Application Note

3 Fluke Corporation How adjustable speed drives affect power distribution
12-pulse converter
If the delta-wye/delta-delta are
packaged together (delta primary,
delta and wye secondary) and
each secondary feeds one of two
paralleled six-pulse converters, a
12-pulse front-end is created
with all the benefits mentioned
above. 18-pulse designs are also
available. Because 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 wa
ve; it then
actively cancels the harmonics it
detects by generating equal and
opposite polarity harmonics, thus
recreating the sine wave.
Commercial packages might pro-
vide voltage regulation as well.
Active PF correction
Another recent solution is for
manufacturers to offer converter
front ends using fast switching
technology that generates a mini-
mum 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
effective and economical in a
particular situation. However,
what is often overlooked by the
end-user, and what should be
clear from the information in this
section, 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).
A comparison of average-responding and true-rms multimeters
Multimeter Reading
Waveform Description Average-sensing DMM True-rms 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
Why True-rms
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 1260729.
LineLoad
Figur
e 5.
Harmonic trap filter
.