Guardian SCC 25, Manual

Page: 44 / 79

Page 44
GUARDIAN SCC-40 Issue (Auto VI) & V3.6t 07/12/00
TWO COMPRESSOR-SINGLE MOTOR OPERATION
Two SCC-40 controllers may be connected together in a Master-Slave
configuration for applications where two stage compressors share a single motor.
The low stage compressor (LS-U) is always the master.
The high stage compressor( HS-U) is always the slave.
The Std. (standard) or LS-U or HS-U selection may be made by pressing 'next'
after SC40 is displayed in Setup Unit .
HARD WIRED INTERLOCKS
The common motor contactor requires both low stage and high stage controllers
to have their individual motor relays energised before the common motor
contactor is energised and the motor started. If any hard-wired safety interlocks
are present for either high stage or low stage then the motor contactor is not
energised. Both compressor pump systems must be running before the
controllers energise their motor relay output to start the motor.
HIGH STAGE SLAVE MODES
The high stage compressor (HS-U) controller may only be selected to operate in
in REMOTE or MANUAL CAPACITY modes. The HS-U controller acts as a slave
to the low stage (LS-U) controller. The low stage controller starts and stops under
LOCAL or REMOTE suction pressure or MANUAL control and issues Start-slave
and Stop-slave signals to the high stage whenever the low stage compressor
starts or stops.
LOW STAGE START HIGH STAGE SLAVE
The low stage controller removes its Stop-slave output whenever the low stage
pump is started. The RESET TRIP RELAY (1 second pulse) is issued by the low
stage immediately after pump running signal is detected. This signal is
connected to high stage start-slave input which initiates a startup sequence as
though the high stage compressor had been started in local suction pressure
control. Only when both controllers have completed their pre-lubrication time and
energised their motor output relay is the motor contactor energised.
If the high stage fails to start its pump, or is in OFF mode, or fails to move its
slide to At_minimum then the motor contactor is not energised and the failure
mode of the low stage is displayed as
'nnot' (motor run fail) whilst the high stage
controller displays the appropriate fail message 'PunP ' or 'S-UP'.
LOW STAGE STOP HIGH STAGE SLAVE
When the low stage suction setpoint is satisfied and the low stage has been at
minimum for longer than the 'stage down time' or a low stage Manual Stop
request is given, then the low stage compressor goes into its stopping sequence
and issues a Stop-slave signal to the high stage prior to removing the low stage
motor relay signal which de-energises the motor contactor. The high stage goes
into its stopping sequence, lowers its slide to At_Minimum and then removes the
high_stage motor relay signal (the motor contactor is probably already
de-energised). Both high stage and low stage compressors run their oil pumps
until the appropriate slides are At_minimum and both controllers display
'Waiting' followed by 'Ready'.