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AC Metrology
11 questions By Tony R. Kuphaldt
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Question 7 of 11
What does it mean if a meter movement is described as being RMS indicating, average responding?
Reveal answerIt means that the meter movement’s indication is naturally proportional to the average value of the measured AC signal, but its calibration is skewed to represent RMS value when measuring a sinusoidal signal.
Challenge question: if one of these meters is subjected to a square-wave AC signal, will its “RMS” indication be falsely low, falsely high, or accurate?
Notes:This concept is confusing to many students, yet important for them to understand. I suggest you illuminate the subject by asking a series of questions:
- What would one of these meters indicate if the amplitude of a sinusoidal signal were doubled? Answer: the indication would double.
- What would one of these meters indicate if the signal amplitude and wave-shape were to change in such a way that the average value of the signal doubled, but the RMS value did not increase as much? Answer: the indication would double.
- What would one of these meters indicate if the signal amplitude and wave-shape were to change in such a way that the RMS value of the signal doubled, but the average value did not increase as much? Answer: the indication would increase only as much as the average value increased.
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Question 8 of 11
Electrostatic meter movements use the physical attraction between metal plates caused by a voltage to deflect a pointer, instead of using electromagnetism as is common with most other meter movement designs. Although electrostatic meter movements are not as sensitive as PMMC mechanisms, they have the advantage of being able to measure both AC and DC with equal ease.
Suppose you calibrated an electrostatic meter movement from 0 volts to 500 volts DC. Then, you connected this meter to a sinusoidal AC source and watched it register a voltage of 216 volts. What is the voltage of this AC source, in volts RMS?
Reveal answer240 V RMS
Hint: most mechanical meter movements naturally indicate the average value of an AC waveform!
Notes:Discuss with your students the reason why most meter movements function on the average value of an AC waveform. Why don’t meter movement naturally indicate RMS value? Ask your students how “average” and “RMS” AC values are defined, and what physical systems represent these mathematical models.
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Question 9 of 11
Most electromechanical meter movements are inherently average-responding. They display their indications in units of volts or amps “RMS” only because they have been calibrated to do so for sinusoidal waveforms.
Some electromechanical meter movements, though, are true-RMS responding. For example, electrodynamometer movements, when connected as either voltmeters or ammeters (not as wattmeters), naturally provide indications proportional to the voltage’s or current’s true RMS value.
Based on the inherent differences between these meter movements, describe how you could use electromechanical meter movements to perform qualitative assessments of waveform distortion. In other words, how could you use electromechanical meters to tell whether an AC waveform was sinusoidal or not?
Reveal answerTake an “average-responding” and a “true-RMS” meter that indicate equally when measuring sinusoidal waveforms, and compare their readings when measuring the AC waveform in question. The greater the difference between the two meter readings, the greater the distortion (from a sine-wave ideal).
Notes:Given the prevalence of harmonics in modern AC power systems, this “trick” can be quite useful in making qualitative assessments of harmonic distortion. Of course, expensive test equipment will give quantitative measurements of distortion, but it’s always nice to know how to use lesser test equipment just in case the expensive equipment is not available.
