I was just curious. My friend and I were talking, and he owns a CNC machining company, and when we were at his shop we were talking electrical stuff and he mentioned that his lathe used three phase power. So I asked him what that was and what was different about it compared to normal residential single phase. He didn't really know. I've been researching it online because I'm curious, but I haven't really gotten a good explanation. So I was wondering, what is three phase, and why would certain machines like his lathe need it, and what is really different about it compared to single phase? And where does 120, 240 volt fit in and is there two phase? I know, a lot of questions about something that I'm sure I will never need, but I'm curious. Thanks pros.

Simple answer. Three phase makes a high Horsepower motor less expensive. Just price a 5 HP single phase motor and a 5 HP 3 phase motor. Try finding a 250 HP single phase motor.
The wierdness that has to happen to get a single phase motor to start is eliminated and there is no start winding, hence less expensive. Capacitor start and centrifical switches are used to start single phase motors. One rule might be if rating is < 1HP single phase is an option. If it's 5 HP, 3 phase probably makes better sense.

Three phase motors get upset when they lose one of their phases and might self destruct, so additional protection methods must be employed. The simplest is a motor starter. Short term startups and short circuit protection are handled by two different systems. Phase imbalance and low voltage protection might be employed on critical loads.

3 phase motors start smoothly under load. The motor has 3 windings and is supplied with voltage 120 degrees apart.

The single phase system is probably a bi-phase system, but all it means is that the center ta of the secondary is grounded so you have 120-0-120. The voltages are 180 degrees apart from ground, but the distribution voltage in the KV range is single phase.

I've actually heard of an obsolete two-phase power system that was used.

The second question is harder. For single phase in the US, 120 and 240 are used for single phase residential. The higher the voltage, the less copper you use.

With three phase, it depends on how the 3 transformers are connected. You can have a "Y" connection or a "Delta" connection and you can transform between them.

With the Y connection, you get voltages like 125, 208, 277 and 460V. 277 is commonly used for lighting. With delta, common voltages are 120, 240 and 480. I hope my numbers are right.

There is a lot more to be learned.

That's cool, you don't have to go into a ton of detail. It makes sense now. He is running a lathe, which employs an electrical motor. So this would make his machine cheaper to run three phase. Interesting thing is, he has 200 amp service for his lights, welders heating and everything else, but uses a three phase gas generator for that lathe. He doesn't have three phase coming from the utility.

If you reverse 2 of the lines for 3 phase, Motor will turn opposite direction.
Remote Controlled AirCraft motors usually have 3 wires from controller to motor, and to reverse direction, Swap any 2 of the 3 wires. Since this is true.
You could take 2 wires from controller and 2 that go to the Motor and connect to Double pole double throw relay, crossing the corners like a polarity switch. This way your Aircraft will have reverse thrust. You could come in for a landing, pull power back, hit the reverse switch, Then a burst of power to stop"SHORT".
I also want to do from an altitude, and see the affect in the air, then recover.

Three phase is usually a "Business only" utility option. Residential three phase does occur, but is extremely rare. Business typically have motors for AC's, Cooling pumps. I building I worked in had 32 heat pumps.

Manuacturing requires even more motors.

There is a relatively easy way to make a three phase motor variable speed and operate on single phase, again for low HP motors.

I'd look at the nameplate of the motor and pay attention to HP and VA. There were probably loots of variable that led to the 3 phase lathe.
1) Cost o convert motor
2) Cost to get 3 phase power from utility
3) Avaiability of power in the panel
4) Cost of the lathe and late generator combination
4) Wheher it was bought used

This is just some ideas.