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

5 - PIC III SYSTEM FUNCTIONS

NOTE: 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 in both 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 letters and bold represent
softkeys on the ICVC (e.g. enteR and eXIt).
See Table 3 for examples of the type of information that can
appear on the ICVC screens. Figures 3-9 give an overview
of ICVC operations and menus. The sequence of screens
that should be selected to view any parameter in the ICVC
operations and menus.

5.1 - Capacity control

Generally the chiller adjusts capacity 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 reset 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 4) or from a CCN device.
The default reset value is 0 K. 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
PROPORTIONAL ECW (Entering Chilled Water) GAIN
in the Service/Equipment Service/SETUP screen. Increas-
ing the PROPORTIONAL INC BAND or PROPORTIO-
NAL 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 34.
Factors and variables used in the capacity control determina-
tion 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.
5.1.1 - 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 deviation from the CONTROL POINT
(see above). During operation, 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 percentage change to
either the guide vane position or TARGET VFD SPEED.
Factors considered in the capacity control algorithm 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),
() ACTUAL GUIDE VANE POSITION, (3) ACTUAL
VFD SPEED, and (4) surge prevention mode. Generally
the controller will maintain the highest inlet guide vane
setting at the lowest speed to maximise 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 position 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 relative 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 4.
Table 4 - Guide vane delta modes
Guide vane delta Normal control mode
IGV
From +0.2 to Increase 1st
+2.0
From –0.2 to Decrease
–2.0 when VFD
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
voltage (400 V), VFD output (motor) current is a few per-
cent higher than line current at full speed (50 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.
Surge prevention mode
VFD IGV VFD
Increase Increase only if Increase 1st
when IGV = VFD speed =
max max and if hot
gas bypass is
present and
open
Decrease Decrease —
1st
33