Nick,
A 3ph motor won't start on single phase without some help...
I'll take a guess, someone has rewired the internal motor connections to delta and conected a capacitor from L1 or L2 to L3.
Then it will start and run fine, but at 1/2hp
Chris, please don't try this. First you need to know exactly what you are doing as either a: you'll kill yourself, b: you'll burnt the motor out in 5 min flat.
It's slightly more complicated than what I suggested above, you need propper test equipment in order to get the right cap values (there will be two of them and some sort of actuator)
Nobody can tell you what the cap value will need to be as they are all slightly different and it's found under testing at load.
It would be a simple matter to take the 3ph motor out and swap it for a single phase unit. Don't look for a new one, second hand you can find them for a tenner or so or even free. I have quite a few spare motors collected over the years.
Nick, sounds like someone has turned your motor into it's own 3ph generator.