3.
LT Family
The Vi control LT Series compressors is accomplished using two slides that operate in tandem with one
another to simultaneously maintain the desired Vi and capacity of the compressor. To achieve the desired Vi
setting, the primary slide is moved initially, with adjustments to the secondary slide being carried out after-
ward to maintain the optimal Vi setting.
There are three
Vi Modes
for LT Family compressors. The current mode the compressor is operating in
determines which slide will be moved to control capacity and which slide will be used to achieve the desired
Vi setting. The calculated optimum Vi is displayed on the
Compressor
tile.
•
Mode #1
In this Vi mode, the primary slide is moved to control the capacity of the machine and the secondary
slide is kept unloaded. The secondary of the compressor in
Mode #1
is therefore a function of the move-
ment of the primary slide in order to maintain the desired suction pressure or process temperature set-
point. The primary slide is used to control the capacity of the machine from completely unloaded up to a
position where the optimum Vi for the operating conditions is located. If the machine is attempting to load
and the primary slide reading exceeds the calculated primary slide position for the optimum Vi, the com-
pressor will transtion from
Mode #1
to
Mode #2
.
•
Mode #2
While in
Mode #2
, the primary slide will only be moved to correct for changes in the calculated primary
slide position. Adjustments in capacity will be handled by the secondary slide. The optimum Vi is con-
stantly recalculated based on the current operating conditions. There is typically no positional feedback
from the Vi slide; subsequently, there is no value for the secondary slide valve position for LT Family
compressors.
When the machine is attempting to load and has initially entered
Mode #2
from
Mode #1
, a thirty second
load timer is started. Continous load signals will transition from
Mode #2
to
Mode #3
in thirty seconds.
Short load pulses will take a proportionally longer time to transition because the timer is only permitted to
advance for the duration of each pulse. The primary slide reading will increase proportionally over this
thirty second period. With this method, the accumulating load signals are used to alter the displayed
slide valve reading from the value of the optimum Vi position to 100%. The primary slide reading will be
approaching 100% at the same time as the equivalent of thirty seconds of loading elapse. When the
timer has accumulated the full thirty seconds, the Vi Mode will transition from
Mode #2
to
Mode #3
, lock-
ing the machine at 100%.
An unload signal that interrupts the thirty second transition period from
Mode #2
to
Mode #3
will auto-
matically set a new transition period for the compressor to move from
Mode #2
back to
Mode #1
. This
transition timer will be set to a period of time equal to the amount of time that the compressor had been
loading while in
Mode #2
. This method allows the panel to determine a relative position for the secon-
dary slide based on the duration of load pulses. For example, if the compressor has accumulated twelve
seconds of Load pulses in
Mode #2
and begins unloading, twelve seconds of unload pulses must accu-
mulate prior to returning to
Mode #1
.
Similarly, if the machine is attempting to unload and has initially entered
Mode #2
from
Mode #3
, a thirty
second unload timer is started. If a request to unload the compressor continues for a total of thirty sec-
onds, the Vi Mode will transition to
Mode #1
. A load signal that interrupts the thirty second
Mode #2
to
Mode #1
transition timer will automatically set a new transition period for the compressor to move from
Mode #2
back to
Mode #3
. This transition timer will be set to a period of time equal to the amount of time
that the compressor had been unloading while in
Mode #2
. This method allows the panel to determine a
relative position for the secondary slide based on the duration of unload pulses. For example, if the com-
pressor has accumulated 18 seconds of Unload pulses in Vi
Mode #2
and begins loading, 18 seconds of
load pulses must accumulate prior to returning to Vi
Mode #3
.
Control
Variable Vi
Instruction Manual | GEA Omni™
Control panel
204
GEA Refrigeration North America, Inc. | E_806550_2 | Generated 12.01.2016
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