Dan I hope this comes up in the right place.
My comments were intended for those who want a normal cheap method of producing ac power. I entirely agree with you that for someone who wants to do it themselves and are prepared to accept the high material cost then the permanent magnet scheme is perfectly viable.
I also agree about belts, vee belts are not too clever except at full load, timing belts are better but have other issues and the pulleys are costly.
Commercial alternators are generally no better than 80% efficient at that power level and the self excited capacitor things that now seem to dominate the market will have no better regulation than a decent pma. They are speed sensitive and as with the pma, the regulation will be significantly influenced by the engine governors.
Many loads are not particularly frequency or voltage sensitive these days with built in regulators so that is not the issue that it used to be.
At 600 rpm, with decent diameter discs, 4kw should be reasonably easy to obtain at 120v ac. May be worth incorporating a battery charger winding as you did on the other one or it may be simpler to use 3 discs with ac and battery charging stators.
It would not be rocket science to add a servo speed adjuster to the engine governor to maintain a very good compromise between voltage and frequency, with a slightly rising frequency with load in place of the normal engine drooping characteristic.
Nice project.
Flux