Emerson VSG & VSSG Software Manual page 29

Setpoints on this screen:
· Suction Pressure On/Off – The compressor will automatically cycle ON and OFF at the setpoints entered.
Suction Pressure On/Off control is only active if the Compressor Control Via Suction Pressure option is
selected on the Setup screen. If a compressor shutdown is desired on a suction pressure drop and a
manual reset is required, set the OFF value below the Low Suction Pressure Safety Trip value. This will
shut down the unit and a reset will be required to restart it.
· Suction Pressure Capacity Increase – The capacity of the compressor will increase when suction pressure
is at or above the Increase ON setpoint, and the increase "off" timer has cycled. Capacity will continue to
increase until the Suction Pressure Capacity Increase OFF setpoint is reached. If closer system control is
desired, set the ON and OFF setpoints at the same values. This will essentially eliminate any differential
between the ON and OFF setpoints.
· Suction Pressure Capacity Decrease – The capacity of the compressor will decrease when suction pressure
is at or below the ON setpoint, and the decrease "off" timer has cycled. Capacity will continue to decrease
until the Suction Pressure Capacity Decrease OFF setpoint is reached. If closer system control is desired,
set the ON and OFF setpoints at the same values. This will essentially eliminate any differential between
the ON and OFF setpoints. While this setting is only available for adjustment on the Control Setpoints
screen when the Compressor Control Via Suction Pressure option is selected on the Setup screen, it has
an override effect when control is via process temperature as described below.
Suction Pressure Control
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