Emerson VILTER MicroVission Operation And Service Manual page 234

Appendix C • Remote Control and Monitoring
Offset is used at Load Step % and is 5%.
Once Compressor is Loading or Unloading a step,
next step would be allowed to Load or Unload
respectively only after 5 seconds.
Cap Hold Operation with VFD:
Using a VFD with a compressor requires considering the
VFD capacity as part of the entire capacity of the com-
pressor. When Capacity Hold % value is varied from 0%
to 50% compressor capacity is varied based on unloader
steps and when Capacity Hold % value is varied from 50%
to 100%, compressor capacity is varied based on the VFD
speed. Hence the unloader steps are considered the first
½ of the total capacity and VFD is considered as the oth-
er ½ of the total capacity. When a VFD is employed, the
normal control method is to first move the steps from
0% - 100% (i.e. first ½ of the total capacity) and when
additional capacity is still required, then the VFD speed
is ramped up from 0% - 100% (i.e. other ½ of the total
capacity) When capacity needs to be removed from the
system, first VFD speed is ramped down till 0% and then
unloader steps are removed.
For example, If the number of Unloaders configured in
configuration screen is 4 (25%, 50%, 75%, 100%), then
Load Steps would be 0%, 25%, 50% & 75%. Progress
bar on Main Screen should display the total capacity at
which the compressor is currently operating. The rules
of capacity hold algorithm with VFD are:
1. If Capacity Hold % value is greater than or equal to
((Next Load Step % - Offset) / 2), step will be added.
When Capacity Hold % value is above 10% ((25 – 5)
/ 2), then unloader steps would be loaded at 25%
and progress bar should be loaded at 12% (25 / 2),
refer Figure C-5. When Capacity Hold % value is above
22.5% ((50 – 5) / 2), then unloader steps would be
loaded at 50% and progress bar should be loaded at
25% (50 / 2). When Capacity Hold % value is above
35% ((75 – 5) / 2), then unloader steps would be load-
ed at 75% and progress bar should be loaded at 37%
(75 / 2). When Capacity Hold % value is above 47.5%
((100 – 5) / 2), then unloader steps would be loaded
at 100 % and progress bar should be loaded at 50%
(100 / 2) i.e. ½ of the total capacity. Once unloader
steps are loaded to 100%, it constitutes ½ of the to-
tal capacity (i.e. 50%), then if Capacity Hold % value
goes above 50%, VFD speed will start ramping up and
progress bar should be loaded at Capacity Hold % val-
ue. For example, when Capacity Hold % is set to 75%,
then unloader steps would be loaded at 100% & VFD
Speed would be set at 50% (VFD Output at 12 mA)
and progress bar should be loaded at 75% ((100 + 50)
/ 2). VFD Speed would be at maximum (VFD Output
C - 4
NOTE
NOTE
MicroVission Controller • Operation and Service Manual • Emerson • 35391MV 1.3
at 20 mA) when Capacity Hold % value is set at 100%.
2. If Capacity Hold % value is varied from 100% to 50%,
VFD speed would be ramped down from 100% till
0%. VFD Speed would be at minimum (VFD Output
at 4 mA) when Capacity Hold % value is set at 50%.
If Capacity Hold % value goes below 50%, then un-
loader steps will start getting removed. If Capacity
Hold % value is less than or equal to ((Previous Load
Step % + Offset) / 2), step will be removed. When
Capacity Hold % value is below 40% ((75 + 5) / 2),
then unloader steps would be loaded at 75% and
progress bar should be loaded at 37% (75 / 2). When
Capacity Hold % value is below 27.5% ((50 + 5) / 2),
then unloader steps would be loaded at 50% and
progress bar should be loaded to 25% (50 / 2). When
Capacity Hold % value is below 15% ((25 + 5) / 2), then
unloader steps would be loaded at 25% and progress
bar should be loaded at 12% (25 / 2). When Capacity
Hold % value is below 2.5% ((0 + 5) / 2), then unloader
steps would be loaded at 0% and progress bar should
be loaded at 0%.
3. If Capacity Hold % value varies within the range of
((Previous Load Step % + Offset) / 2) and ((Next Load
Step % - Offset) / 2), then unloader step would not
vary and would stay at current step. When Capacity
Hold % value is greater than 15% ((25 + 5) / 2) and
lesser than 22.5% ((50 - 5) / 2), then unloader steps
would be loaded at 50% and progress bar should be
loaded at 25% (50 / 2). This is only applicable when
Capacity Hold % value is varying from 0% to 50%.
Modbus Register 40177 - Active Setpoint
This register is used in conjunction with Register 40172
= 0, Auto Control mode.
• 1 = Setpoint #1 Active
• 2 = Setpoint #2 Active (note: you must enable two
setpoints from configuration screen first).
Sometimes compressors are switched from Suction
Pressure control mode to Process Temp control mode or
vice versa. This can be done via communications using
the following register.
Modbus Register 40173 - Auto Capacity Control Type
• 0 = Suction Pressure (if enabled from Configuration
screen)
• 1 = Process Temp (if enabled from Configuration
screen)
• 2 = D ischarge P ressure ( if e nabled f rom C onfiguration
screen)