What does RMS actually mean?

Answer 1

RMS stands for root mean square. RMS is a method for measuring complex AC waveforms. It produces a voltage reading where the constantly varying AC voltage would do the same work as the equivalent DC voltage.

For instance, the standard US outlet voltage is listed as 120V. If you look at the AC waveform on an oscilloscope, you would see the voltage vary from +170 volts to -170 volts for a total of 340 volts peak-to-peak.

Now, if you apply the voltage to, say, a heating element, the voltage would do the same work whether it was positive or negative, so the 170 volts is the value that we would use, not 340.

BUT, being a sinusoidal AC wave, the voltage is not always 170. It varies all the way from near zero to the peak value of 170, so the voltage would not heat the element as hot as a constant (DC) 170 volts would.

If you measure the voltage at many points along the waveform, square each reading, average or calculate the mean of all the squared readings, then take the square root of the average (mean), you end up with a number that represents an equivalent DC voltage that would do the same work. In our example, it would heat the heater as hot. Voila! RMS!

This average is well known for simple sine wave voltages, such as what you would find at your receptacle. The RMS value is 0.707 X the peak voltage. So our 170V example: 170 X 0.707 = 120.19. The opposite is RMS X 1.414 = Peak, or RMS X 2.828 = peak-to-peak. So, 120 X 1.414 = 169.68.

But what if the AC waveform is non-sinusoidal, such as music coming out of an amplifier or impuse noise, or whatever? This is where RMS measurements are really useful, enabling us to find the equivalent power, or noise figure, etc.

For example, the voltage that comes out of a lamp dimmer is all chopped up, definitely non-sinusoidal. If you try to measure this voltage with various standard meters, you will get readings all over the map, because each meter will interpret the screwy waveform differently. If you use a "True RMS" meter, the results will be repeatable and stable, allowing valid comparisons to be made.

By the same token, amplifiers that are rated in "RMS Watts" are fairly well representative as to the actual power you can expect them to produce. Some manufacturers rate their amps in what they call "Peak Power". Remember above how an RMS voltage of 120 is also a peak voltage of 170? This lets the unscrupulous manufacturers put bigger numbers to their amps, making them appear more powerful. Now you know better!

Answer 2

Watts root mean square is the effective value of alternating current electrical power compared to direct current power. In AC there is a rms or effective value of voltage and a rms or effectice value of amperage. Power is volts rms times amps rms. That is the AC watts compared to direct current power. There cannot be RMS power or watts RMS. Scroll down to related links and look for "Why there is no such thing as 'RMS watts' or 'watts RMS' and never has been". RMS watts is meaningless, but we use that term as "an extreme shorthand" for power in watts calculated from measuring the RMS voltage.

Answer 3

RMS = root mean square. The RMS of an AC voltage is VAC / sqrt(2) where VAC is the voltage peak to neutral. typical home voltage is 110 - 120volts RMS, if you took an oscilloscope and looked at it, you'd notice the voltage peak is closer to 170 volts.

Answer 4

RMS stands for Root Mean Square.

It is a method of evaluating the "average" level of a sine wave such as an AC voltage.

If we take readings of a voltage throughout a complete period of an AC voltage (i.e. one cycle) we will have positive readings and negative readings because the signal is negative for as long as it is positive on each cycle. Use these figures to obtain the average voltage and the result will be zero. So any AC voltage has an average voltage of 0V. It's absolutely right but completely useless as it tells us nothing about the size of the sine wave.

If we take the same values and square each of them we will end up with a set of positive values because negative numbers multiplied together will always produce a positive result. If we take the square root of each of the values, we get the same figure as we started with for all positive voltages and we have a positive value for all negative numbers but the same magnitude.

Taking an average of the values gives us a useful result to define the voltage of an AC signal.

Having taken the time to understand RMS, for most cases it is enough to know that the RMS value of a sine wave is about 0.71 x the peak. Conversely, the peak voltage is 1.41 x the RMS voltage. 110V AC therefore will have a peak voltage of 155V.

Alternative Answer

An r.m.s. value is the amount of a.c. current that will do precisely the same amount of work as a given value of d.c. current. For example, 10 A (rms) will do the same amount of work as 10 A d.c. Since voltage and current are proportional, r.m.s. values also apply to voltages. It provides a practical means of comparing the values of a.c. and d.c. quantities.

Answer 5

Root Mean Square. It's useful primarily when comparing DC to AC