Pic Iii System Functions; Capacity Control; Control Point Deadband - Carrier 19XRV Start-Up, Operation And Maintenance Instructions Manual

PIC III System Functions

IMPORTANT: Words not part of paragraph headings and
printed in all capital letters can be viewed on the ICVC
(e.g., LOCAL, CCN, RUNNING, ALARM, etc.). Words
printed both in all capital letters and italics can also be
viewed on the ICVC and are parameters (CONTROL
MODE, TARGET GUIDE VANE POS, etc.) with associated
values (e.g., modes, temperatures, pressures, percentages,
on, off, enable, disable, etc.). Words printed in all capital let-
ters and in a box represent softkeys on the ICVC (e.g.,
and ). See Table 4 for examples of the type
ENTER EXIT
of information that can appear on the ICVC screens. Figures
14-20 give an overview of ICVC operations and menus.
CAPACITY CONTROL — Generally the chiller adjusts ca-
pacity in response to deviation of leaving or entering chilled
water temperature from CONTROL POINT. CONTROL
POINT is based on the configured SETPOINT (in the
SETPOINT screen: LCW SET POINT or ECW SET POINT or
ICE BUILD SET POINT), and CONTROL POINT is equal to
this SETPOINT plus any active chilled water reset value. A re-
set value may originate from any of the three chilled water/brine
reset options configured in the ICVC Service/Equipment
Service/TEMP_CTL screen (see page 45) or from a CCN de-
vice. The default reset value is 0° F. CONTROL POINT may be
viewed or manually overridden from the MAINSTAT screen.
Minor adjustments to the rate of capacity adjustment can be
made by changing PROPORTIONAL INC (Increase) BAND,
PROPORTIONAL DEC (Decrease) BAND, and PROPOR-
TIONAL ECW (Entering Chilled Water) GAIN in the Service/
Equipment Service/SETUP2 screen. Increasing the PROPOR-
TIONAL INC BAND or PROPORTIONAL DEC BAND, or
decreasing PROPORTIONAL ECW GAIN will reduce the
amplitude of the capacity control response (within limits). See
also Proportional Bands and Gain on page 38.
Factors and variables used in the capacity control determi-
nation are displayed in the Service/Control Algorithm Status/
Capacity screen and in the Status/COMPR screen. Viewing this
data will aid in troubleshooting and understanding current
operation.
Variable Speed (VFD) Application — The PIC III controls
the machine capacity by modulating both motor speed and
inlet guide vanes in response to chilled water temperature devia-
tion from the CONTROL POINT (see above). During opera-
tion, when the CONTROL POINT is not met within 1/3 of the
width of the CHILLED WATER DEADBAND, the controller
will calculate a GUIDE VANE DELTA which will effect a per-
centage change to either the guide vane position or TARGET
VFD SPEED. Factors considered in the capacity control algo-
rithm include: (1) the sign and magnitude of GUIDE VANE
DELTA (based on deviation from CONTROL POINT adjusted
for the error trends and CHILLED WATER DEADBAND), (2)
ACTUAL GUIDE VANE POSITION, (3) ACTUAL VFD
SPEED, and (4) surge prevention mode. Generally the control-
ler will maintain the highest inlet guide vane setting at the low-
est speed to maximize efficiency while avoiding surge.
First, the calculation of GUIDE VANE DELTA is performed.
If GUIDE VANE DELTA is positive, the response will be an
IGV or VFD position increase (within limits). If GUIDE VANE
DELTA is negative, the response will be an IGV or VFD posi-
tion decrease (within limits). Next, the surge prevention mode is
determined based on location of the present operating point on
the CHILLED WATER DELTA T/ACTIVE DELTA P map rel-
ative the configured surge prevention line. This mode will either
be Normal or Surge Prevention. The following table indicates
which output is modulated first. When the first output reaches its
limit (ACTUAL GUIDE VANE position reaches maximum),
the second output is modulated. See Table 5.
Table 5 — Guide Vane Delta Modes
GUIDE VANE
DELTA
IGV
From +0.2 to Increase
+2.0 1st
Decrease
From –0.2 to when VFD
–2.0 speed =
min
Normal Control mode occurs when ACTIVE DELTA T
> SURGE/HGBP DELTA T.
Surge Prevention Mode occurs when ACTIVE DELTA T
≤ SURGE/HGBP DELTA T.
The VFD GAIN parameter allows for additional adjustment
of the VFD response. Increasing VFD GAIN will increase the
rate of speed change.
Generally for the case of line voltage equaling motor volt-
age (460 volts), VFD output (motor) current is a few percent
higher than line current at full speed (60 Hz). As drive speeds
decrease from maximum, drive output voltage decreases
linearly with output frequency, and motor current continues to
increase relative to line current.
The TARGET VFD SPEED, ACTUAL VFD SPEED and the
VFD GAIN can be viewed and modified in the CAPACITY
display screen. The TARGET VFD SPEED can be manually
overridden by the operator from the COMPRESS screen. The
VFD MINIMUM SPEED, VFD MAXIMUM SPEED, VFD
GAIN and VFD INCREASE STEP can be selected and modi-
fied in the SETUP2 display screen. TARGET and ACTUAL
VFD SPEED can be viewed in the COMPRESS screen.
ECW CONTROL OPTION — If this option is enabled, the
PIC III uses the ENTERING CHILLED WATER temperature to
modulate the vanes instead of the LEAVING CHILLED
WATER temperature. The ECW CONTROL OPTION may be
viewed on the TEMP_CTL screen, which is accessed from the
EQUIPMENT SERVICE screen.
CONTROL POINT DEADBAND — This is the tolerance
range on the chilled water/brine temperature control point. If the
water temperature goes outside the CHILLED WATER DEAD-
BAND, the PIC III opens or closes the guide vanes until the tem-
perature is within tolerance. The PIC III may be configured with
a 0.5 to 2 F (0.3 to 1.1 C) deadband. CHILLED WATER DEAD-
BAND may be viewed or modified on the SETUP1 screen,
which is accessed from the EQUIPMENT SERVICE table.
For example, a 1° F (0.6° C) deadband setting controls the
water temperature within ±0.5° F (0.3° C) of the control point.
This may cause frequent guide vane movement if the chilled
water load fluctuates frequently. A value of 1° F (0.6° C) is the
default setting.
DIFFUSER CONTROL — On all units with Frame 5 com-
pressors and those Frame 4 compressors with the variable (split
ring) diffuser option, the PIC III adjusts the diffuser actuator
position (DIFFUSER ACTUATOR on the COMPRESS screen)
based on the ACTUAL GUIDE VANE POSITION. This is
done in accordance with a compressor build-specific "schedule"
entered in the SETUP2 screen. The schedule consists of guide
vane and diffuser positions for three points (designated as the
25%, 50%, and 75% Load Points). In order for the schedule to
be valid, the guide vane values must be ascending and the dif-
fuser values must be descending for the three points. Diffuser
actuator output is controlled by a 4 to 20 mA output from CCM
terminals J8-3(+) and J8-4(–). Figure 21 shows the relationship
between diffuser-related parameters for a typical build.
37
NORMAL SURGE
CONTROL PREVENTION
MODE MODE
VFD IGV
Increase
only if VFD
speed =
Increase
max and if
when IGV
hot gas
= max
bypass is
present
and open
Decrease
Decrease
1st
VFD
Increase
1st
—