Circuit Functions
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IOM 1242-8 • PATHFINDER
®
MODEL AWV CHILLERS
Capacity Control
The compressor will vary capacity via changes to the motor
speed (frequency).
Auto Capacity Control
The normal speed change is 1 Hz for all compressor
configurations.
Circuit State = Off
- Speed command is 0 Hz.
Circuit State = Preopen
- Speed command is set to the
minimum for the configuration.
Circuit State = Start
- Immediately after starting, the
compressor speed will be set to the minimum for the
configuration and held there while circuit state is Start.
Circuit State = Run
- After the circuit enters the Run state,
changes to the speed are performed based on load and unload
commands coming from the unit capacity control logic (see
Unit Capacity Control section). The speed is constrained to a
range from the minimum based on the configuration up to the
Compressor Maximum Speed set point.
When a capacity increase occurs, a time delay starts; and
when a capacity decrease occurs, a separate time delay starts.
While either of these delays is active, no normal capacity
changes will occur. The load and unload delay times are
calculated values.
Circuit State = Pumpdown
- Speed command will drop 2 Hz
every second until reaching the minimum for the configuration.
Manual Speed Control
The speed of the compressor may be controlled manually.
Manual speed control is enabled via a set point with choices
of Auto or Manual. Another set point allows setting the
compressor speed. However, the resulting compressor speed
is still limited to the range from the minimum speed based on
configuration up to the Compressor Maximum Speed set point.
The compressor speed will be stepped up or down until it is
equal to the speed that corresponds to the manual speed set
point. Changes to the speed will be made as fast as allowed by
the calculated load and unload delays. Speed control may be
set to Manual only when circuit state is Start or Run.
Capacity control shall revert back to automatic control if either:
• circuit state changes from Start or Run to another state
• speed control has been set to Manual for four hours
• an unload event occurs on the circuit
Load and Unload Delay
LWT Error determines the delays for load and unload
commands.
Table 54: Load and Unload Delay Conditions
LWT Error
Delay
LWT Error > Startup Delta
T set point
Load command delay is 5
seconds
0.1°C ≤ LWT Error ≤
Startup Delta T set point
Load command delay will vary
from 5 seconds to 10 seconds
– (Stage Down Delta T
set point) ≤ LWT Error ≤
-0.1°C
Unload command delay will
vary from 3 seconds to 6
seconds
LWT Error < -(Stage Down
Delta T set point)
Unload command delay is 3
seconds
VR Solenoid Valve Control
There are three solenoid valves for changing volume ratio of
the compressor:
• 50 VR solenoid valve
• 75 VR solenoid valve
• 100 VR solenoid valve
The compressor should start with all three solenoid valves off.
After compressor has been running for at least 20 seconds, the
control logic will “stage” the solenoid valves.
which VR solenoid valves are on at each stage as well as
stage up and stage down conditions. These staging conditions
must be active for 30 seconds to trigger each stage up or
down.
Table 55: VR Solenoid Valve Control Stages
Stage
VR
50
VR
SV
75
VR
SV
100
VR
SV
Stage Up
Condition
Stage Down
Condition
0
1.6
Off
Off
Off
Pressure
Ratio > 2.10
n/a
1
1.8
On
Off
Off
Pressure
Ratio > 2.85
Pressure
Ratio < 1.90
2
2.4
Off
On
Off
Pressure
Ratio > 3.85
Pressure
Ratio < 2.65
3
3.1
On
On
On
n/a
Pressure
Ratio < 3.65
When the circuit state becomes Pumpdown or Off, all VR
solenoids should be turned off.
