240 volt 20 amp electric motor, run on 120 whats the rpm?

I have this motor with the following specs

i hooked it up to run on 120 volt, runs just fine, but i was wondering if there was a way i could determine the rpm, if it is different or the same. or if you can point me to some equations i could use to figure it out?

That motor is only designed to run on 208-230 volt single phase power. If you attempt to run it very long on 120 volt power you will burn out the windings. The speed depends on the number of poles inside not the voltage applied. The lowering of the voltage will increase the amperage draw to try and compensate for this. The internal wires are designed to handle no more than .92 amp at 230 volts and if you try to run it on 120 volts it will draw almost 2 amps of power. It may appear to run just fine now but it won't last long.

well bummer, good thing it was almost free, so my rpm will be constant, dont really care if i burn out the motor. but that tells me what i need to know, i was looking at it for attaching my barley crusher to.

If you keep the load low, the motor should last. The full current draw is only at full load (1/8 HP) so halving the load will bring the current more inline with the 0.84 amps at 60 Hz that it lists.

I doubt if it would last long enough to even be worth installing. The amp draw will increase by a massive amount as soon as you attempt to put a load on it. Stick it on craigslist for a couple bucks to help offset the cost of the right one. If you're looking for a cheap and fairly reliable motor for your crusher take a peak at Grizzly.com. They're made in china or taiwan but seem to be of a better quality than the typical stuff you get from there. I've been using one on my bandsaw for 3 years now and I abuse the heck out of that thing.

Can't you just install a dedicated 220v outlet for it? If not, you may be able to find a step-up transformer at a surplus place....

i had a dedicated circut , but i want to use it for a grain mill motor. so i was hoping to make it useful for 120.

If the motor is synchronous then you'll have trouble operating at a lower voltage as yjfun has said. However, if it's an induction motor then you're safe operating at a lower voltage, it will just operate at a lower speed and deliver less power. I looked around briefly, but I couldn't find a good place to find the model of the motor from the serial number.