Application Note

Power Quality Troubleshooting Fluke Corporation 27
12-pulse converter
If the delta-wye/delta-delta are
packaged together (delta pri-
mary, delta and wye secondary)
and each secondary feeds one
of two paralleled six-pulse con-
verters, a 12-pulse front-end is
created with all the benefits
mentioned above. 18-pulse
designs are also available. Be-
cause of the extra cost, this type
of solution tends to only get
used on high HP loads.
Active filters
This relatively new technology
is based on an elegant concept–
using power electronics to solve
the problems created by power
electronics. It senses the instan-
taneous ac sine wave; it then
actively cancels the harmonics
it detects by generating equal
and opposite polarity harmon-
ics, thus recreating the sine
wave. Commercial packages
might provide voltage regula-
tion as well.
Active PF Correction
Another recent solution is for
manufacturers to offer converter
front ends using fast switching
technology that generates a
minimum amount of harmonics
and has near unity power factor
(both Total PF and DPF).
There is room for discussion
on which approach to harmonic
mitigation might prove most ef-
fective and economical in a par-
ticular situation. However, what
is often overlooked by the end-
user, and what should be clear
from the information in this sec-
tion, is that the total cost of a
drive system should include
both the cost of the drive itself
and the harmonic mitigation
(whether part of the drive or
installed separately).
True-rms test tools are necessary for accurate
measurements of distorted waveforms. For more
information, see the Fluke application note Why
True-rms, document number B0294A.
A comparison of average-responding and true-rms multimeters
Multimeter Reading
Waveform Description
Average-sensing True-rms
DMM DMM
Sine wave Correct Correct
Square wave
(flat-top voltage)
10% high Correct
Current to single
phase diode rectifier
40% low Correct
Current to 3 phase
diode rectifier
5-30% low Correct
Line Load
Figure 7.5 Harmonic trap filter.
Why True-rms