CB-110-4 37
avoided by maintaining the compressor to operate
with minimum frequency before the compressor
switches to normal operation.
Operation with force frequency (P2-03)
In case of long pipelines and rising lines or a suction
gas line with design flaws, there is a risk of oil migration
during the long partial load phases. If the frequency
and therefore the mass flow is increased, the oil can be
circulated back and a compressor shut-off can be
avoided. This can be realized via the "Force" function
of the VARIPACK frequency inverter. If a lack of oil in
the oil separator or compressor is signalled, the digital
input DI2 of the VARIPACK frequency inverter can be
activated to operate the compressor at the configured
force frequency. As standard, a value of 50Hz is set.
"Avoid resonances" (P2-05 .. P2-08)
If vibrations that are not easy to eliminate occur in a
specific range of operating frequencies, these frequen-
cies can be excluded. For this, two frequency ranges
can be defined in which the compressor cannot remain
during operation. When the setpoint exceeds the
defined initial value, the frequency remains at this value
until the setpoint has exceeded the end value and the
frequency range has been skipped. When the setpoint
falls below the defined end value, the frequency is
maintained until the setpoint has fallen below the initial
value.
Automatic limitation of the maximum frequency
During operation in field weakening mode, the fre-
quency inverter automatically limits the operating fre-
quency as soon as the maximum operating current of
the compressor or the frequency inverter is reached.
This allows economic dimensioning of the combination
of compressor and VARIPACK without running the risk
that the frequency inverter switches to fault due to over-
current under exceptional operating conditions (high
condensing temperature, start or start after a long
standstill).
Fig.18: Graphical representation of the general control functions
6.1 Extension module for pressure control
Graphical explanations of the control functions of the
extension module for pressure control described in the
following sections, see figure 19, page 39.
Basic function and minimum settings
If the extension module for pressure control is connec-
ted to the VARIPACK frequency inverter, the operating
mode "Pressure control" can be selected via BEST
SOFTWARE or via display parameter P1-12. This con-
figuration enables the relevant parameters for pressure
control.
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