Application Note

5 Fluke Corporation Troubleshooting power harmonics
To determine whether you have
a harmonics problem you need to
measure the true-rms value and
the instantaneous peak value
of the wave shape. For single
applications, you need either a
clamp meter like the Fluke 335,
336 or 337 or a multimeter like
those in the Fluke 80, 170 and
180 Series that makes true-rms
measurements. For three-phase
applications, you’ll need a power
quality analyzer like the Fluke
430 Series.
“True-rms” refers to the
root-mean-square, or equiva-
lent heating value of a current
or voltage wave shape. “True”
distinguishes the measurement
from those taken by “average
responding” meters. The vast
majority of low-cost, portable
meters are average responding.
These instruments give correct
readings for pure sine waves
only and will typically read low
by as much as 50 percent when
confronted with a distorted
current waveform. True-rms
meters give correct readings
for any wave shape within the
instrument’s crest factor and
bandwidth specifications.
Crest factor
The crest factor of a waveform is
the ratio of the peak value to the
rms value. For a sine wave, the
crest factor is 1.414. A true-rms
meter will have a crest factor
specification. This spec relates to
the level of peaking that can be
measured without errors.
A quality true-rms handheld
digital multimeter has a crest
factor of 3.0 at full scale. This
is more than adequate for most
power distribution measure-
ments. At half scale the crest
factor is double. For example, the
Fluke 187 DMM has a crest factor
spec of up to 3.0 when measur-
ing 400 V ac and a crest factor
of up to 6.0 when measuring
200 V ac.
Using a true-rms meter with a
“Peak” function—like the Fluke
187—the crest factor can be
easily calculated. A crest factor
other than 1.414 indicates the
presence of harmonics. In typical
single-phase cases, the greater
the difference from 1.414, the
higher the harmonic content. For
voltage harmonics, the typi-
cal crest factor is below 1.414;
i.e., a “flat top” waveform. For
single-phase current harmonics,
the typical crest factor is much
higher than 1.414.
Three-phase current wave-
forms often exhibit a “double
hump” waveform, therefore the
crest factor comparison method
should not be applied to three-
phase load current.
After you’ve determined that
harmonics are present, you can
make a more in-depth analysis
of the situation with a harmonic
analyzer such as the Fluke 43B
Power Quality Analyzer.
Meter Type
Measuring
Circuit
Sine Wave
Response*
Square Wave
Response*
Distorted Wave
Response*
Average
Responding
Rectified
Average x 1.1
Correct 10 % high Up to 50 % low
True-rms RMS Calculating
converter. Cal-
culates heating
value.
Correct Correct Correct
*Within multimeter’s bandwidth and crest factor specifications.
Multimeter performance comparison average
responding vs. true-rms
Troubleshooting tools