The initial phase reflects the starting point of alternating current, which is related to the choice of starting time. The initial phase can be positive or negative. If the instantaneous value of sine is positive when t=0, its initial phase is positive angle; If the instantaneous value of sine is negative when t=0, its initial phase is negative angle.
From the sine voltage expression u(t) = Um? Sin (Ω t+θ) indicates that the initial value (when t = 0) reflecting sine quantity is
u(0) = Um？ sinθ
Here, θ reflects the initial value of sinusoidal voltage, which is called initial phase.
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Different phases correspond to different instantaneous values, so the phase reflects the sinusoidal change process.
The initial phase θ and the phase (Ω t+θ) are in radians, which are commonly used in engineering in degrees. In sinusoidal AC circuits, sinusoidal quantities with the same frequency are often encountered, but their amplitudes and initial phases are different. The two sinusoidal voltages shown on the right have the same frequency, different amplitudes and initial phases, which are respectively expressed as
u 1(t)= u 1 msin(ωt+θ 1)
u2(t) = U2m？ sin(ωt +θ2)
Different initial stages show that their speed of change with time is inconsistent. For example, they cannot reach their respective positive maximum or zero at the same time. θ 1? & gtθ2，u 1 vs u2？ The positive maximum value is reached first, and u 1 is one phase ahead of u2 (θ 1? -θ2), or u2 lags behind u 1 an angle (θ 1- θ2).
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