RMS (Root-Mean-Square) is the term used to
describe the effective or equivalent DC value of an
AC signal. Most digital multimeters use average
sensing RMS calibrated technique to measure RMS
values of AC signals. This technique is to obtain the
average value by rectifying and filtering the AC
signal. The average value is then scaled upward
(calibrated) to read the RMS value of a sine wave.
In measuring pure sinusoidal waveforms, this
technique is fast, accurate, and cost effective.
Inmeasuring non-sinusoidal waveforms, however,
significant errors can be introduced because of
different scaling factors relating average to RMS
values.
Average sensing RMS calibrated
AC True RMS, normally refers as True RMS,
identifies a DMM function that is AC coupled, and
responds accurately only to the effective RMS AC
component value regardless of the waveforms.
However, DC component plays an important role in
the distorted non-symmetrical waveforms, and will
also be of interest sometimes. A full wave rectified
sine waveform is a good example, and the AC true
RMS function will only give the AC component
reading which is at 43.6% of the total effective
DC+AC RMS reading.
AC True RMS
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DC + AC True RMS calculates both of the AC and
DC components given by the expression
2
2
DC +(AC rms) When making measurement, and
can respond accurately to the total effective RMS
value regardless of the waveform. Distorted
waveforms with the presence of DC components
and harmonics may cause:
DC+AC True RMS
AC bandwidth of a DMM is the range of
frequencies over which AC measurements can
be made within the specified accuracy. It is not
the frequency measurement function, and is the
frequency response of the AC functions. A DMM
cannot accurately measure the AC value with
frequency spectrums beyond the AC bandwidth
of the DMM. Therefore, wide AC bandwidth plays
an important role in high performance DMMs. In
reality, complex waveforms, noise and distorted
waveforms contain much higher frequency
spectrum than its fundamental.
1) Overheated transformers, generators and
motors to burn out faster than normal
2) Circuit breakers to trip prematurely.
3) Fuses to blow
4) Neutrals to overheat due to the triplen
harmonics present on the neutral
5) Bus bars and electrical panels to vibrate
AC Bandwidth
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