Simple AC circuit calculations

Chapter 1.4 Simple AC circuit calculations

Lessons In Electric Circuits Volume II – AC Book
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Lessons In Electric Circuits Volume II – AC Book

  • 18CHAPTER 1. BASIC AC THEORYand below the “zero” line on a graph is zero. However, as a practical measure of amplitude,a waveform’s average value is often calculated as the mathematical mean of all the points’absolute values (taking all the negative values and considering them as positive). For a sinewave, the average value so calculated is approximately 0.637 of its peak value.• “RMS” stands for Root Mean Square, and is a way of expressing an AC quantity of voltage orcurrent in terms functionally equivalent to DC. For example, 10 volts AC RMS is the amountof voltage that would produce the same amount of heat dissipation across a resistor of givenvalue as a 10 volt DC power supply. Also known as the “equivalent” or “DC equivalent” valueof an AC voltage or current. For a sine wave, the RMS value is approximately 0.707 of itspeak value.• The crest factor of an AC waveform is the ratio of its peak (crest) to its RMS value.• The form factor of an AC waveform is the ratio of its RMS value to its average value.• Analog, electromechanical meter movements respond proportionally to the average value ofan AC voltage or current. When RMS indication is desired, the meter’s calibration must be“skewed” accordingly. This means that the accuracy of an electromechanical meter’s RMSindication is dependent on the purity of the waveform: whether it is the exact same waveshapeas the waveform used in calibrating.1.4Simple AC circuit calculationsOver the course of the next few chapters, you will learn that AC circuit measurements and calcu-lations can get very complicated due to the complex nature of alternating current in circuits withinductance and capacitance. However, with simple circuits (figure 27,1.23) involving nothing more thanan AC power source and resistance, the same laws and rules of DC apply simply and directly.10 VR1R2R3100 Ω500 Ω400 ΩFigure 1.23: AC circuit calculations for resistive circuits are the same as for DC.