the same high load on it all the time. So the stator will still run hot but it looks like the MOSFET R/R are much more reliable therefore less like to fail or partly fail and take the stator out. Interesting reading other bike sites that are reporting doing the MOSFET upgrade, plenty of other bike models with the 3-phase permanent magnet stator like the XZ have the same stator failures.

Ultimately I feel the solution to keeping the stator cooler will be an electronic one that controls the output of the stator to provide the minimum power needed for the bike at all times rather than the current system that pulls full power from it all the time. So I think part of the problem is it's delivering 20amps all the time but can't quite cope with the heat involved when you include heating from the engine too. And yes if we could keep it cooler by some means it probably will cope at 20amps but it's in a difficult place to keep cool. Once I get the bikes out of storage I'll be in a better position to measure the temps and see if different R/R, heat sinks on case etc do make the difference we need.

Sorry for the long post, have been thinking about the bikes too much lately and would like to help solve some of these problems. Much more interesting that what I'm currently working on. Looking at the photos I have of the stator rebuild it's been almost six years since I've had the bikes out :'(

[Rikugun]

No need to apologize for the lengthy post. I appreciate the time and thought that went into it. It's also nice to see someone addressing the core cause rather than trying bandaid methods to control the symptoms. Of course attacking on both fronts increases the chances for a workable solution but I'd rather see one that doesn't add unnecessary complexity. An electronic fix seems more elegant than some of the other methods suggested. Hope to hear more from your investigations.  :)

Treedragon, the XV stator seems to be the simplest solution in that it only requires choosing the correct replacement when the time comes.  :D I'm a bit confused by your post though...   Do you have one in your bike but just don't recall the model it came from?  If there is a more robust stator from an older model I guess supply comes into play. If new production for replacements for that model exist, one can only hope the same quality materials are being used to give the same performance/life expectancy.

[supervision]

  Check this out,  Google magneti marelli catalogue 2011    This has really high quality stuff.  they show a permeant mag alt for MOTOGP.  They show how it is done, first spinning the magnet in the center , not around the outside.  It also show size and all. 

[supervision]

  The cut in speed is 2,500 rpm, and the thing only makes 15 amp at 4,500, max out at 22amp past 6,000.  The reason I thing this would run cooler is the way the winding is not captured by the magnet, it is in the center of the winding. 

[treedragon]

Rikugun

no I don't have one in my bike but I did have one in my hand.

They are available aftermarket new around NZ$200 and something as I recall, my current situation doesn't run to having any spare coin to spend so I "make do" in this case I have one out of a CX Honda I think with the excess wires clipped. There was only one XV model they came out on and unfortunately I seem to have a memory like a sieve with a hole in it just now so details are sparse.

Who knows what the future might bring so I will relook should this one cook  ;D

 

[kiwibum]

  The cut in speed is 2,500 rpm, and the thing only makes 15 amp at 4,500, max out at 22amp past 6,000.  The reason I thing this would run cooler is the way the winding is not captured by the magnet, it is in the center of the winding.

I think the requirement of higher RPM (other than GP stuff revs high anyway) is because the rotor is in the centre and a small diameter.  It will have to turn faster to get the same surface speed (magnetic flux cutting the stator coils) that if it was on the outside and larger diameter like the XZ.

And yes having the flywheel enclose the stator will definitely hold the heat in. This makes me wonder if cutting a diagonal slot between the magnets or even drilling holes through the flywheel to enable more air movement round the stator as it spins would help with cooling. Would lighten the flywheel a little which wouldn't be a big issue but it would have to be balanced correctly.

[supervision]

  Kiwi, did you see the pictures of the stators, with the center mag? 

[kiwibum]

  Kiwi, did you see the pictures of the stators, with the center mag?

Yes I did, that's what my last reply was about. These wouldn't work on the XZ because the flywheel on the XZ is the rotor, the mass of the centre mag on the magneti marelli one would be too small I think. It's a great idea to have the stator on the outside like that to get better cooling which made me think about what could be done to the flywheel on the XZ to help cool the stator windings.