Emerson VILTER MicroVission Operation And Service Manual page 237

Appendix C • Remote Control and Monitoring
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 Remote Capacity % value is varied from
0% to 50% compressor capacity is varied based on un-
loader steps and when Remote Capacity % value is varied
from 50% to 100%, compressor capacity is varied based
on the VFD speed. Hence the unloader steps are consid-
ered the first ½ of the total capacity and VFD is consid-
ered as the other 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 capac-
ity) 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 re-
moved 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 Remote Capacity % value is greater than or equal to
((Next Load Step % - Offset) / 2), step will be added.
When Remote Capacity % 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-7. When Remote Capacity % 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 Remote Capacity % value is above
35% ((75 – 5) / 2), then unloader steps would be
loaded at 75% and progress bar should be loaded at
37% (75 / 2). When Remote Capacity % 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 un-
loader steps are loaded to 100%, it constitutes ½ of
the total capacity (i.e. 50%), then if Remote Capacity
% value goes above 50%, VFD speed will start ramp-
ing up and progress bar should be loaded at Remote
Capacity % value. For example, when Remote
Capacity % 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 at 20 mA) when Remote
Capacity % value is set at 100%.
MicroVission Controller • Operation and Service Manual • Emerson • 35391MV 1.3
2. If Remote Capacity % 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 Remote Capacity % value is
set at 50%. If Remote Capacity % value goes below
50%, then unloader steps will start getting removed.
If Remote Capacity % value is less than or equal to
((Previous Load Step % + Offset) / 2), step will be re-
moved. When Remote Capacity % 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 Remote Capacity % 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 Remote Capacity % 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 Remote Capacity % 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 Remote Capacity % 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 Remote
Capacity % 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
Remote Capacity % value is varying from 0% to 50%.
C - 7