给你找了一下wiki

https://en.wikipedia.org/wiki/Split-phase_electric_power

The two 120 V AC lines are supplied to the premises from a transformer with a 240 V AC secondary winding which has a center tap connected to ground. This results in two 120 V AC line voltages which are out of phase by 180 degrees with each other. The system neutral conductor is connected to ground at the transformer center tap. 240 V AC can be obtained by connecting the load between the two 120 V AC lines.