Regarding 8-pole generators:Because it costs much more to make a 8 pole head than a four pole this is justified as the really big engines just don`t spin fast. Since most prime movers in the small sizes run fast enough to spin 1800 RPM, finding small 8 pole heads is tough. There is little need for them to justify the higher manufacturing complexity and cost. As a result they are quite rare.
I don't know very much about how generators are constructed, could you explain why an 8-pole unit would be more complicated than a 4-pole? I am aware that slow speed motors tend to have larger frame sizes for the same Hp, and I assume this would be the case with a generator too, but what other factors would go towards the higher cost?
Regarding toothed belts:I once coupled a 150 Hp electric motor to a 250 KVA generator using a 4" wide cog belt. (This made a rough and ready frequency convertor with a variety of output voltage options--and it's still in service some 8 years later.) Even though the shaft speeds were relatively low (6-pole motor running on 60 Hz with a 4-pole generator producing 50 Hz power), the "singing" of the belt was amazingly loud and was unpleasant to be around. I do understand that there are a variety of different tooth profiles available and it may be possible to minimize that problem.
Also regarding toothed belts:It would be easy to make a 2:1 ratio using standard automotive camshaft drive parts. This would work OK with the higher speed Listeroids (900 RPM engine speed). One would have to investigate the power handling capacity, of course. On the other hand, my 4" belt setup was rated to carry 300 Hp, so a 1" belt could probably handle 6 Hp easily even with the handicap of small pulleys and the consequent low belt speed.
A final question about toothed belts:How do the loses compare between serpentine and toothed belts anyway?
Best regards,
Andy Hall