This means a displacement between signals. One period is 360° and the phase is expressed as how an angle is a displacement between signals.
This derives from one period of a sine wave that the turbine of a dynamo generates when it revolves once. If the revolution of the turbine is viewed from a side, an end of the magnet moves up and down and the generated voltage accordingly moves up and down. If this is expressed on a graph as changes in time, sine waves are formed. These waves show that the winding direction of the coil alternates and that the phases of the generated voltage differ 120° from one another.
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Since power is generated as three-phase AC, one set of power lines consists of three phases. Household lighting uses one of these phases. Industrial power systems may use the three phases.
Periodical change of the same signal comes from the concept of such a revolution system. The same concept of phase is also applied to digital signals. For example, if a clock waveform with a duty factor of 50% is reversed, the phase shifting is 180°.
Although the above discussion focuses on the phase of a voltage change, there are also phases between a voltage and a current. The current lags behind the voltage 90° (+) in an inductance and the current leads the voltage 90° (-) in a capacitance.
Consequently, the phase of the voltage and current changes depending on the combined resistance, or impedance.