Emerson Vission 20/20 Operation And Service Manual page 202

Appendix B • Vission 20/20 Application Procedures
scale) away from the current position, then en-
ergize the capacity slide for a time that is propor-
tional to the amount it is away from the new target
position. To say it another way – the further you are
away from the new target position – the longer the
slide is energized in the proper direction. This con-
trol region is called the proportional band region.
3. If new target is > 10% (full scale) away from current
position (greater than the proportional band re-
gion) – then energize continuously.
There are mechanical properties that limit the speed at
which the capacity of the compressor can be changed.
The capacity slide actuators can only turn so fast. The
proportional change to the compressor capacity occurs
at a fi xed rate.
Caphold and Operation with VFD
• Using a VFD with a compressor requires considering
the VFD capacity as part of the entire capacity of
the compressor. Typically, a VFD is operated from
50% to 100% speed, therefore the VFD is considered
½ the total capacity and the slide valve movement
is considered the other ½ of total capacity. When
a VFD is employed, the normal control method is
to fi rst move the capacity slide from 0-100% when
additional refrigeration is required. When the capac-
ity slide is at 100% and additional capacity is still
required, then the VFD is ramped up in speed. It
follows then that the 4-20ma Caphold signal is bro-
ken down into two parts:
• 4-12 mA = 0 -100 slide valve position
• 12-20ma = VFD minimum speed to VFD maxi-
mum speed.
Consideration should also be given that when moving
the capacity slide valve from 0-100%, the actual corre-
sponding capacity of the compressor is not changing in
a linear relationship of 0-100%. The last 15% travel of the
slide valve results in a greater change of capacity than
15%. Integrators should realize that the caphold value
sent relates to slide valve "position" and not actual ca-
pacity of the compressor. For most purposes however,
assuming a linear relationship is adequate.
The rate at which the capacity slide moves from 0-100%,
and the rate at which the VFD increases speed from min-
imum to maximum is not the same. So there are two
different capacity profi les that the integrator needs to
consider.
B - 46
Remote Enable Output
• When the compressor is off and in Remote mode, a
REnable output will provide a signal to indicate that
the 20/20 is in a condition where it is ready to be
started. No start inhibit conditions exists, the 20/20
is not in anti-recycle mode, and there are no trips
active. If the compressor is able to be started, then
Remote Enable output will go on. When the output
is on, then closing the Remote Start/Stop input will
initiate a compressor start.
Once the compressor has started, the state of the
Remote Enable Output is indeterminate, and has no
CONTROL SCENARIO
Once the Confi guration Screen has been confi gured for
the desired type of Digital I/O control the Vission 20/20
needs to be placed in REMOTE mode. To do this, press the
green UNIT START button, then REMOTE. The Remote
Start-Stop input is now active. The state of the Remote
Enable Output should be determined by the controlling
device. When it is determined to be on, then the control-
ling device can energize the Remote Start-Stop input.
After the compressor has started, then the compressor
capacity is controlled by the selected option. Thought
should also be given as to how the compressor will be
restarted after a power failure occurs.
RESTART ON POWER FAILURE
• The Vission 20/20 allows for selection of different
operations to occur after a power failure has oc-
curred. The selections determine what mode of
operation the 20/20 will be placed after the power is
restored to the panel. This should decided upon and
setup prior to communicating to the 20/20 panel.
1.
2.
3.
4.
5.
1. Always
• If compressor was off prior to power failure, it will
stay off after power is restored.
• If compressor was running prior to power failure,
it will begin an Auto-restart sequence as soon as
power is restored.
• In both cases, the Remote Lock will be ON after
Vission 20/20 • Operation and Service Manual •Vilter/Emerson • 35391SC 2.0
NOTE
meaning.
Always
Never
Timed
Remote Lock Off
Boot in Remote (Direct I/O)