If you are acting as a fabricator and are purchasing the driver for the generator and making up the unit then:
I'd likely start with looking at the mfg design data - probably then call the mfg if not clear specs on what they recommend for a driver.
If this is a curiousity issue, then:
T = Hp x 5252/N, where N = rpm, T is in ft-lbs
Hp = kw/.746
So, T(required) = [kw(gen)/.746] x 5252/N
for a 1800rpm, 100kw gen:
T(required) = (100/.746) x 5252/1800 = 391 ft-lbs
That's a driver producing 134hp at 1800rpm.
If you are using this to size a driver, don't forget there some efficiency issues involved - you will need to oversize the driver.
cf
