Section 3
SEQUENCING WITH VARIABLE
SPEED DRIVE COMPRESSORS
16
the VSD Unload time in AUTO mode and the fixed-speed restart times, rapid cycling and
hunting can be minimized.
The pressure regulator of a fixed-speed L (100 psi) compressor does not begin to throttle
until 101 - 102 psi. As a result, the pressure regulator has no effect, and the machine will
operate Load/Unload with or without the full load solenoid valve. The net result is that the
VSD compressor will always be running and able to modulate; the fixed-speed machines
would come on and go off as needed but operate Load/Unload.
Note that the normal load delta becomes narrower while operating a sequenced system.
The settings above will load at 97 and unload at 101 psi. The system will typically
maintain 98-100 psi.
3.3 SETUP FOR MULTIPLE VSD COMPRESSORS
With pressure-controlled VSD circuits, configure all compressors by Sequence Hours or
Com Number, as desired. Enable the Max. Speed Input function. All but one of the
compressors will be full-loaded, thus limiting the pressure to the rated value of the
machines.
With serial-controlled VSD circuits, configure all compressors by Sequence Hours or
Com Number, as desired. The controllers allow all machines to run at reduced speeds to
match delivery to demand. Unneeded machines will be stopped automatically. If the
compressors are L (100 psi) machines, set the program points as shown in Table 3.2:
Target system pressure at discharge of compressors: 95 psi
Table 3.2 Multiple VSD System Settings
Parameter VSD Compressor Settings
Unload Setpoint 102
Load Delta 8
VSD Setpoint 98
Lowest Pressure 90
Recover Time 10 sec.
Parameters may require adjustment based on system capacity and application.
Unload Setpoint should NEVER be more than 2 psi above package rating.
Note that the normal load delta becomes narrower while operating a sequenced system;
the settings above will load at 95 and unload at 101 psi. The system will typically
maintain 97-99 psi.
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