Phase

When there are two or more waveforms with matching periods in voltage or current, the numerical value that indicates the temporal shift of the starting point of each waveform is expressed in angle (degrees or rad). In AC voltage, phase is important, and when operating generators in parallel, not only the voltage waveforms but also their phases must match completely. When the phases completely match, it is called in-phase. In three-phase AC voltage, the phase of each voltage waveform is offset by 120 degrees (2/3π rad).