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Old March 16th, 2017, 09:58 AM   #1
rgx107
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Bad rotor, poor magnetism

It seems the rotor or to be specific the magnets are bad, not strong enough. I did the measurements per the workshop manual, and it all points to the rotor magnets. It is also consistent with how it runs, it's fine at high revs but when idling or at low revs the battery (charge) indicator comes on - meaning the voltage drops too low. I have around 7-8 V on the generator leads at 4000 rpm - should be 35 V with no load.

And the rotor is expensive, around 500 EUR locally. Maybe slightly cheaper online. And it seems to require special tools to replace, so I'm hesitating. Since the old rotor is scrap one might not need to be that careful when removing it, but when installing they recommend a "flywheel holder" that looks like an oil filter removal tool, only bigger. Anyone has done this? What did you use.

Other than the special tools it seems to be fairly straightforward. The bike is from 2013 and has done around 55 000 km - I guess most 300s have less milage so you can start worry about yours now.
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Old March 16th, 2017, 01:33 PM   #2
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So all the windings have a resistance that's within spec?

I've heard of one or two cases of 40 year old motorcycles having the alternator rotors lose enough magnetism to have trouble, but something unusual would have had to happen to one that's only four years old to do that to it. Like it got loose and banged around on the poles, or someone took a torch to it, etc..
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Old March 16th, 2017, 03:51 PM   #3
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Yes all windings are within spec, but the spec is fairly generous: 0.05 - 0.6 ohm. I measured them to 0.5-0.6 ohm - though if I just put the test leads together the instrument shows 0.3 ohm (the resistance of cables and connectors), so I guess the actual resistance is around 0.3 ohm. And also they have similar voltages which makes me think they are OK. I can't believe the coincidence of all three of them being partly shorted in the exact same way. And there is no isolation fault either. Magnets however, since all of them rotate past the coils, a few of them could fail and others be good, that would give lower voltage reading on a simple instrument. Same reading on all coils.

And please share, the failed stators you have seen, have they had lower voltage? Similar to this case? Sure windings in the coil could get shorted especially where the laps overlap. But all three? I will need to measure again.

The shop manual says magnets can go bad by "age", but what I have learned earlier about magnets is that most common cause is overheating. Yes I don't understand how they could overheat, or age that quickly either.
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Old March 16th, 2017, 05:28 PM   #4
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The problems I've heare about with weak magnets were with the magneto windings, so the spark was weak. In one case I specifically remember, it was a drag bike and the charging coils had been removed, but if the magnets were weak enough to cause weak spark, I'm sure the alternator's charging output would have been low too. Sometimes if windings are bad, you can poke at them with a stick and the resistance will change.

Is it possible that some repair work was done and the polarity of one or more coils is reversed? The voltage you're reading sounds so low that I can't imagine the magnets got that weak. Do they behave like magnets, like sticking to a screwdriver, for example?
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Old March 16th, 2017, 06:56 PM   #5
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Are you sure that all coils are OK?
A bad regulator can damage those really quickly.

Sorry I cannot help much, maybe this link could help:
http://faq.ninja250.org/wiki/How_doe...system_work%3F
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Old March 17th, 2017, 01:38 AM   #6
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The first thing is that measuring such low ohms is very difficult to be accurate.
The magnet is very unlikely to be faulty, as it has to be removed any way there is no extra work involved.
You can rewind the coils I have done this on an ER6F nine coils three per phase just make a note of the position and direction of the winding.
A partial short will not show up on the ohm meter but will kill the voltage output but should be obvious when its dismounted, discoloration due to heat.
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Old March 17th, 2017, 05:39 AM   #7
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Maybe buying just the coils would be more economic.
A bad rectifier could damage replacement coils.

The tools for removing the rotor are not too expensive.
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Old March 18th, 2017, 02:34 AM   #8
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Thanks guys, your comments are very valuable. I haven't done any more work yet, but it is clear that the generator is bad. I will remove the cover with the stator to inspect, if you're right that is good news because the stator is easier to replace.

Steve you are right that there is no chance ever that the resistance as measured on the connectors can be below 0.05 ohm. It must be a typo. I have some experience of using a milliohm-meter to verify ground connections, thick cables (100 mm2) bolted to a vehicle and they are usually around 10 milliohm or slightly below. To measure below 50 milliohm the stator leads would need to be short-circuited, if even then.

I am tempted to rewind the coils but I will first buy a new stator most likely, if needed. The season is short here, have to benefit from the few warmer months of the year. From the pictures it looks like there are 18 coils.

The rectifier/regulator must be verified as well but I do know it gives some charge so at least part of the rectifier is OK. The regulator might be OK, otherwise it's shorted and all regulation has been done by the stator gradually failing until current was so low it couldn't degrade further.

The thread should have a new title as well.
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Old April 14th, 2017, 06:11 AM   #9
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Out of curiousity, how many miles does your 300 have? My stator has gone twice now both times at almost exactly 20k miles
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Old April 14th, 2017, 02:43 PM   #10
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55 000 km.
I haven't gotten around to tear off the stator yet, but I will report back for sure. And yes, I was wondering if my bike had a unique issue so it's interesting to hear about others faulty stators. And without even knowing yet if it is the stator that is bad on mine, I can't help wondering about the cause. Too high temperature? Overload? Vibrations or shock?
If the stator itself is the cause, it might be possible to rewind it with higher grade wire, or with higher grade coating of the wire rather. Slightly thicker wire might also be possible when you manually wind the coils. If the stator is not the cause but only the victim, one might not want to reinforce it because it could trigger worse problems. Maybe it would be wise to monitor the stator current? Or its temperature?
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Old April 17th, 2017, 06:19 PM   #11
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I'm finding that I have the exact same problem at the moment, receiving the same results as you have stated here. I spoke with an old-school motorcycle mechanic a few days ago, and he recommended to check the Rectifier first, as in his experience, they are the most likely thing to fail (at least in Australia, as the headlights on our modern motorcycles are wired to be permanently on, over-working the rectifier).
I did all of the other tests, and it appears that my bike has a loss of magnetism too, as the output is no where near the recommended 35V.

I did remove and resistance test the Rectifier, and it appears that the W pin (referring to the workshop manual) is dead, as I couldn't get any reading from it at all, let alone a result within spec.

Have you tested this as well? Could it be the cause of the problem, or yet another victim?
How have you gone so far, removing the stator?
Hopefully you've had a breakthrough!
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Old April 17th, 2017, 06:55 PM   #12
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I've seen warnings in motorcycle service manuals about the need for careful handling of the rotor and that the magnets in the rotor are delicate. Something as simple as accidentally dropping the rotor on a hard surface, like a concrete garage floor, can ruin the magnetism of the rotor magnets.
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Old April 17th, 2017, 07:21 PM   #13
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I have also seen the warning to handle the magnets carefully. I do know that the ones in my bike have never been dropped themselves, though the bike itself has unfortunately been in a bit of a tumble, only resulting in a broken mirror and a bent left-hand clip-on, easy fixes.
That tumble was a good 3-5 thousand kilometres if not more from the problem arising, but do you think that could be a cause to the problem?
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Old April 17th, 2017, 07:22 PM   #14
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Very unlikely.
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