Map”. Surge may be detected in two ways, by monitoring the pressure differential across the
compressor, or by monitoring the compressor motor current. Either detection will light the Red
LED on the ACC, indicating a surge was detected. 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
counter, 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 increment 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
for stability requirements).
Once the system again becomes unstable, no additional speed reduction can occur.
The most common causes for instability are:
•
High condenser liquid temperature
•
Dirty condenser tubes
•
Chillers with very light loads
•
Rapid changes to chilled or condenser liquid flow
•
Valves on air-handler coils closing rapidly causing changes in heat-load
•
Extremely short chilled liquid loop
•
Parallel chiller with poor control causing temperature variations
•
Parallel chiller with poor control of chilled or condenser water flows
•
Incorrect evaporator refrigerant level
If an OSCD does not reduce speed at all, ensure that 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. The chiller must achieve stability,
which is signaled 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 properly function with larger amounts of
temperature instability. Consult YORK Service to make this adjustment.
Surge margin adjustment
Surge Margin Adjustment allows the Service Technician to increase the speed of the drive for all
mapped surge points. This parameter is rarely used, and it decreases the efficiency of the OSCD
chiller system.
The ACC board is no longer in production. The functions 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.
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Liquid-Cooled OptiSpeed Compressor Speed Drive
