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AC phase
24 questions By Tony R. Kuphaldt
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Question 19 of 24
In this phasor diagram, determine which phasor is leading and which is lagging the other:
Reveal answerIn this diagram, phasor B is leading phasor A.
Follow-up question: using a protractor, estimate the amount of phase shift between these two phasors.
Notes:It may be helpful to your students to remind them of the standard orientation for phase angles in phasor diagrams (0 degrees to the right, 90 degrees up, etc.).
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Question 20 of 24
Is it appropriate to assign a phasor angle to a single AC voltage, all by itself in a circuit?
What if there is more than one AC voltage source in a circuit?
Reveal answerPhasor angles are relative, not absolute. They have meaning only where there is another phasor to compare against.
Angles may be associated with multiple AC voltage sources in the same circuit, but only if those voltages are all at the same frequency.
Notes:Discuss with your students the notion of “phase angle” in relation to AC quantities. What does it mean, exactly, if a voltage is “3 volts at an angle of 90 degrees”? You will find that such a description only makes sense where there is another voltage (i.e., “4 volts at 0 degrees”) to compare to. Without a frame of reference, phasor angles are meaningless.
Also discuss with your students the nature of phase shifts between different AC voltage sources, if the sources are all at different frequencies. Would the phase angles be fixed, or vary over time? Why? In light of this, why do we not assign phase angles when different frequencies are involved?
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Question 21 of 24
Determine the total voltage in each of these examples, drawing a phasor diagram to show how the total (resultant) voltage geometrically relates to the source voltages in each scenario:
Reveal answer
Notes:At first it may confuse students to use polarity marks ( and -) for AC voltages. After all, doesn’t the polarity of AC alternate back and forth, so as to be continuously changing? However, when analyzing AC circuits, polarity marks are essential for giving a frame of reference to phasor voltages, which like all voltages are measured between two points, and thus may be measured two different ways.
