OM AGSB-5
19
Load/Unload Select
The load/unload selector determines which solenoid will be pulsed for a change in capacity.
When unloading is required, the load/unload select output should be off. When loading of
the compressor is required, the output should be on.
Slide Pulse
The slide pulse output moves the compressor slide in order to reach the capacity reflected
by the slide position target. The output will pulse for 200 ms every 3 seconds until the slide
position is within a 3% deadband around the target.
Condenser Fan Control
The compressor must be running in order to stage its fans on.
VFD (Standard)
Condenser pressure trim control is accomplished using a variable frequency drive (VFD) on
the first two fans that turn on. This VFD control uses a proportional integral function to
drive the saturated condenser temperature to a target value by changing the fan speed. The
target value is normally the same as the saturated condenser temperature target setpoint.
The VFD will start the fans when the saturated condenser temperature goes above the
temperature target. Once the VFD fans are on, they will not shut off until the saturated
condenser temperature is less than the minimum saturated temperature plus 5 degrees F.
Stage up Compensation
In order to create a smoother transition when another fan is staged on, the VFD compensates
by slowing down initially. This is accomplished by adding the new fan stage up deadband
to the VFD target. The higher target causes the VFD logic to decrease fan speed. Then,
every 10 seconds, 0.5
o
F is subtracted from the VFD target until it is equal to the saturated
condenser temperature target setpoint. This will allow the VFD to slowly bring the
saturated condenser temperature back down.
Condenser Target
This logic is only used with VFD = Yes. Most applications will benefit from using the
default values. In the software versions previous to AGSU30101F, there was only one
setting for condenser target setpoint, with a default of 110°. Beginning with AGSU30101F
software there are two setpoints used to set a minimum (Min) and a maximum (Max) range
for the saturated condenser target. This can be found on the circuit controller at Set Fan
Sps(5). This will allow for a floating condenser target based on saturated evaporator
temperature. The default values of the minimum and maximum are both set to 110°
saturated condensing temperature. This will allow for the most stable unit operation.
Adjusting the Min or Max setpoint at each circuit controller will vary the condenser target
along a line determined by two points which are, 85° saturated condenser at 20° saturated
suction and 110° saturated condenser at 50° saturated suction. Note that the chiller system
is designed for specific refrigerant flow capacities, which may be exceeded by decreasing
the condenser target. The result will be at lower ambient temperatures, the chiller may
attain the maximum unit tonnage capacities while compressor loading will be limited on
low discharge superheat.
Fan Stages with VFD Option
The VFD option must always be enabled. The first two fans are controlled by the fan VFD.
This leaves 6 stages of FanTrol available with 8 fan circuits, and 4 stages available on 6 fan
circuits. Although fans 5/6 and 7/8 are controlled by one contactor each, more stages are
created by using virtual stages. See the table below:
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