Here is some of my disagreement with the following statements you made:
You stated . . .
"the pulley used on a alternator can be sized for higher rpm at idle and all the way to max rpm of the engine. This increases the power available at all engine rpm."
The reality is, you can change pulley ratios on either for more output. Going by actual shaft RPM at the generator or alternator - a typical 12 volt tractor generator can output 5-20 amps at 1600 shaft RPM. A typical alternator, e.g. a Delco 10SI makes 23 amps at 1600 shaft RPM, a 12SI 30 amps at 1600 shaft RPM, and a 15SI 40 amps at 1600 shaft RPM. My point is that alternators make more power at the SAME RPMs as generators. But yes, alternators are rated for higher revs, and they are often on engines that might spin 7000 RPMs. Not a problem on a 2400 RPM tractor.
You stated about alternators . . .
"Another advantage is that the field is all winding and no magnets. This enables full excitation at low rpms "
What we are calling "generators" and "alternators" both use electromagnets for field power, and both also carry residual magnetism. Just happens that generators rely on that residual magnetism to get started in most setups (not all), whereas alternators use battery power. That being said, many alternators can self-field if you spin them fast enough.
You stated . . .
"The major disadvantage of the alternator is the diodes and the voltage regulator are built in. Very few people can take an alternator apart and replace the diodes or brushes and or the voltage regulator"
Very few people ?? In what context? I can change the brushes in a Delco 10SI in a few mintues without even taking it apart. When it comes to people who actually have mechanical expertise, there is little to NO difference. Especially when it comes to very simple alternators like Delco 10SIs or 12SIs. A few bolts and they come apart very easy. A person can swap in a new regulator and diode trio in a few minutes. Granted, there are newer models that require desoldering to take apart (e.g. the CS series), but they rarely appear on old farm tractors. How many people can turn the armature in an old generator or change out the field coils?
You stated . . .
"Generators seldom break down suddenly. Voltage regulators fail more often , but if you pull the cover and push the right coil you can start charging and drive home or finish your job. "
Seems like you've gone in a circle here. You say "most" people can't take apart a simple alternator, yet you assume these same people know how to work on a mechanical regulator? That is a huge stretch. The reality is, they all fail now and then - but alternators tend to be more reliable overall for several reasons. Brushes last much longer since they don't carry charge current, and the regulators are fully electronic (Delco 10SIs and 12SIs).
Getting away from tractors and speaking on cars. I was a mechanic when half the cars on the road had generators instead of alternators. If anything, we had more problems with them, not less.
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