I have a 240V Roto-Phase converter (ARCO Electric model M) in my shop installed by an electrician. A wire came loose last week and while debugging that I started thinking about the way it was wired and confused myself. My understanding of 3 phase power is that each leg is 120 degrees out of phase with the other legs. But based on the way my converter is wired, the single phase input passes straight the load unmodified. These legs appear to be 180 degrees out of phase to me if my thinking is correct instead of 120. Is this correct?

All single phase is two wires that conduct AC power. Consider a alternator with only one coil. That would be one sine wave. We don't consider the two wires out 180 degrees out of phase of each other. Just two conductors that are needed to complete a circuit from power source through a load.

3 phase, with adding two additional coils on an alternator, due to the rotating of those additional coils and the placement of those coils on a rotating shaft, the voltage output lags behind 120 and 240 degrees, or in other words, those other "phases" don't produce power until later, in the beginning, and continue to lag each other as the rotating continues.

A phase converter fakes a "phase" on a third conductor by starting and stopping a voltage by use of capacitors and relays.

A statement from:

Static phase converter, what is a static phase converter? Static vs rotary to generate three phase power



The static phase converter does not actually generate 3-phase power continuously as a rotary phase converter does, but only long enough for it to start up. Once the motor has started the static phase converter circuitry disconnects itself and the motor continues to run on single-phase power; because only two of three windings receive power during running,

I think the popular misconception is from the typical diagrams of single and three phase, each showing different data, for different purposes.

The single phase is showing how AC power starts at zero volts, increases to peak, back to zero, and so on, as related to voltage.

The three phase is really trying to show the same 3 AC power but each beginning at a different time, thus each "phase" 120 degrees apart.

I hope this helps.

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It's correct. Phase is a relative thing. When you're thinking about 3-phase power, you're thinking of the phases of each leg relative to the common neutral wire. In that case, it's true; their phases differ by 120 degrees. However, if you were to take any two of the three wires and measure the voltage between them, you simply get a sine wave, just as if they were 180 degrees apart (which they ARE -- relative to some OTHER reference). In other words, all three legs are mutually 120 degrees apart relative to the neutral line, but any two of them are 180 degrees apart relative to some other reference. Since a three-phase load (such as a motor on a tablesaw or a milling machine) doesn't actually use the neutral line, the reference is completely arbitrary. Hence what you see as 120 degrees of phase separation, the motor sees as 180 degrees.

In a typical 240-volt 3-phase system, each leg has an RMS voltage of 138.5 volts relative to the neutral, and they differ by 120 degrees, as you said. But if you take any two of the three legs and measure the voltage difference between them, it's 138.5*sqrt(3) = 240 volts. 208-volt 3-phase is more common; in that case all three poles are 120V from neutral and 208V mutually from each other.

Your RPC is simply using your two existing single-phase hot lines as two of the three legs for a 240-volt 3-phase system, as they meet the criterion that they differ from each other by 240 volts. The third leg is generated by the rotating 3-phase motor in the RPC and will differ from BOTH of the other two legs by 240 volts. As far as the 3-phase load on the business end of the RPC is concerned, this is indistiguishable from any other 3-phase power source. It doesn't care that the arbitrary reference voltage doesn't happen to be the same as the neutral leg in your single-phase system.

Does that make sense?

Tkrussel beat me to it!

Maybe, let me mentally chew on that awhile.

Note that I made a couple of minor corrections to my original answer with respect to the voltages I listed.