TOPIC:

Surely it's the current being shunted to ground that causes the heat, not the shunting circuit itself?

I think that the point was the "shunting" circuitry in a "Shunt" type V/R IS the cause of the heat which in turn eventually burns out the key-components in the V/R which will then burn out the stator. ......unless the stator shorts out on it's own.
Then there's the other "Can-0-worms": Which is better.....an "exposed-wind" stator or one that has the winding's epoxy coated? One could argue that the epoxy coated stator would hold the winding's securely, reducing inter-wire wear from vibration which could eventually fail due to shorting. OR ....the epoxy coating doesn't allow the coils to cool and that could lead to failure?

I'm still not getting it. Seems both systems are 'shunting' or 'short circuiting' current to ground. The heat produced by the current being shorted to ground would be more prominent than the heat generated by the circuit turning the shunt on an off. Or, what am I missing?

There's quite a learning curve here when you study "controling the output of permanent magnet alternators" If you trigger the output on and off as an alternative. the periods when the alternator is turned off, the voltage builds up in the stator, then when the alternator is turned on, the spike in high voltage dumps into the system and can cause havoc to the electronics. So instead the excess voltage is "shunted" to ground. This keeps the voltage under control.
Bottom line is that with a permanent magnet rotor, as long as the magnet is turning, it's generating electricity at maximum output. The regulator in this case has to decide where to send all that electricity, either to the system (battery or to supply the needs of the running systems) or to ground. The alternative would be an energized rotor which is a rotating iron core with windings that are energized creating a rotating magnet of varied intensity. By controlling the amount of electricity going to the rotor one controls the input of magnetism applied to the stator and that controls the output of the stator. All that happens after the stator is the rectifcation of the AC to DC. This type of alternator runs much cooler since it only generates as much electricity as required.

Kawboy wrote: ....The alternative would be an energized rotor which is a rotating iron core with windings that are energized creating a rotating magnet of varied intensity. By controlling the amount of electricity going to the rotor one controls the input of magnetism applied to the stator and that controls the output of the stator. All that happens after the stator is the rectifcation of the AC to DC. This type of alternator runs much cooler since it only generates as much electricity as required.

My 80's DOHC Honda 4's have excited-field alternators in them. Some folks are on a mission to convert them into the permanent magnet type - seems crazy to me to convert something that is marginally more complex (and which improves performance) into something that likes to burn out.

Having said that, the reason that some folks want to convert the excited type to the permanent magnet type is because the windings in the 'excited part' - the rotor - rotate, and can chafe leading to open or short circuits. Open-circuiting isn't that big of a deal, the bike's charging is impaired. However short-circuits - which are not uncommon - cause the R/R to pump more current into the rotor in an attempt to maintain the magnetic field (which is impaired due to the shorted winding) and the heat from that extra current can (and generally does) take out the R/R leaving both a faulty rotor and R/R. This is a lot more prominent on bikes that are run a lot at high RPM (since that rotor is spinning on the end of the crankshaft). A design improvement would have a circuit to limit rotor short circuit current (or fusible links) which would prevent the R/R from burning out.

It's an interesting arguement that has been made but I question it. Automotive alternators spin at or near the same rpm's due to the fact that the belt pulley on the alternator is usually about 2.5 inches and the crank pulley is about 6 inches which gives the alternator an increase in speed of 2.4 times that of the crankshaft. Most auto engines tend to run around the 1800 - 2000 RPM range and can speed up to 5000 to 6000 rpm on acceleration. That gives the alternator a running speed of around 4300 rpm at cruising speeds to as much as 13200 rpm on acceleration. The biggest problem that I've seen over my many years wrenching cars is the alternator failures are due to worn slip rings and worn out carbon brushes. Quite often I could save an alternator by just cleaning up the slip rings with a file and dropping in a new set of carbon brushes. Nowadays, the manufacturers have gone to copper plated slip rings instead of solid copper slip rings so there's not as much material to work with. Alternator rebuilders have access to replacement slip rings and provided that the windings are intact, they can just replace the slip rings and Bob's your uncle.

Still makes for an interesting conversation and thanks for sharing Bucko.
KB

Of course there's the question of Oil vs. brushes unless there's a way of keeping the two separated.