Ummm....Mr. Maine, our bikes have a 3 phase alternating current magneto. The permanent magnets provide a constant excitation to the armature windings. Electricity is generated by the rotating magnetic field passing across the conductors of the stator armature winding. The (cheap) shunt regulator uses high shorting currents in the stator windings to create an extra rotating magnetic field counteracting the rotating field from the permanent magnet. The resulting magnetic field is reduced and so are the induced voltages. The high shorting currents causes extra heat dissipation in the stator windings.
The MOSFET regulator is a better shunt regulator with more accurate control, but it still shunts excess power back to the windings.
An even better regulator is the series type which opens the armature circuit when voltage rises above the set point.
What our bikes do not have is an alternator with variable excitation. The voltage regulator does not control an excitation current to an electromagnetic field.
I haven't worked in a rewind shop, although I have spec'ed work and visited a very good one (DeMaria
in L.A. where they were doing a $20k rewind on an irreplaceable* 50 hp DC motor for us). I've run generators and synchronous motors up to 4.5 megawatt, many AC generators in the 2 or 2.5 megawatt range, replaced voltage regulators on these where the regulator requires two men just to lift it into position, cleaned windings and slip rings and commutators, renewed brushes, replaced diodes, etc. When a 2 meg generator burns up the automatic voltage regulator and we have to run it with manual control of the field current for a couple of days--one gets an understanding of voltage regulation. Running one generator and bringing another into parallel and balancing the load was also a normal operation--match the speed and phase sequence of the on-coming generator, close the circuit breaker and put it on the bus, adjust the governors to balance the load, adjust the voltage regulators to balance the power factor.
*Why was that 50 hp DC motor irreplaceable? Well, nothing is, actually, but there were no more to be found, the manufacturer, Reliance, would not make any, and we really needed it--it had a brake on one end, a gear on the other, was waterproof, and had a frame that fit the winch foundation. I asked a good hydraulics shop about converting these winches to hydraulic, and they suggested staying with electric. I asked Tony DeMaria Sr., owner of the rewind shop, about a different electric set up, and he suggested hydraulics! My thought that if we were stuck was to go to a 75 hp AC motor, current limited to 50 hp (that's what the frame and gearing could handle), soft start, two speed, reversible.