Frequency and phase measurement

Chapter 12.2 Frequency and phase measurement

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

  • 366CHAPTER 12. AC METERING CIRCUITSFigure 12.10: Toroidal current transformers scale high current levels down for application to 5 Afull-scale AC ammeters.• So-called true RMS meters use different technology to provide indications representing theactual RMS (rather than skewed average or peak) of an AC waveform.12.2Frequency and phase measurementAn important electrical quantity with no equivalent in DC circuits is frequency. Frequency measure-ment is very important in many applications of alternating current, especially in AC power systemsdesigned to run efficiently at one frequency and one frequency only. If the AC is being generated byan electromechanical alternator, the frequency will be directly proportional to the shaft speed of themachine, and frequency could be measured simply by measuring the speed of the shaft. If frequencyneeds to be measured at some distance from the alternator, though, other means of measurementwill be necessary.One simple but crude method of frequency measurement in power systems utilizes the principleof mechanical resonance. Every physical object possessing the property of elasticity (springiness)has an inherent frequency at which it will prefer to vibrate. The tuning fork is a great example ofthis: strike it once and it will continue to vibrate at a tone specific to its length. Longer tuning forkshave lower resonant frequencies: their tones will be lower on the musical scale than shorter forks.Imagine a row of progressively-sized tuning forks arranged side-by-side. They are all mountedon a common base, and that base is vibrated at the frequency of the measured AC voltage (orcurrent) by means of an electromagnet. Whichever tuning fork is closest in resonant frequency tothe frequency of that vibration will tend to shake the most (or the loudest). If the forks’ tines wereflimsy enough, we could see the relative motion of each by the length of the blur we would see aswe inspected each one from an end-view perspective. Well, make a collection of “tuning forks” outof a strip of sheet metal cut in a pattern akin to a rake, and you have the vibrating reed frequencymeter: (Figure 376,12.11)