Specifications

44
A diffuser pressure transducer is installed on compressors
with a split ring diffuser. The CCM monitors pressure fluctua-
tions at the inlet to the inner diffuser ring. Excessive pressure
fluctuations may indicate that the impeller is experiencing ro-
tating stall. A Diffuser Position Fault (Alarm 247) is declared
if pressure fluctuations exceed acceptable limits.
DIFFUSER CONTROL (Software Version 10 and
Later) — On all units with Frame 5 compressors and those
Frame 4 compressors with the variable (split ring) diffuser op-
tion, the PIC II adjusts the diffuser actuator position using Vari-
able Diffuser Optimization (VDO). Configurations for VDO
are produced by the ECAT chiller selection program and are
entered into the VDO_SRD screen. Diffuser control output is
enabled whenever the DIFFUSER OPTION is enabled, wheth-
er the machine is running or not.
The diffuser position, rather than following a specific rela-
tionship to the guide vane position, is determined based on sev-
eral factors: guide vane position, lift, rotating stall and surge
conditions.
Diffuser actuator output is controlled by a 4 to 20 mA out-
put from CCM terminals J8-3(+) and J8-4(-).
The diffuser and guide vanes will both move in Guide Vane
Control Test but the relative positions do not represent the rela-
tive position of these actuators during operation; the diffuser
and the guide vane operate independently.
The configurations for VDO Diffuser Control are found on
a label on the back of the control panel. See Fig. 11.
If you do not have the correct settings for VDO Diffuser
Control contact a Carrier Engineering Representative.
DEMAND LIMITING — The PIC II controls provide a fea-
ture for limiting AVERAGE LINE CURRENT or LINE
KILOWATTS (demand) by limiting capacity via guide vane
control. The limit applied is called ACTIVE DEMAND
LIMIT, which is equal to a BASE DEMAND LIMIT value
(set in the SETPOINTS Screen, page 30, default value 100%),
or that determined by AUTO DEMAND LIMIT INPUT (an
optional 4 to 20 mA input, described below). ACTIVE
DEMAND LIMIT may also be forced to be different from
BASE DEMAND LIMIT by manually overriding the value
(forcing) from the MAINSTAT screen or writing a value via a
CCN network device, or controlled by another chiller in Lead
Lag operation (see page 57).
The demand limit may be based on either line current or
kW, as indicated by DEMAND LIMIT SOURCE in the
EQUIPMENT SERVICE/RAMP_DEM table. The default is 0,
for demand limiting based on AVERAGE LINE CURRENT
(percent of RATED LINE AMPS, as displayed on the default
screen). Setting DEMAND LIMIT SOURCE to 1 makes de-
mand limiting based on PERCENT LINE KILOWATTS (dis-
played in the MAINSTAT screen). MOTOR PERCENT KILO-
WATTS is equal to (MOTOR KILOWATTS ÷MOTOR RATED
KILOWATTS ) ×100. MOTOR KILOWATTS is measured by
the ISM and the MOTOR RATED KILOWATTS value (100%
rated kW) is set on the RAMP_DEM screen.
If the DEMAND LIMIT SOURCE (average line current)
exceeds the ACTIVE DEMAND LIMIT by 5% or less, in-
creases in guide vane opening will be prevented. If the DE-
MAND LIMIT SOURCE (percent line current) exceeds the
ACTIVE DEMAND LIMIT by more than 5%, the guide vanes
will be forced to close. Also, as the DEMAND LIMIT
SOURCE approaches the ACTIVE DEMAND LIMIT from a
lower value, allowable capacity increases become increasingly
more limited, beginning when the DEMAND LIMIT
SOURCE is within the DEMAND LIMIT PROP BAND (con-
figurable in the RAMP_DEM table).
Demand Limit Control Option
— The demand limit control
option (20 mA DEMAND LIMIT OPT) is externally con-
trolled by a 4 to 20 mA signal from an energy management
system (EMS). The option is set up on the RAMP_DEM
screen. When enabled, 4 mA will set ACTIVE DEMAND
LIMIT to 100% of the DEMAND LIMIT SOURCE (regard-
less of the value of BASE DEMAND LIMIT), and 20 mA will
set ACTIVE DEMAND LIMIT to the value configured as
“20MA DEMAND LIMIT OPT” in the RAMP_DEM table.
Wire the auto demand limit to terminals J5-1 (-) and J5-2
(+) on the CCM. In order to use a 1 to 5 vdc input instead of 4
to 20 mA, install a 25 ohm resistor in series with the + lead at
terminal J5-2. The associated dip switch should remain in the
up (on) position despite the notation on the CCM board. See
Fig. 24.
A DEMAND KILOWATTS monitoring feature is also avail-
able. This feature provides a display of average demand
(power) in kilowatts (in the POWER screen). This value is
continuously updated and averaged over the preceding time
interval specified as DEMAND WATTS INTERVAL in the
SERVICE / EQUIPMENT SERVICE/RAMP_DEM screen.
CHILLER TIMERS AND STARTS COUNTER — The PIC
II maintains two run time clocks: COMPRESSOR ONTIME
and SERVICE ONTIME. COMPRESSOR ONTIME indicates
the total lifetime compressor run hours. SERVICE ONTIME is
a resettable timer that can be used to indicate the hours since
the last service visit or any other event. A separate counter tal-
lies compressor starts as TOTAL COMPRESSOR STARTS.
All of these can be viewed on the MAINSTAT screen on the
ICVC. Both ontime counters roll over to 0 at 500,000 hours.
Manual changes to SERVICE ONTIME from the ICVC (or
forced via the Service Tool) are permitted at any time. If the
controller is replaced, one opportunity, before the first startup
with the new controller, is provided to set COMPRESSOR
ONTIME and TOTAL COMPRESSOR STARTS to the last
readings retained with the prior controller. The SERVICE
ONTIME timer can register up to 32,767 hours before it rolls
over to zero.
The chiller also maintains a start-to-start timer and a stop-
to-start timer. These timers limit how soon the chiller can be
started. START INHIBIT TIMER is displayed on the MAIN-
STAT screen. See the Start-Up/Shutdown/Recycle Sequence
section, page 62, for more information on this topic.
OCCUPANCY SCHEDULE — The chiller schedule, de-
scribed in the Time Schedule Operation section (page 23), de-
termines when the chiller can run. Each schedule consists of
from 1 to 8 occupied or unoccupied time periods, set by the op-
erator. The chiller can be started and run during an occupied
time period (when OCCUPIED? is set to YES on the MAIN-
IMPORTANT: Do not alter the factory settings for VDO
Diffuser Control without consulting with Carrier Engineer-
ing. Incorrect settings could cause the machine to repeat-
edly shut down on rotating stall alarm or surge.
Fig. 24 — Auto Demand Limit Wiring (1 to 5 vdc)
a19-1885