With the mechanical regulator the alt. is only asked to produce 12.2v @ 1,500 rpm....
That is incorrect. The alternator is asked for far more power, but is incapable of providing enough power for both the bike electrical loads and also put energy in the battery. The voltage being read here is the battery in the process of depleting.
it's still the same battery and field/ stator so why would it not be bad to ( new reg. ) ask it to produce 14.5v at the same rpm ??
It >IS< "asking". The alternator simply has production limits. (not knowing the "new" regulator design, it must be assumed that it controls the alternator field strength to keep the battery near its peak set point, and reduces the alternator filed strength when that limit is exceed. You have no data or rooted theory as to why the voltage would go as high as your suspicions predict.)
About the original design...
The mechanical regulator applies full voltage to the alternator field coil whenever the battery falls below a certain level.
At idle, the battery is draining, and so requires charging. The mechanical relay is then closed and whatever voltage the battery has, gets applied to the alternator field. The output of the alternator is all it can be, given the RPM of the rotor. Spin it faster and it will make more power, up to a point.
When the battery achieves enough charge (14.5v) the regulator then opens the relay contact, and allows the field coil to be driven via an in-line resistance (if I recall correctly, a 10 ohm resistor). This drops the voltage to the field coil, reducing the strength of the magnetic field and the output capability of the alternator at whatever RPM it is spinning. If the battery voltage continues to climb above the 14.5v set point, the relay closes to the opposite contact, which applies a direct short across the alternator field winding. This kills the magnetic field in the alternator entirely, and the output is then nil at any alternator RPM. However, if there is a load on the battery, the battery voltage falls, the regulator then selects one of the other relay positions (and alternator field strength) based on the voltage level the battery presents to it. At some RPM/load conditions, the mechanical relay points "chatter" or oscillate, which provides and "average" magnetic field strength to satisfy battery charge state requirements.
The alternator output capability is non-fixed and non linear with RPM. At Zero RPM there is no output. At 5000 RPM it is capable if 210 Watts output (CB750). The upper output level is limited by the magnetic field strength saturation and of the alternator component configuration (core permeability, number of flux lines crossing the number of wires, frequency and time duration of flux crossings, resistance of the wires, etc.) You can't take more energy out of it the it can convert.
With the RPM at idle, the output capability is reduced from the 210 Watt peak. (1/2 or lower) simply due to the frequency of the flux crossings across the windings. Further, loading the alternator output reduces the peak voltages developed within the windings. This effects the time duration of the flux crossing (due to the sinusoidal nature of the AC waveform). This means that the power developed is limited/reduced.
You could actually short the output winding together and the alternator would survive as the sinusoid peaks developed would be so low as to make little power to heat the windings (perhaps not the external shorting wires, unless you make them really big).
As a DC load on the rectifier, the diodes would fail far before any damage to the alternator could occur. (assumes stock rectifier. But, I expect further analysis would show any rectifier that could survive the current limits placed upon it, would STILL not damage the alternator stator.)
To damage the stator, at least 2 rectifier diodes would have to short, and survive the current flow from the battery into the stator windings (along with the associated wiring, which is highly unlikely.) I seriously doubt the 750 alternator could make enough power to damage itself due to physics limitations of the components and design.
A "new" regulator, likely replaces the relay with a solid state device which still controls the battery voltage via the alternator field coil strength, using the voltage fed to the field coils as a means toward that control. There is no valid reason why a "new regulator" would allow the battery or distributed system voltage to rise higher than 14.5 V. (Unless the "new regulator" designer was as daft as you are suggesting.)
Sorry, this is quite a thread-jack, off the original topic.
