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Discussion Starter #1
Good afternoon, my system quit charging so I pulled the stator as coils were shoting to ground. The picture sbhows a burnt spot that just crumbled off. Any theories as to why ?
jerry
 

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Hi Jerry, I've only seen one burnt stator back in 70s. The wires to rectifier got pinched to ground near battery box.

What year is bike & motor? What is charging system? Stock rectifier & Zener or something else? Has wiring harness been obviously modified.

Can you see where wires shorted to ground or is it just inside stator. The chassis wiring needs to be checked for shorts to ground if cause is not obvious. The charging system is not fused.

On an aside be sure to store rotor inside stator to prevent magnetism loss. Or use keeper strips on rotor.
Don
 

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Hi Jerry, Don,

Any theories
My monitor is showing the stator potting as black? If so, that's a pattern stator.

What is charging system? Stock rectifier & Zener or something else? Has wiring harness been obviously modified.
Can you see where wires shorted to ground or is it just inside stator. The chassis wiring needs to be checked for shorts to ground if cause is not obvious. The charging system is not fused.
Bear in mind, assuming standard stator, output is only a maximum of 11-12 Amps even if the engine's being revved like a small 2-stroke.

A fuse in a battery-supplied DC circuit is wise because any battery is capable of producing very high current even if only for a short period.

What year is bike & motor?
'70 Daytona according to Jerry's Forum Profile?

Hth,

Regards,
 

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Discussion Starter #5
Thanks for the interest.
The bike is a T100r 1968 engine in a 70 frame , came that way when I got it 43 years ago. The stator is stamped Lucas in the epoxy stuff and as a 3 71 , 47205 A stamped into the edge of the frame plates. The system is fused but has never blown in my past memory and always charged.
I have had an intermittent dead or mostly discharged battery this past riding season and spent many hours searching for why. What I finally found was that the clutch cable had slipped under the zip tie on the upper frame tube to a point where it rubbed through the plastic coating and shorted on the coil terminal. Hence the dead battery. But not every ride as sometimes it was fine . By the time I solved that problem the alternator was not charging. So would the dead short be a reason for the demise of the stator? I finished the ride season by trickle charging the battery between rides.
jerry
 

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Hi Jerry
I would be so tempted to upgrade the system to 3 phase, if I had to change the rotor anyway.
Regards
Peg.
 

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Discussion Starter #9
Yes I am considering the upgrade but exploring the why it happened first
thankyou jerry
 

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Hi Jerry,

clutch cable
rubbed through the plastic coating and shorted on the coil terminal.
Which coil terminal (as in "-" or "+")?

Is the bike still using points or have they been replaced with an electronic ignition?

If electronic ignition, what was the coil terminal connected to (e.i. 'box', other coil, 'ground')?

would the dead short be a reason for the demise of the stator?
Very unlikely; the rectifier separates AC (alternator) and DC (including ignition); that'd have to be faulty as well for there to any chance of a DC component affecting an AC one.

rotor is tight
Risking telling you something you know already, @oldgoldie means check the outer part of the rotor hasn't come loose from the centre - the central hex. can be tight on the crankshaft but the metal around it - that holds the magnets in place - can come loose from the central hex. The way to check is, with the centre secured to the crankshaft, grab the outside of the rotor with both hands or a strap wrench and attempt to turn it, If you detect any movement between rotor outer and centre, 'fraid it's toast.

Outside possibility but also check for any movement (other than rotation) in the crankshaft relative to the crankcase?

Hth.

Regards,
 

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Discussion Starter #11
Morning,
The cable shorted to the positive making a solid ground at times as the ammeter would go to a full negative, otherwise during its demise it would swing wildly. and intermittently show a charge as it shorted or not . Yes I still use points and the bike had been a reliable starter and runner. So I think the rotor is sound (had one way back with a loose centre) or it would not have started and run as well as it did but I will check again later. Maybe it was just coming 40 years old and said enough?
jerry
 

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A likely cause of the fried stator winding is the intermittent short circuiting that Jerry describes. Even though the maximum current is only 10 or 12 amps, that's into a load at a nominal 12V, but the alternator is trying to drive current through the rectifier into a near short circuit and so the current may therefore be well in excess of 10 amps. The higher current could raise the temperature of the windings beyond their designed range to the point where the enamel insulation fails (normally 150-170 degrees C, unless Lucas used really high spec wire!) Once the insulation fails in a small area of a coil you can get a "shorted turn" which will then self-heat the insulation of more and more turns until destruction of the whole coil occurs.
I would also check the rectifier after this sort of shorting problem as it may have one or more damaged diodes.
 

