stores the conditions surrounding the surge into memory, so that it can avoid the stored operating
point anytime in the future.
Early versions of the ACC software required that the drive always start and run up to full speed. ACC
software starting with version C.ACC.01.04 applies anew slow ramp up of the drive speed. This new
software lowers the peak current demand from the drive during start up, saves additional energy,
and reduces the possibility of the chiller running in a stall condition.
The new software will quickly ramp the compressor speed up to 1/2 speed, and then it takes 5
minutes to ramp up to full speed. During this slow ramp up period the vanes will open to meet
the cooling demand. If the leaving chilled liquid temperature is 0.5 or lower of the leaving
chilled liquid temperature setpoint, then the drive speed will stop increasing the RPM of the
compressor motor, and start to search for a surge map point. On extremely hot days the chiller
may surge during the slow ramp period. The new software has a method to limit the surging. If two
surges were to occur during the slow ramp period, then the speed of the drive will increase to full
speed.
Now that the ACC function is provided by the microboard in the OptiView panel future control
changes will be covered by the operation manual for the chiller model of interest. All versions of
software require two conditions to be met for speed reduction to occur. These two conditions are:
Setpoint requirements
The leaving chilled liquid temperature must be 0.5°F or lower from the leaving chilled liquid
temperature setpoint. A programmable value is now available through the OptiView panel on
software versions C.OPT.01.21.307 for the YK chiller. This programmable value is not available on
the YT chiller. Speed reduction will not occur until the leaving chilled liquid temperature reaches
this range.
Stability requirements
The leaving chilled liquid temperature must be stable. Lack of stability will be indicative of the
vanes hunting, the leaving chilled liquid temperature varying, and the green LED on the ACC will be
on. Once the above conditions are met, the ACC may begin to lower the speed of the compressor
motor 1/10 of a hertz at a time. As the ACC lowers the speed, the leaving chilled liquid temperature
will begin to creep up. As this occurs, the control center will begin to open the vanes slightly, just
enough to maintain the leaving chilled liquid temperature /- 0.5°F of the leaving chilled
liquid temperature setpoint. The ACC will continue to lower speed, with the leaving chilled liquid
temperature control in turn driving the vanes to a more open position. This process will continue
until one of three following situations occur. This setting is no longer available after software
version C.OPT.01.21.307 for the YK chiller.
Full open vane operation
Once the vanes reach the full open position, the ACC knows it can no longer reduce speed and
maintain the leaving chilled liquid temperature setpoint. The ACC will maintain operation at this
point, with the vanes full open, and the speed at the last point reached when the vanes hit 100%. If
there is an increase in load while at this point, the ACC will increase speed until the vanes are closed
to 95% of open. The ACC will then be allowed to continue to reduce speed again.
Effects of surge
If in the process of reducing speed and opening vanes the compressor should surge, the ACC will
boost the speed up by 0.8 Hz. The ACC will store in memory a value that represents the ratio of
condenser pressure to evaporator pressure, the vane position, and the speed of the drive. The ratio
of condenser pressure to evaporator pressure is displayed as Delta P/P on the Control Panel.
The ACC will then know not to reduce speed this low again, if the same delta pressure, and the vane
position 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
Liquid-Cooled OptiSpeed Compressor Speed Drive
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