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OPERATION
occur. The load and unload delay times are calculated values.
Whenever the speed command is less than the Compressor
Minimum Frequency value, the speed command should be
increased at the same rate allowed for normal capacity increases
until it reaches that value. If there is an unload command from
the unit capacity control logic, the speed command should not
decrease if it is less than or equal to the Compressor Minimum
Frequency value.
Circuit State = Shutdown - Speed will be ramped down to 20 Hz.
The ramp rate will be 1 Hz/second, but the maximum ramp time
should be 30 seconds so the ramp rate will adjust up as needed
to complete the speed ramp within 30 seconds.
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 and Manual. With manual control speed can be set in
0.1 Hz increments, in a range from 14 Hz to the Compressor
Maximum Speed set point.
When setting a manual speed, the compressor speed will
generally be stepped up or down using 1 Hz speed changes. If
the change required to reach the manually set speed command
is less than 1 Hz, the compressor speed will change directly to
the manually set speed command. Changes to the speed will
be made as fast as allowed by the calculated load and unload
delays. Capacity control may be set to Manual only when circuit
state is Run .
Capacity control shall revert back to automatic control if either:
• the circuit state changes from 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.
IOM 1264-4
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
Whenever the VR of the compressor needs to be reduced, all
three solenoid valves must first be turned off for 15 seconds.
After that, the necessary solenoid valve can be turned on.
When the circuit state becomes Shutdown or Off, all VR
solenoids should be turned off.
Manual VR Control
VR can be controlled manually using settings on the variable
VR screen. VR control can be changed to Manual only when the
circuit state is Run. When VR Control is set to Manual, the VR
can be selected with choices of 1.6, 1.8, 2.4, 3.1. The solenoid
valve outputs will be set according to the table in the previous
section on automatic control. However, there will not be any
time delays or special operation for staging the VR down as in
automatic control.
Evaporator EXV Control
The Evaporator EXV is installed in the liquid line feeding the
evaporator and the chiller controls use specific logic to set the
position.
EXV will normally be positioned to control suction superheat
(SSH) with a PI loop. In some conditions, the evap EXV will be
positioned to control evaporator pressure to keep the compressor
within its operating envelope.
EXV States
The control of the EXV follows a sequence of states. The
transitions from one state to another is based on the running
conditions of the circuit. The states are as follows:
• Closed – EXV fully closed.
• Startup – The pressure target ramps up to bring the liquid
level up in the evaporator.
• Superheat – EXV controls to a calculated pressure target
in order to maintain evaporator liquid refrigerant level at an
optimal level.
39
50 VR SV
75 VR SV
100 VR SV
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