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Discussion Starter #13
Thank you BrianG,
Heat does destroy many things and given the shorting problem taking so long to trace and age of the stator it gave up the ghost. So I will be getting a new one by spring. I will do some research and test the rectifier also. I have another one on the shelf i need be. I will continue to see if other solutions come up here. I appreciate the experience of the riders at triumph
jerry
 

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Hi Jerry,

The cable shorted to the positive making a solid ground at times as the ammeter would go to a full negative,
Uh-uh, 'fraid that wasn't the fault (unless the bike's electrics are 'negative ground' and/or your bike is otherwise peculiarly-wired?) ...

A standard DC electrical circuit is always from battery -ve to battery +ve via a resistance (the circuit can also include a switch to interrupt the circuit but it isn't essential).

A "short [circuit]" is still from battery -ve to battery +ve, just not via a resistance; this results in a huge (100-plus for a short period) Amps draw from the battery. If that doesn't blow a fuse, because most original or pattern wires on Britbikes can't conduct more than about 10A without damage, the huge Amps of a "short" result in great heat in the conductors, which results in - at minimum - melted insulation within a few seconds, followed by smoke and fire shortly afterwards.

The resistance in a points-switched ignition circuit is an ignition coil. If your bike has anything like standard pre-'79 electrics, if the clutch cable is electrically connected to anything, it's to battery +ve, same as the coil's "+" terminal. In that case, any "short" simply cannot have been there.

Another clue is, "The system is fused but has never blown", in an earlier post. '68-'78, Lucas supplied harnesses with the only fuse in the Brown/Blue wire attached to battery -ve (not good but not relevant here). Is the fuse original British "35A" or a 15A or 20A US equivalent? If it is, the ignition coils are supplied from battery -ve, the fuse would've blown if there really had been a "short" between coil terminal and clutch cable.

The only ways the above is wrong is things like the electrics are 'negative ground', there are wires without fuses attached to the battery -ve terminal, coils are connected arse-about-face, possibly if the fuse is US "35A"?

the ammeter would go to a full negative, otherwise during its demise it would swing wildly. and intermittently show a charge as it shorted or not .
Mmmm ... I've got Ammeters on my T100 and my T150 ... T150 'cos I was aiming for 'conkers', T100 because it's '69 and originally standard ... as diagnostic tools, both are about as much use as a chocolate fireguard ...

However, you can't show me what you saw; if you're absolutely convinced the Ammeter was indicating an intermittent short-circuit - and not normal behaviour on Brit twin - absent anything electrically non-standard on your bike, there might be an intermittent short-circuit, but not where you thought it was. :(

A likely cause of the fried stator winding is the intermittent short circuiting that Jerry describes. Even though the maximum current is only 10 or 12 amps, that's into a load at a nominal 12V, but the alternator is trying to drive current through the rectifier into a near short circuit and so the current may therefore be well in excess of 10 amps.
(n) 'Fraid there is simply and absolutely no possibility whatsoever that Brian's explanation is correct.

The alternator generates electricity by spinning some magnets beside some coils of wire - waving a magnet near a wire to induce electron movement in the wire is a basic physical principle.

Like most motorcycle alternators, the one on your bike is "permanent magnet" - the rotor is permanently-magnetised; how much electron movement it induces in a given stator's coil depends on the magnetic strength, the speed of the magnet's approach to the coil, the coil's wire diameter and the distance between magnet and coil.

A permanent-magnet alternator is not physically capable of responding to a 'demand' (Brian is possibly confused with a car alternator?). It cannot "drive" anything; if it could, we could ride around with as many headlamps on the bike as we wanted and more-powerful alternators would never have been needed.

The alternator AC Volts have absolutely no connection whatsoever with the regulated DC Volts - look in any pre-'79 Triumph workshop manual and note the test Voltages @ 3,000 rpm; neither 6V nor 12V, which were/are the DC systems Volts, which depended solely on the battery fitted.

The higher current could raise the temperature of the windings beyond their designed range to the point where the enamel insulation fails (normally 150-170 degrees C, unless Lucas used really high spec wire!) Once the insulation fails in a small area of a coil you can get a "shorted turn" which will then self-heat the insulation of more and more turns until destruction of the whole coil occurs.
As there cannot be a "higher current", similarly this cannot be the explanation for any higher temperature.

Nor is there any such thing as a ""shorted turn", Brian has invented this term from his basically-flawed explanation.
This is an older Lucas stator:-



... however, it's exactly what your bike's looks like inside the potting and is basically what any stator looks like inside. Do you see any insulation on the coil wires? So how can there be any sort of "short" if a coil wire breaks?

