oscilloscope, or with a high-end DVOM that
also has a “peak capture” feature, you can get a
pretty good indication of whether excessive
distortion exists in an AC power supply. This
“peak” is the maximum point we see when look-
ing at the sine wave on an oscilloscope; i.e., the
highest voltage measured in the AC waveform.
Crest factordescribes the ratio of the peak
value of a measured waveform to its root mean
square (RMS). A pure, undistorted sine wave’s
crest factor, therefore, is described by the
equation:
1 0.7071.414
In other words, the peak voltage value of an
undistorted sine wave is 1.414 the true RMS
value. Any variation from this value indicates a
distorted waveform and bad harmonics. Using
a peak-capture-capable DVOM, here’s how to
perform these measurements, step by step:1 First, measure the true RMS value of the
voltage at a receptacle. When using a true
RMS meter, the reading will be automatic
if you are set to the VAC scale on the meter.
2 Multiply that reading by 1.414 to estab-
lish the theoretical peak value.
3 Check the actual peak value with the peak
capture feature on your meter.
4 Compare the actual value with the theoret-
ical value. If they differ by more than 2%
or 3%, excessive harmonic distortion exists.To differentiate between voltage and cur-
rent harmonics, consider these points:- For voltage harmonics, the typical crest
value will be less than the product of the
RMS value 1.414. On an oscilloscope,
this will appear as a “flat top” on a sine-wave
curve, as shown in the top photo opposite. - For current harmonics, the typical crest
value will be greater than the product
of the RMS value 1.414. On an
oscilloscope, this will appear as an abnor-
mally high peak.As an example, let’s use the numbers dis-
played in the bottom left photo opposite to
calculate distortion. To keep things simple,
we’ll round the RMS value down to 121.
We begin by determining the theoretical
peak by multiplying the RMS value by 1.414:
121 1.414171 VAC
Our actual measured peak (see Peak Max
on scope), at 161 V, is lower than that value,
which would appear as a flat-top waveform on
an oscilloscope.
Subtract the measured peak from the theo-
retical peak:
171 161 10 V
Divide the difference by the theoretical peak:
10 173 6%
Since 2% to 3% is considered the allow-
able limit, this system has excessive distortion.Current Multiplication
Excessive current in neutral conductors is a
common phenomenon in the three-phase
power supplies (where three sinusoidal volt-
ages are generated out of phase with one
another) typical of many marinas. Before we
leave this discussion of harmonic distortion,
we need to understand how and why it occurs.
In a well-balanced, three-phase AC circuit
(which is how all AC power begins in shore-
power installations), there are always har-
monics as the wave of the current alternates.
Even-numbered harmonic currents cancel
each other out; odd-numbered harmonics do
not—in fact, they add algebraically, causing
the neutral conductors in a three-phase system
(the white wire in North America) to carry as
much as 180% of the amperage carried by
each of the three individual phase conductors.
This particular problem has been identified in80 electrical systems troubleshooting