06/04/2026
INSTANTANEOUS WAVEFORM EQUATION
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⚡ Instantaneous Waveform Equation
📌 Where:
🔹 Amax – is the maximum amplitude of the waveform.
🔹 ωt – is the angular frequency of the waveform in radian/sec (could also be: 2πƒt)
🔹 Φ (phi) – is the phase angle in degrees or radians that the waveform has shifted either left or right from the reference point.
⬅️ If the positive slope of the sinusoidal waveform passes through the horizontal axis “before” t = 0 then the waveform has shifted to the left. So Φ > 0. Thus the phase angle will be positive in nature, +Φ giving a “leading phase angle”. In other words it appears earlier in time than 0° producing an anticlockwise rotation of the vector.
➡️ Likewise, if the positive slope of the sinusoidal waveform passes through the horizontal x-axis some time “after” t = 0 then the waveform has shifted to the right. So Φ < 0. Thus the phase angle will be negative in nature -Φ producing a “lagging phase angle” as it appears later in time after 0° producing a clockwise rotation of the vector. Both cases are shown below.
🔄 Phase Relationship of a Sinusoidal Waveform
📌 Firstly, let’s consider that two instantaneous quantities such as a voltage, v and a current, i have the same frequency ƒ in Hertz and both start at t = 0. As the frequency of the two quantities is the same their angular velocity, ω must also be the same. So at any instant in time we can say that the phase of voltage, v will be the same as the phase of the current, i.
📊 Then the angle of rotation within a particular time period will always be the same. The difference between the two quantities of v and i will therefore be zero. That is: Φ = 0.
✅ As the frequency of the voltage, v and the current, i are the same they must both reach their maximum positive, negative and zero values during one complete cycle at the same time (although their amplitudes may be different). Then the two alternating quantities, v and i are said to be “in-phase”.
📉 Two Sinusoidal Waveforms – “in-phase”
⚡ Now let us consider that the voltage, v and the current, i have a phase difference between themselves of 30°. Therefore, (Φ = 30° or π/6 radians).
📌 As both alternating quantities rotate at the same speed, they will have the same frequency. Therefore this phase difference will remain constant for all instants in time. Thus the phase shift of 30° between the two waveforms is represented by phi, Φ as shown below.
📐 30° Phase Difference of a Sinusoidal Waveform
📊 The voltage waveform above starts at zero (0°) along the horizontal reference axis. However, at that same instant of time the current waveform is still negative in value and does not cross this reference axis until 30° later.
📍 Then we can see that there exists a difference between the phases of the two waveforms as the current waveform crosses the horizontal reference axis reaching its maximum peak and zero values 30° after the voltage waveform.
❓ What are Leading and Lagging Waveforms?
⚠️ Since the two sinusoidal waveforms are no longer “in-phase”, they must therefore be “out-of-phase” with each other by an amount determined by phi, Φ and in our simple example this is: 30°. So we can say that the two waveforms are now 30° “out-of-phase” with each other.
⏱️ Then the current waveform can also be said to be “lagging” behind the voltage waveform by the phase angle, Φ of 30°. Thus the two waveforms have a Lagging Phase Shift and the expression for both the voltage and current above is given as:
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✍️ Sisira Senevirathna