Dunham-Bush ACXi 165-5SP-LN, Installation, Operation And Maintenance Manual

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4.0 ELECTRICAL
NOTE:
The flow switch or pump interlock
cannot be used for normal control of the
unit. (See Section 4.5.2).
4.5.2 Customer Control interlock
Control contacts from an external
controller can be used to enable or
disable operation of the unit. The wiring
diagram specifies the terminals to which
the contacts must be wired. To enable
the unit, the contacts must be closed. To
disable the unit, the contacts must be
opened.
4.5.3 Anti-Recycle Timer
(Microcomputer)
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 microcomputer will not
restart the compressor motor until the 15
minutes have elapsed. COFF is displayed
when the compressor control point (1 CP
or 4CP for compressors 1 or 2
respectively) is addressed, and when
other conditions for compressor start are
satisfied. See Section 4.3.
4.5.4 Load Control (Microcomputer)
The microcomputer 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 microcomputer
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 current limit function
(see Section 4.5.7) overrides the
temperature control.
The status of the compressor can be
observed by displaying the compressor
control point (1CP or 4CP). One of the
following messages will be displayed:
COMP # LOAD Automatic load
COMP # HOLD Automatic hold
COMP # UNLD Automatic unload
COMP # OFF Off on temperature or
customer control
COMP # COFF Off on timer (C lock
off)
COMP # LOFF Manual off or safety
shutdown
Where # is 1 or 4 for compressors 1 or
2 respectively.
4.5.5 Ramp Control (Microcomputer)
Another feature of the microcomputer 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
0.4, smaller values producing slower
loading rates. The ramp start B setpoint
can be set anywhere between 10 and
50%. The compressor will load quickly
to this value and then follow the ramp
slope from there. See Table 4.5.5 for
ramp rates at various settings.
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