JOHNSON CONTROLS
14
FORM 160.00-O10
ISSUE DATE: 8/24/2020
SECTION 1 - GENERAL INFORMATION
DRIVE AND CHILLER OPERATION
When the chiller enters a start command, the OSCD
is commanded to precharge, the pre-charge contactor
will close and slowly charge the DC bus capacitors
through the pre-charge resistors. This is called the
precharge period, which will last for 12-seconds. Af-
ter the 12-second time period has expired, the supply
contactor will close and the pre-charge contactor will
open. The supply and pre-charge contactors will re-
main in this state for the duration of the run cycle. The
OSCD logic board directly provides the commands for
the precharge contactor and a pilot relay to provide
power to the supply contactor's coil. The coil power
requirements for the supply contactor it too large to be
directly driven by the drive logic board.
The cooling fans and pumps will turn on after the
precharge time is complete, and continue to run for 2
minutes after the stop command is provided from the
OptiView panel.
During normal chiller run conditions the OptiView
panel provides a speed command to the OSCD. The
speed command is converted into the proper turn-on
and turn-off commands for the inverter by the drive
logic board.
OPTISPEED COMPRESSOR DRIVE CONTROL
SYSTEM OVERVIEW
The HYP model of OSCD can only be controlled by
the OptiView Control Panel with a 031-03630 mi-
croboard installed.
The OSCD control system is composed of various
components located within both the OptiView Control
Panel and the OSCD. Thus integrating the Control Cen-
ter with the OSCD. The OSCD system utilizes various
microprocessors, which are linked together through a
ModBus communication link.
Data such as output currents, baseplate temperatures,
and input voltage are communicated from the OSCD
to the microboard for display through the ModBus
communications link. The OptiView panel collects
data from the many chiller sensors to determine the
optimum speed for the compressor. This information
is communicated to the drive logic board. The drive
logic board then determines the output voltage and fre-
quency to control the compressor motor.
OptiView Control Panel VSD Functions
Refer to the specific OptiView™ Control Panel service
and operation manual for detailed information. All of
the OSCD related information is contained under the
Motor and Compressor Screens.
Drive Not Reducing Speed
The ACC may begin the process of reducing speed, but
may stop speed reduction if instability in the leaving
liquid temperature is encountered. Once the system
again becomes unstable, no additional speed reduction
can occur. The most common causes for instability are:
• High Condenser liquid temperature.
• Dirty Condenser tubes.
• Chillers with very light loads.
• Rapid changes to chilled or condenser liquid flow.
• Valves on air-handler coils closing rapidly caus-
ing changes in heat-load.
• Extremely short chilled liquid loop.
• Parallel chiller with poor control is causing tem-
perature variations.
• Parallel Chiller with poor control of chilled or
condenser water flows.
• Improper evaporator refrigerant level.
If you experience a problem with an OSCD not re-
ducing speed at all, make certain the system is not
in manual speed control, or locked into fixed speed.
Either situation will cause the chiller to maintain full
speed. If the OSCD is reducing speed, but not running
as slow as you expect it should, it is likely because it is
either in an unstable condition, or running just above a
mapped surge point.