What does True RMS mean?

Answer 1

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.

Have fun

Answer 2

True Rms Calculation

Answer 3

When measuring the value of an alternating current signal it is often necessary to convert the signal into a direct current signal of equivalent value (known as the RMS, Root Mean square, value). This process can be quite complex (see Root mean square for a detailed mathematical explanation). Most low cost instrumentation and signal converters (for example handheld multimeters of the sort used by maintenance engineers) carry out this conversion by filtering the signal into an average value and applying a correction factor.

The value of the correction factor applied is only correct if the input signal is sinusoidal. The true RMS value is actually proportional to the area under the curve, and not the average value of the curve itself. For any given waveform the ratio of the average value to the area under the curve will be constant and as most measurements are carried out on what are (nominally) sine waves the correction value assumes this waveform, but any distortion or offsets will lead to errors. Although in most cases this produces adequate results, a correct conversion or the measurement of non sine wave values, requires a more complex and costly converter, known as a True RMS converter.

Answer 4

RMS = Root Mean Square

Answer 5

Less expensive voltmeters and ammeters assume voltage and current is in phase and sinusoidal. If it is not you will get false results from them if the load on a circuit is reactive such as a motor or if the wave shape is not a pure sine wave.
True RMS instruments can correctly resolve various phase relationships and wave shapes to give you a true RMS reading of voltage and current.

Answer 6

RMS voltage is the effective voltage of a periodic signal. One of the previous posters gave the shortcut equation for finding the RMS voltage of a sine wave.

The actual formula for RMS Voltage for any periodic signal is:

Vrms = sqrt { 1/T * integral [ v^2(t) dt ] } over the interval 0 to T
where v(t) is the equation for your signal, and T is the signal's period. The result usually simplifies to an easier equation, but it's different for different types of signals.

Examples:

For a sine wave, the formula is Vp/sqrt(2) where Vp is the peak voltage

For a sawtooth wave, the formula is Vp/sqrt(3)

For a full-wave rectified signal, the formula is Vp/sqrt(2)

For a half-wave rectified signal, the formula is Vp/2

Answer 7

Root Mean Square (velocity)

Answer 8

Root mean square.

If my memory serves me coorectly it is calculated something like this:

Vrms=V*SQURT(2)/2

Answer 9

root-mean-square :)