Dunham-Bush ACDS Installation, Operation & Maintanance Manual Download Page 45

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4.0 ELECTRICAL

4.3.6.3 Anti-Recycle Timer

The compressor motor requires an anti-
recycle time delay which prevents
restart for 15 minutes after a start. The
purpose of this feature is to avoid
frequent starts which tend to elevate
the motor winding temperature and
impose undue wear on contactors. The
controller will not restart the
compressor motor until the 15 minutes
have elapsed.

4.3.6.4 Load Control

The controller controls the leaving
water temperature within a narrow
deadband by pulsing load and/ or
unload solenoids on the compressor.
The load and unload solenoids position
the slide valve within the compressor to
control its capacity. The controller
determines a desired level of loading
and varies pulse duration depending on
difference between load target and
actual load. The load target is varied
based on rate of approach to desired
temperature preventing significant
temperature oscillations. The status of
the compressor can be observed by
displaying the compressor control point.

4.3.6.5 Ramp Control

Another feature of the controller is ramp
control, which is the ability to vary load
time of the machine from start. Often
when the machine is started, the water
in the chilled water circuit is warm, and
the unit will go to full load quickly. With
ramp control, the user can program the
computer so that it loads at a
predetermined rate. This is a valuable
tool, since it can help reduce power

consumption and demand charges.
Two variables are used to define the
ramp profile: Ramp rate and start point.
Ramp rate defines the length of time
the unit takes to load from start point to
full load. Start point is the percent of full
load at which the ramp begins. The
ramp rate A setpoint can be set
anywhere from 0.1 to 1.0, smaller
values producing slower loading rates.
The ramp start B setpoint can be set
anywhere between 0 and 50%. The
compressor will load quickly to this
value and then follow the ramp slope
from there. See Table 4.3.6.5 for ramp
rates at various settings.

4.3.6.6 Staging Control

On multiple-compressor machines,
when the controller determines that a
compressor is fully loaded and
temperature is not being maintained,
another compressor is added. When
unloading, a compressor is taken off
line when the computer determines that
the remaining compressors can control
water temperature.

4.3.6.7 Pumpdown

Control

This feature is to stored refrigerant in
condenser when compressor is
stopped. Pumpdown control will take
place when temperature set point is
achieved before compressor is being
cycled off or unit enable input is off.
The compressor will stop when suction
pressure is lower than pumpdown
setpoint, 65PSI [4.5BAR] (for R22) or
15 seconds (setpoint) delay timer is
elapsed.

TABLE 4.3.6.5 Ramp Rates for Several Setpoints (In Minutes)

Ramp

Rate

Setpoint

1 Comp.

Start Point Setpoint

2 Comp.

Start Point Setpoint

3 Comp

Start Point Setpoint

4 Comp

Start Point Setpoint