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Discussion Starter · #1 ·
Hi all,
Can anyone throw light on the following.....
We know that these alternators are 3 phase. Like many other bike makes, they have three wire from the alternator carrying AC current to the RR where it is converted to DC and regulated to produce +/_ 14 volts into the bike's electrical system. The AC current voltage generated by each of the 3 phases (wire) from the alternator to the RR varies depending on the type of alternator and the engine's RPM but around 10 volts per 1000 RPM so ( 5000 RPM = 50 VAC before reaching the RR).
Question is: what happens if anything in the alternator if one or more phase/wires breaks or disconnects ? The 50 volts has no where to go. How could this affect the alternator ? Is burned out stator coils a possibility ?
All advice is appreciated.
Cheers.
JMB
 

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Discussion Starter · #2 ·
ERROR !
Sorry guys, this inquiry was posted on the wrong forum ! Sorry about that ......notwithstanding that, anyone with knowledge of Jap design alternators is more than welcome to opine.
Cheers.
JMB
 

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The system is not a simple linear regulator as you describe. There is feedback in that control system - the voltage regulator adjusts the alternator field current in order to maintain ~13.8V at the output to the battery. If you turn things on, lights, indicators, what ever, current is drawn from the battery, dropping the voltage. The regulator senses this and increases the field winding current to raise the voltage back to where it needs to be. The system is fairly slow to react though, because the field winding is an inductor, and they don't like to change the current flowing through them. This is fine if you add or remove load slowly, but if you do it quick, bad things can happen. I know of an alternator that was wrenched of it's moorings when a bank of spotlights was switched on! Worse still, is the dreaded "load dump". Modern alternators can deal with this, but if you've an old one, or have swapped out the "special" diodes for standard ones, the voltage can easily reach 50V if the load is suddenly disconnected, before the regulator can deal with the sudden loss. Many systems designed to run on 12V will let the smoke out if 50V is applied - eg. incandescent lamps.

If a phase winding breaks, or a diode fails, that phase can no longer contribute, and the system will not be able to deliver the same power. This happened to me on a trip from Adelaide to Darwin back in the days most of it was gravel. The corrugations caused the connections to the diodes to break off. I had an ammeter in the old HR panel van, so I knew I had lost some power, but with one phase out of action, the system could still maintain the battery. With two gone, it couldn't quite. When the third went, we were close enough to Andamooka to get there on the battery alone.
 

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Discussion Starter · #4 ·
The system is not a simple linear regulator as you describe. There is feedback in that control system - the voltage regulator adjusts the alternator field current in order to maintain ~13.8V at the output to the battery. If you turn things on, lights, indicators, what ever, current is drawn from the battery, dropping the voltage. The regulator senses this and increases the field winding current to raise the voltage back to where it needs to be. The system is fairly slow to react though, because the field winding is an inductor, and they don't like to change the current flowing through them. This is fine if you add or remove load slowly, but if you do it quick, bad things can happen. I know of an alternator that was wrenched of it's moorings when a bank of spotlights was switched on! Worse still, is the dreaded "load dump". Modern alternators can deal with this, but if you've an old one, or have swapped out the "special" diodes for standard ones, the voltage can easily reach 50V if the load is suddenly disconnected, before the regulator can deal with the sudden loss. Many systems designed to run on 12V will let the smoke out if 50V is applied - eg. incandescent lamps.

If a phase winding breaks, or a diode fails, that phase can no longer contribute, and the system will not be able to deliver the same power. This happened to me on a trip from Adelaide to Darwin back in the days most of it was gravel. The corrugations caused the connections to the diodes to break off. I had an ammeter in the old HR panel van, so I knew I had lost some power, but with one phase out of action, the system could still maintain the battery. With two gone, it couldn't quite. When the third went, we were close enough to Andamooka to get there on the battery alone.
Hi Enginia,
Thanks for the quick and comprehensive reply. I now know a bit more about the inner workings of alternators !
I guess what I really need to find out,and you might have the answer is this...... we are restoring an old Yamaha L2 (single cylinder, 2 stroke) museum exhibit. We have no workshop manual and no wiring diagram for the 6 volt magneto direct lighting generator. The flywheel rotor energises a lighting coil and an ignition coil, both low voltage ( 6 volts) through 2 separate leads. So that the bike can be started and run as a static exhibit we hooked up the ignition side lead to a new remote HV ignition coil. It seems to run just fine with the AC current. The quandary is that the lighting cable is not connected to anything. The magneto generator produces 5 VAC at idle and 12 VAC at half throttle down this single wire. The questions are 'the current has nowhere to go, is this likely to affect the stator's lighting or ignition coils because there is no load on the lighting circuit' ? And, should we put a load on the lighting ?
Your opinion and advice is appreciated.
JMB
 

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Not connecting the lighting circuit will cause no problems. Just don't let anything short it out.
The coil will develop a voltage (AC) but will flow no current unless you load it.
If you could spin the rotor up fast enough, the voltage would build up to the point where the terminals would ionize the air and flash over, but at 1000 V/mm and possibly 20 mm between them (20,000 V), that would to be well outside the capability of the engine.
 
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