What is the meaning of "true RMS"?
I have a fluke multimeter with true RMS but nowhere in the manual dose it explain what true RMS is.
To understand the meaning of true Root-Mean Square (RMS) one need to go to its mathematical and a more pragmatic physical definition.
Since we are dealing with a changing quantity we need to compute its average value over a period, however it also has its positive and negative components. This is why we need RMS.
An alternating voltage can be expressed as V(t)=V(peak) sin(2Pi f t). This is an equation for a signal of frequency f and amplitude V(peak)
The RMS term comes from statistics and can be stated as
V(rms)=sqrt((1/N)(sum of (V(i)^2) form 1 to N) for single frequency N=to the period.
When it comes to a real world of periodic signals the signal can vary in frequency and complexity. Fortunately any complex periodic signal can be decomposed into sine-waves of many amplitudes and frequencies. As you probably have guessed this could be a computational nightmare.
A practical approach would be to apply its physical definition.
V(rms)=sqrt(P(avg)/R)
The true RMS voltage is measured using the following method.
P(avg) is the average power dissipated in a resistor R when a RMS voltage V(rms) is applied. The power is measured by a thermistor and the scale of the meter is calibrated as a ratio of temperature to resistance. This is true for any waveform and frequencies.
Note that most meters you use are not true RMS meters. They measure only an average or a DC equivalent of a 60Hz pure sine wave.
For a sine wave
V(rms)=0.707 V(peak) See references supplied.
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