Owner`s manual

34
RMS, or Root Mean Square, is the measurement used for any time varying signal’s effective value: it
is not an “Average” voltage and its mathematical relationship to peak voltage varies depending on the
type of waveform. By definition, RMS Value, also called the effective or heating value of AC, is
equivalent to a DC voltage that would provide the same amount of heat generation in a resistor as the
AC voltage would applied to the same resistor.
Since an AC signal’s voltage rises and falls with time, it takes more AC voltage to produce a given
RMS voltage. In other words, the grid must produce about 169 volts peak AC that turns out to be 120
volts RMS (.707 x 169). The heating value of the voltage available is equivalent to a 120 volt DC
source. (This is for example only. This does not mean that AC and DC are interchangeable!).
The typical multi-meter is not a True RMS reading meter. As a result it will only produce misleading
voltage readings when trying to measure anything other than a DC signal or sine wave. Several
types of multi-meters exist, and the owner’s manual or the manufacturer should tell you which type
you have. Each handles AC signals differently.
A rectifier type multi-meter indicates RMS values for sine waves only. It does this by
measuring average voltage and multiplying by 1.11 to find RMS. Trying to use this type of
meter with any waveform other than a sine wave will result in erroneous RMS readings.
Average reading digital voltmeters are just that, they measure average voltage for an AC
signal. Using the equations for a sine wave (see below), average voltage (V
avg
) can be
converted to Volts RMS (V
rms
), and doing this allows the meter to display an RMS reading
for a sine wave.
A True RMS meter uses a complex RMS converter to read RMS for any type of AC
waveform.
When taking reading with a non-True RMS reading meter, a 120 Volt RMS sine wave will still
measure about 120 Volts RMS. This is because the meter uses the mathematical relationships
shown below to give a proper RMS reading for a sine wave. However, if used with a modified sine
wave or square wave, these meters will only read about 90-105 volts. Don’t be mislead, there is
nothing wrong with the inverter or the meter. To prove this, try the following test. Connect a normal
light bulb to the AC output and allow the inverter to power the bulb (AC source power turned off). If
there is only 90-105 volts RMS available, the bulb will glow orange as it would during a brown out. If
it appears normal, the voltage is approximately 120 VAC RMS. You can see that improper
measurement can easily lead someone to believe that the inverter is not putting out its rated power.
Normally, True RMS reading meters are very expensive, such as the Fluke 87 series meters.
However, there are now models of True RMS reading meters available on the market for under
$100.00. Check with Radio Shack for information on the True RMS meters that they carry.
A few things to keep in mind about RMS values that apply when dealing with a sine wave are as
follows:
Peak Volts AC x .707 = V
rms
V
rms
= 1.11 x V
avg
1.414 x V
rms
= Peak Volts AC
V
avg
= .637 x Peak Volts AC
For a modified sine wave or square wave these equations do not apply and the easiest way to deal
with this is to invest in a True RMS reading meter. (For a square wave, V
avg
, V
rms
and V
peak
are all
equal).
6.1 RMS VALUES, METERS and MEASUREMENT