York VSD 292 Series Operation Manual page 18

SECTION 1 - GENERAL INFORMATION
The ACC will then know not to reduce speed this low
again, if the same delta pressure, and the vane posi-
tion conditions are encounter again in the future. As
the chiller encounters various conditions, which result
in surge, it will store more points, and eventually this
storing of points creates a "Surge Map". Surge may be
detected in two ways, by monitoring the pressure dif-
ferential across the compressor, or by monitoring the
compressor motor current. Either detection will light
the Red LED on the ACC, indicating a surge was de-
tected. The chiller may surge 6 to 8 times before the
ACC can raise the speed enough to get the chiller back
out of surge. Each surge is counted on the surge coun-
ter, which may be viewed on the control center. This
surge counter will always display the total number of
surges encountered by the chiller as determined by the
ACC. Surging which occurs at fixed speed will incre-
ment the surge counter as well, but only surges that
occur when speed reduction is possible are recorded in
the surge map.
Drive Not Reducing Speed
The ACC may begin the process of reducing speed, but
may stop speed reduction if instability is encountered.
This is the same instability discussed as one of the two
conditions which must be met to begin reducing speed
initially (See "Stability Requirements"). Once the sys-
tem again becomes unstable, no additional speed re-
duction can occur.
The most common causes for instability are:
• High Condenser liquid temperature.
• Dirty Condenser tubes.
• Chillers with very light loads.
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• Rapid changes to chilled or condenser liquid flow.
• Valves on air-handler coils closing rapidly caus-
ing changes in heat-load.
• Extremely short chilled liquid loop.
• Parallel chiller with poor control is causing tem-
perature variations.
• Parallel Chiller with poor control of chilled or
condenser water flows.
• Improper evaporator refrigerant level.
If you experience a problem with an OSCD not re-
ducing speed at all, make certain the system is not
in manual speed control, or locked into fixed speed.
Either situation will cause the chiller to maintain full
speed. If the OSCD is reducing speed, but not running
as slow as you expect it should, it is likely because it
is either in an unstable condition, or running just above
a mapped surge point. As described above, the chiller
must achieve stability, which is evidenced by the Green
LED being extinguished, before speed reduction will
commence. Instability will cause the Green LED to be
illuminated.
Stability Limit Adjustment
Stability Limit Adjustment allows the system to prop-
erly function with larger amounts of temperature insta-
bility. Consult YORK Service to make this adjustment.
Surge Margin Adjustment
Surge Margin Adjustment allows the Service Techni-
cian to increase the speed of the drive for all mapped
surge points. This parameter is rarely used, and it de-
creases the efficiency of the OSCD chiller system.
The ACC board is no longer in production. The func-
tions of the ACC board were transferred to the OptiView
panel in 2008 with software version C.OPT.01.19.307
and the 031-02430-xxx board.
FORM 160.00-O4
ISSUE DATE: 07/31/2019
JOHNSON CONTROLS