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ScopeMeter
®
Test Tool
MDA-550-III Test Tool
65
5. Use
h
i
E to switch the vertical scale between % of fundamental frequency and
linear voltage or current value.
6. In Scale Options, toggle between the TDD reading and the THD reading for a current
waveform.
TDD or Total Demand Distortion is the ratio of the rms value of all the current harmonic
components to the maximum demand current entered as a value. This is useful when running
at low load conditions. In that case THD would be relatively high, but the harmonic currents
generated would be low, and the effect on the supply system is negligible.
The distortion caused by the harmonics can affect the operation of other electrical equipment on
the same circuit. Other loads such as motors and transformers can overheat, have shortened life,
and ultimately fail due to the presence of harmonics.
Tips:
• Voltage and current harmonics are closely related, but the percentage levels are usually very
different. Voltage are a low number and current harmonics are a higher number.
• Voltage THD exceeding 6 % on any phase can require more investigation. Harmonics can be
reduced by modification to the drive, installation of harmonic filters, or other solutions for
harmonic mitigation. When installing a filter, the harmonics measurement can be done before
and after the installation to verify the performance of the filter.
• Scale Options show the higher frequency components by selecting 2 kHz to 9 kHz or 9 kHz to
150 kHz as the horizontal scale. The horizontal scale shows frequencies instead of
harmonics numbers.
• The frequency components are calculated using an FFT algorithm based on the acquired
waveform. The horizontal scale is linear as the values are not related to the fundamental
frequency.
• Use these frequency ranges to determine to what extent a drive (for example, with an active
front end) that operates on the same input power is affecting the input section of the drive
under test with high frequency components. This also can influence filters on the input of the
drive.
1.888.610.7664 sales@GlobalTestSupply.com
Fluke-Direct.com
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