JOHNSON CONTROLS
120
FORM 160.84-OM1
ISSUE DATE: 9/21/2017
SECTION 4 - VSD OPERATION
FIGURE 46 - RECTIFIER SIDE OF THE POWER UNIT HYP0774 AND HYP1278A
LD16292
OPTISPEED COMPRESSOR DRIVE AND
CHILLER OPERATION (0490 AND 0774 AMP
MODELS)
When the chiller enters a start command, the OSCD
is commanded to pre-charge, the Aux. power devices
are gradually turned on to slowly charge the DC Bus
capacitors. This is called the pre-charge period, which
will last for 12-seconds. After the 12-second time pe-
riod has expired, the AUX. power devices are gated
fully on. After precharge is complete the AUX. power
device will remain turned on for the duration of the run
cycle. The OSCD logic board provides the turn on, and
turn off commands for the AUX. power devices.
The cooling fans and pumps will turn on after the pre-
charge time is complete, and continue to run until the
rectifier is turned off.
In April of 2016 the control for the fans and pumps was
changed so that they would continue to run 2 minutes
after the stop command. This change occurred in soft-
ware version C.HYP.03.04.01, and C.HYP.04.04.01.
During normal chiller run conditions the control panel
provides a speed command to the OSCD. The speed
command must take into account minimum speed
command from the anti-surge control, and the speed
required by the leaving chilled liquid temperature con-
trol. The OSCD logic board will determine the proper
turn-on and turn-off commands to the inverter to pro-
vide the correct RPM to the compressor motor.
When the chiller enters a normal stop command the
OSCD will continue to follow the speed command
from the control panel. The control panel begins to
unload the chiller. As the load and pressure across the
compressor starts to decrease, the output speed of the
OSCD will slow down. When the capacity control
devices (PRV or VGD and sometimes hot gas bypass
valve) reaches minimum load position, the OSCD will
start to decelerate the motor to 50 Hz. When 50 Hz is
reached all rectifier and inverter power devices will be
turned off except for the AUX. The OSCD will remain
in a precharge state for 60 seconds, and then the AUX.
is turned off. The DC link capacitors will start to dis-
charge through the bleeder resistors.
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