HP means horsepower, i.e. power. Obviously, a 3/4HP motor is 3x as powerful as 1/4HP one. Power is the capacity to do work - more powerful means 'can work harder'.
It's got nothing to do with RPM as such. For a sander that would be how aggressively it can sand - in any given setup, a small motor will stall more easily than a bigger one. Gearing won't necessarily help - you'll get more torque in exchange for a slower speed, just like a car's gearbox. You might find a heavier disc will help, as its inertia will mean smoother sanding at higher speeds, but if it does slow it will take a while to pick up again - you can't create power where none exists.
If they're induction motors (and they most probably are), the RPM is determined by the motor design and the mains frequency. I'd expect it to be around 1400 RPM (wot my pillar drill motor is), but it should be on the data plate of the motor. BUT...
... there may be a gotcha: 10-pin bowling alleys use a lot of power. It is entirely possible that these are 3-phase motors: I'd expect the heavier equipment to be 3-phase (the pin handling system, for example). It makes sense for the whole system to be 3-phase for consistency.
Domestic wiring is single-phase, and you can't cheaply use a three phase motor on a single phase system (you have to generate the three phases yourself, and that needs an expensive bit of kit between the mains and the motor in question).
Suggestions:
1. Post photographs of the motor and its data plate, so we can all have a look.
2. Have a look at the specs for commercial disc sanders - what size motors do they typically have?
3. If they do turn out to be single-phase (but all the designs use 3/4 HP), you might still use them for a small lathe or similar. Lathes usually have a chuck or a faceplate that acts as a flywheel, and the cutting force is small compared to a pillar drill doing woodwork (my drill is 1 HP).
That said, I think you're being a bit optimistic about them being powerful enough for anything woodwork-ish.
Sorry to put a dampener on things.
E.
PS: You might want to delete the duplicate post in Design - that section is really for furniture designs, etc. rather than machine-making (although Mr. Wandel's stuff probably fits in both places!).
