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6.1 If the phase angle between the voltage and the current is 30 degrees, what is the value of the real power as a percentage of maximum?
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A: cos30degrees = 0.866; therefore, the real power = 86.6% of maximum
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6.2 Why is the amount of power that is used zero when the voltage and current are 90 degrees out of phase with each other?
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| A: Because the cosine of 90 degrees is 0. (b) because the product of two sinewaves that are 90 degrees out of phase with each other is a third sinewave of twice the frequency, half of which is positive and half of which is negative, and the two halves cancel each other out. |
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6.3 What does a negative value of power imply?
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| A: It implies that the load is sending back power to the supply. |
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6.4 When is the value of AC power and the value of DC power the same?
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| A: When the DC voltage and current match the AC RMS voltage and current, and the phase angle is 0 degrees. |
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6.6 What is the power factor if the phase angle between the voltage and current is 50 degrees?
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6.7 Is it possible to have a power factor greater than 1? Why or why not?
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| A: No, because the cosine of an angle can never be greater than 1. |
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6.8 Why is a low power factor undesirable?
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| A: Because it requires more current- handling capability, including bigger transformers, switches, cables, transmission towers, labor, and other resources, for the same amount of power transferred. |
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6.9 Why is a luminaire with a high power factor smaller and lighter than an equivalent one with a lower power factor?
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| A: Because there is less current flowing through it, it can use smaller wires, fuses, components, and hardware. |
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6.10 How can a low power factor be corrected?
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A: If it's an inductive load, add a power factor correction capacitor; if it's a capacitive load, add a power factor correction inductor. |
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6.11 If, in a highly reactive load, the current is high but the consumed power is low, what happens to the power in the system?
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A: It moves back and forth between the load and the source. |
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6.12 What is wattles power?
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| A: Power that does no work, i.e., reactive power |
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A: Power that does work; the power consumed by a load |
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6.14 What is complex power?
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| A: The vector sum of real and reactive power. |
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