On a '2-wire' stator, one end of each stator coil is connected to one wire exiting (to connect to the rectifier), the other end of each stator coil is connected to the other wire exiting. Your bike's stator coil wires are electrically- and vibe-insulated by the potting; if a wire breaks and loses contact with every other wire in the coil, the coil stops generating, showing as reduced output.

check the rectifier
The only vaguely-useful thing in Brian's post ...

Nevertheless, while AC 'leaking across' old rectifiers to connected DC systems can be checked, I still have difficulty with the possibility there was intermittent conducting from your bike's DC system (the battery is the only thing on the bike capable of overheating the stator) but without blowing the fuse? :unsure:

Hope at least some of the above helps but I'm still nonplussed ... I know the explanation isn't Brian's but I'm not sure what it is. :( Possibly the explanation could be no more difficult than the stator took a whack at some point in the last 49 years and the vibes finally cracked off that piece of potting?

Regards,
 

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Discussion Starter #15
Well don't I feel silly. I have 3 british bikes and I KNOW they are positive ground. Just ran out to the shop to confirm the coil position and the cable was shorting to the negative blade on the coil. Thank you for pointing out this imp point. I feel a little silly but do not mind correction. Still the fuse never blew. I will check tomorrow the fuse rating. Possibly to high a rating? The piece missing on the stator was just crumbles I picked off with my finger nail. The stator winding are definitely grounded to the stator frame plates. I do appreciate any help.
jerry
 

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Nor is there any such thing as a ""shorted turn", Brian has invented this term from his basically-flawed explanation.
This is an older Lucas stator:-



... however, it's exactly what your bike's looks like inside the potting and is basically what any stator looks like inside. Do you see any insulation on the coil wires? So how can there be any sort of "short" if a coil wire breaks?
Stuart, I made no mention of wires breaking. Look at your picture. What do you think is the brown colour on all the wire turns of each coil that you can see? It is enamel INSULATION, and that stops one layer of wires shorting out to the next. Failure of this enamel due to excess temperature causes shorted turns, and this can cause devices with windings like transformers, motors, alternators to "burn out".
 

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Stuart, yes, it is copper coloured because it's enamel copper wire - just Google that term. It is the "raw ingredient" of any wound component (coil): separate visible insulation would be too thick to use. The colour can vary a bit depending on who makes it, usually somewhere between orangey and dark brown, but it is essential to any coil winding as it allows turns to be touching/close together without shorting. So, if it was not enamelled each turn of bare copper wire would short out to the next one, or would short out between one layer and the next (each alternator coil is the many turns that you can see in your picture and is also several layers of turns in depth).

As jerry has said he found that the stator it connected to the frame/ground, instead of "floating", this will cause large currents to flow through part of the bridge rectifier to ground/battery positive on each positive half cycle, which does not go through the fuse. The battery will still receive a small charge on the negative half cycle, but probably not sufficient to keep it charged.
 

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Hi Brian,

stator it connected to the frame/ground, instead of "floating", this will cause large currents to flow through part of the bridge rectifier to ground/battery positive on each positive half cycle, which does not go through the fuse.
This is wrong again:-

. When "stator it connected to the frame/ground" - an Ohm-meter displays zero or a low number when connected between stator mounting and either stator lead - it cannot generate anything; stator coils, rectifier and the two wires between form a circuit, not independent 'halves'.

As well:-

. On Jerry's bike, the Brown/Blue wire from battery -ve to Ammeter, the Brown/White wire from Ammeter to rectifier, the rectifier and the Red wire from rectifier to battery +ve form another circuit. "large currents" could not "flow through part of the bridge rectifier to ground/battery positive" without similarly flowing from battery negative to the rectifier. Absent Jerry posting otherwise, the Brown/Blue wire includes a/the fuse.

. No RM21 stator is capable of generating "large currents" even if the rotor is brand-new (in post #5, Jerry posted the stator is stamped "47205"; this is a standard RM21 stator).

Regards,
 

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Stuart, the stator can generate current into a short circuit: the power is dissipated as heat in the internal resistance of the windings. I suggest you draw out the circuit of the alternator, bridge rectifier showing the four separate diodes and the battery. Then draw a short from one wire of the alternator - either will do - to ground, then trace the current flow when the un-grounded wire is first positive then negative (as it's a.c) with respect to the other wire.

You must think separate the a.c. and d.c. circuits here. Forget the battery in this situation: no current can flow from the battery into the alternator; you will see this if you draw out the circuit and understand each diode's action within the bridge. The ammeter showing large discharge that jerry reported will be due to a short circuit on the d.c. side of the rectifier and is due to current flowing from the battery into his intermittent short with the clutch cable.
 
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