A 10 hp 50 rpm generator will be as large as a 360 hp 1800 rpm induction motor. Maybe divide by two for the efficiency and power density scaling according to the 4/3rds power law... yeah, there's ways to make it smaller with nothing but the most expensive electrical steel and a halbach array of neodymium magnets.. but i don't think that will get it anything smaller than the physical equivalent of a 100 hp 4 pole motor.. going to weigh about a ton, half of it at 9$ a pound for copper and you don't want to know what the magnets will cost..
and the efficiency of such a machine would be nearly the same as a gearbox to take that 50 rpm and turn it into 1800, or 1200.
I suggest you try and find the largest commercially available timing belt for a ~500 hp engine, and cast a couple aluminum pulleys as large as practically possible for another 5:1 increase, and then another 5:1 increase with smaller belting.. you could then back feed the grid with a 1200 rpm induction motor.. and such an arrangement can be legal if you can find an inspector who can figure out that induction motor's need reactive power from the grid to maintain excition.
if a 900 rpm induction motor can be found then obviously use that.. less efficiency loss in the belting, but the 900 rpm motors are less efficient than 1200 rpm units.
