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Discussion Starter #1
I've spent the past few hours looking for this issue and can't find any reference so here goes:

3 weeks ago I was leaving work and my '07 900 Custom wouldn't start. The starter would spin, but not engage. This is my 4th season on the bike with 20,000+ miles on it so I figured I needed a new battery. 40 minutes and $95 dollars later it fired right up and I drove it home.

On an extended trip that weekend (two days later), same problem. I let it sit a half hour and it started up. Had to push start it once that day but otherwise it started every time.

Got back from the trip and started running some tests. I get 12 volts across the battery no matter what rpm I'm running. The stator resistance seems to check out (though I'm using a cheap analog gauge), but my tests show that I'm only getting 2-4 V/AC off the alternator (should be 54+) . I'll have it checked out by professionals but it looks to me like the rotor's shot.

Since this is a $500+ service I'm wondering if the 900 is prone to this problem. Anyone else have this? Is it a maintenance issue or do these things just go bad for no reason?
 

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Discussion Starter #5
If a weak battery will take out a stator, what might a regulator plug full of water do? One of the things I discovered and will be asking my mechanic about is water protection for that plug. When I unplugged the reg to test the alternator circuits about a 1/4 oz of oily water ran out of the plug. I had ridden home in a downpour a couple days before this problem surfaced.
 

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Could you explain that a little more as I do not quite understand.
sfair, in most cases a lead acid battery will begin to fail because over time material has flaked off of the plates and accumulated at the bottom of the battery. These piles of residue build up until they touch the bottom of the plates and when they do, they start slowly shorting out the battery. The battery begins to loose the ability to keep a charge at the level where the regulator is happy. Thus the regulator demands power (voltage & current) keep coming from the alternator. The windings in the stator begin to overheat and the insulation fails thus shorting, or burning out, coils within the stator. SteveJB's problem sounds like it started with the regulator however. The stator and the regulator can be tested easily with a VOM before spending a lot of money.
 

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Discussion Starter #8
Sojourner, what you're saying makes a lot of sense to me. If you're right, the problem probably started by keeping the original battery too long and storm water in the reg was the last straw. The book calls for a digital VOM and mine is analog. Testing the stator with my meter was a bit sketchy. The spec for resistance is .11 - .17 ohms. "Significantly less than that" means a short in the windings. I can't read hundredths on my $12 WallyWorld special.

I seem to be in the middle of an electronic mojo right now. Trailer lights won't work so I'm not getting this to a shop 'til I fix that! Must be that pesky Perseid meteor shower ;)
 

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sfair, in most cases a lead acid battery will begin to fail because over time material has flaked off of the plates and accumulated at the bottom of the battery. These piles of residue build up until they touch the bottom of the plates and when they do, they start slowly shorting out the battery. The battery begins to loose the ability to keep a charge at the level where the regulator is happy. Thus the regulator demands power (voltage & current) keep coming from the alternator. The windings in the stator begin to overheat and the insulation fails thus shorting, or burning out, coils within the stator. SteveJB's problem sounds like it started with the regulator however. The stator and the regulator can be tested easily with a VOM before spending a lot of money.
But a permanent magnet excited shunt regulated alternator ALWAYS outputs maximum power at the rpm it is running at, so how can a weak battery cause the stator to burn out? I am still missing something.
 

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But a permanent magnet excited shunt regulated alternator ALWAYS outputs maximum power at the rpm it is running at, so how can a weak battery cause the stator to burn out? I am still missing something.
Ohms law, W(P)=VxA. If the battery is fully charged the regulator is satisfied and tapers off the current (not the voltage), thus reducing the work the alternator needs to do. Can you envision an alternator/regulator being the same as a Battery Tender? If the battery cannot maintain 13.8VDC then the regulator thinks the battery still needs to be charged and continues sending current to the battery thus overworking the alternator and boiling the electrolyte out of the battery at the same time.
 

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But a permanent magnet excited shunt regulated alternator ALWAYS outputs maximum power at the rpm it is running at, so how can a weak battery cause the stator to burn out? I am still missing something.
Ohm's law tells us less resistance = more current. A battery that is bad is actually constantly in a low charge state. When the battery is low on charge it's internal resistance drops, causing it to draw more current from the stator. The limiting factor in ANY DC circuit is resistance. As the battery resistance goes down the stator tries valiantly to fill the circuit, this increases the heat (due to more current flow), thus slowly destroying the stator insulation by heat breakdown. The regulator in this system sees a battery that is constantly low and leaves the shunt resistor out of the circuit, since the shunting resistor is a reasonably high resistance it causes the stator current to be low. As for the stator always putting out max current, it is actually only trying to "fill up" the circuit, based on the circuit resistance.

