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Compressor Control
The compressor is controlled via:
• commands to the VFD via Modbus to start/stop the
compressor
• a speed command to the VFD via Modbus to control the
compressor speed
• a digital output to control the oil return solenoid valve and
jet pump solenoid valve
• digital outputs for compressor VR solenoids to control VR
Cycle Timers
A minimum time must pass between starts of each compressor.
When the compressor starts, a timer starts which will run for a
time determined by the Start-Start Timer set point.
A minimum time must pass between the stop and start of each
compressor as well. When the compressor stops, a timer starts
which will run for a time determined by the Stop-Start Timer set
point.
While either timer is running the compressor cannot start. Both
cycle timers will be enforced even through cycling of power to
the chiller. These timers may be cleared via the Clear Cycle
Timers set point.
Starts Per Hour Limit
In addition to the cycle timers, a limit of four starts per hour
is enforced. A buffer of start times for the last four starts is
maintained. If the current time is an hour or less after the first
timestamp in the buffer, the next start will be delayed. This limit
is cleared if the Clear Cycle Timers set point is set to Yes.
Capacity Control
The compressor will vary capacity via changes to the motor
speed (frequency). The speed is controlled via writing to a
specific modbus register in the VFD. The VFD should initiate
the compressor to run when the circuit state is Start, Run, or
Pumpdown.
Auto Capacity Control
The minimum running speed is set to 14 Hz and the normal
speed change is 1hz for all compressor configurations.
Circuit State = Off - Speed command is 0 Hz.
Circuit State = Start - Immediately after starting, the
compressor speed will be set to a frequency between 14 Hz
and 60 Hz and held there while circuit state is Start.
Circuit State = Run - After the circuit enters the Run state
for 30 seconds, changes to the speed are performed based
on load and unload commands coming from the unit capacity
control logic. 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
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will occur. The load and unload delay times are calculated
values.
Circuit State = Pumpdown - Speed will be set to 50 Hz or the
calculated Compressor Minimum Frequency value, whichever
is higher.
Manual Capacity Control
The capacity target of the compressor may be controlled
manually. Manual capacity control is enabled via a set point
with choices of Auto and Manual. Capacity control may be set
to Manual only when circuit state is Run.
When Manual control is selected, a manual speed in units
of Hz can be entered using the Manual Speed set point with
a range from 14 Hz to the Compressor Maximum Speed set
point.
When setting a manual speed, the compressor speed will be
stepped up or down using 1 Hz speed changes until it is equal
to the speed that corresponds to the selected value. Changes
to the speed will be made as fast as allowed by the calculated
load and unload delays.
Capacity control shall revert back to automatic control if either:
• the circuit state changes from Start or Run to another
state
• capacity control has been set to manual for four hours
VR Solenoid Valve Control
There are three solenoid valves for changing volume ratio (VR)
of the compressor:
• 100% solenoid valve
• 50% solenoid valve
• 75% solenoid valve
Note: The VR position % does not equal the % capacity load of
the compressor.
Automatic VR Control
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. The table below
shows which VR solenoid valves are on at each stage as well
as stage up and stage down conditions. The staging conditions
must be active for 30 seconds to trigger each stage up or
down.
Stage
VR
0
1.6
1
1.8
2
2.4
3
3.1
33
IOM 1264-1 • NAVIGATOR
C
IrCuIT
50 VR SV
75 VR SV
100 VR SV
Off
Off
On
Off
Off
On
On
On
MODEL WWV CHILLERS
®
f
unCTIons
Off
Off
Off
On
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