Page 65 - Energize November 2021

TECHNICAL
Figure 6(a): A twelve-pulse bridge (Yaskawa)
Figure 6(b): Current waveform and harmonic content (WEG)
Figure 7(a): 18 pulse bridge (Yaskawa)
Figure 7(b): Current waveform and harmonic content (WEG)
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The twelve-pulse system uses two
transformer secondaries, in a delta and
Y formation to produce two three-phase
waveforms which are 30° apart. These are
applied to three-phase rectifier bridges to
produce the waveform shown in Figure 5.
The configuration of the 12-pulse system
is shown in Figure 6(a), while Figure 6(b)
shows the waveform and the rectified
waveform. The phase shift transformers
produce six AC waveforms with a 30°
shift between each phase. The 12-pulse
system has a lower ripple content and a
lower harmonic content than the 6-pulse
system.
18 pulse system
The 18-pulse system uses a transformer
with three secondaries, with phase
differences of +20°, 0° and -20°. The
system has the lowest harmonic content
of all three systems.
Higher pulse systems
Systems using 24 and 36 pulses are also
available in the market but use large,
complicated transformer systems.
Active front-end
The active front end uses high power
transistors such as IGBT to control the
input current. Unlike thyristors which can
only be switched on by a gate signal and
rely on the zero crossing to switch off,
the power transistor can be switched on
and off in any required pattern to give an
input current waveform with the desired
harmonic level.
Active front-end (AFE) also offers
bidirectional switching which allows
power feed back to the grid during motor
braking. Although it does not affect other
harmonics and cannot be retrofitted
into existing drives, it does reduce total
harmonics at any load to 2 to 3% THD.
The AFE is immune to voltage
imbalance, is available in most sizes, and
can be used on multiple drives with a
single front end. It provides voltage sag
ride through capabilities, a unity power
factor and a regenerative power flow.
It requires an inductive-capacitive
inductive filter to filter the high frequency
IGBT switching from the line. Because of
the added technology introduced by the