Phase is used in electrical systems to refer to alternating current, where voltage and current oscillate in a sinusoidal fashion. A single phase power supply has one live wire that connects to the load and one neutral wire that connects to the power source. A phase is the voltage change that occurs in a specific period of time as a power generating unit rotates. If the voltage and current peak simultaneously on a resistive load, they are said to be in phase. The voltage and current won’t peak when a capacitor and/or inductor are added to the load. They will be out of phase. In conjunction with the rotation of the power generating coil, the phase lead and lag are expressed in angular units.
A three-phase supply is when there are three power generating coils connected in either a Y (star), or delta configuration. Each are physically displaced at a angle of 120 degrees.