Do you feel like an Electrical Engineer yet?????
 

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The stator reliability should be rated for full power at 100% duty cycle. I haven't seen any data that says the stator has been derated for any of the variables found in MIL-STD-217F. If it fails it's a design flaw from using inferior components, not due to constant output caused by circuit resistance. Insulation should last 10 years or more, not short in the first year or so of operation. The infant mortality rate for the stator is not in proportion to typical electrical components. Way to many stators have died with probably less than 100 hours on them. The stator is a crappy design build to poor requirements, and no doubt made by the lowest bidder for the job.
 

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Ohm's law tells us less resistance = more current. A battery that is bad is actually constantly in a low charge state. When the battery is low on charge it's internal resistance drops, causing it to draw more current from the stator. The limiting factor in ANY DC circuit is resistance. As the battery resistance goes down the stator tries valiantly to fill the circuit, this increases the heat (due to more current flow), thus slowly destroying the stator insulation by heat breakdown. The regulator in this system sees a battery that is constantly low and leaves the shunt resistor out of the circuit, since the shunting resistor is a reasonably high resistance it causes the stator current to be low. As for the stator always putting out max current, it is actually only trying to "fill up" the circuit, based on the circuit resistance.

Do you feel like an Electrical Engineer yet?????
How can I put this, the above is not true.
1. The alternator, alone, always generates as much voltage as it can when it turns. It HAS to because there is no internal way to stop it. It is not like a car that has a controlable field. So the faster it spins, the more voltage it develops. When you check the stator when the regulator is disconnected, you get 50 plus volts. If the engine could rev higher, you would get even more voltage.

Do you agree with me, so far?
 

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The stator reliability should be rated for full power at 100% duty cycle. I haven't seen any data that says the stator has been derated for any of the variables found in MIL-STD-217F. If it fails it's a design flaw from using inferior components, not due to constant output caused by circuit resistance. Insulation should last 10 years or more, not short in the first year or so of operation. The infant mortality rate for the stator is not in proportion to typical electrical components. Way to many stators have died with probably less than 100 hours on them. The stator is a crappy design build to poor requirements, and no doubt made by the lowest bidder for the job.
I have not actually had one apart, so this is a guess. I think what is happening is during production, one or more of the field coils were wrapped to tight over the laminates, possibly contacting a sharp laminate corner. Then when the engine runs, vibration causes the corner to cut through the varnish for that field and, presto, you have a field that is shorted to ground. What happens next is that field cooks and melts the rest of the varnish off of that winding and possibly the other fields, also, resulting in a pile of shorted out goop, depending on how soon it is caught.
Does that sound possible?
 

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How can I put this, the above is not true.
1. The alternator, alone, always generates as much voltage as it can when it turns. It HAS to because there is no internal way to stop it. It is not like a car that has a controlable field. So the faster it spins, the more voltage it develops. When you check the stator when the regulator is disconnected, you get 50 plus volts. If the engine could rev higher, you would get even more voltage.

Do you agree with me, so far?
That is absolutely what Oms law, and my post says. I am very familiar with shunt controlled devices. My statement stands on it's own that the stator winding only puts out current based on the circuit resistance. In a normally functioning charging circuit, yes the stator puts out a virtually steady current, and the regulator shunts the excess off thru the shunt resistor to keep from "boiling the water out of the battery. The problem I was describing involving a bad battery is that the stator is loaded way above its normal 100% rating, thus making much more heat and failing it. In your second post, fully agree with the crappy undersized design. The alt in a 900 will only put out somewhere around 100 watts of accessory power (if memory serves me correct). You can load the system beyond that up to the point where you drop the volts and start to not keep the charge in the battery but the additional heat will burn down the stator insulation.
 

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That is absolutely what Oms law, and my post says. I am very familiar with shunt controlled devices. My statement stands on it's own that the stator winding only puts out current based on the circuit resistance. In a normally functioning charging circuit, yes the stator puts out a virtually steady current, and the regulator shunts the excess off thru the shunt resistor to keep from "boiling the water out of the battery. The .
Nopies on that. The output VOLTAGE is what varies according to speed and must remain constant, not the current. The current varies constantly according to system demands. The output "voltage" must be regulated, not the output current. That is why they use a voltage regulator and not a current regulator.
Also, SCR's are used, not resistors in the regulator.
 
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