Specifications

34
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 ADDED EXAMPLES,
etc.) with associated values (e.g., modes, temperatures, pres-
sures, percentages, on, off, enable, disable, etc.).Words printed
in all capital letters and in a box represent softkeys on the
ICVC (e.g., and ). See Table 3 for examples of
the type of information that can appear on the ICVC screens.
Figures 15-21 give an overview of ICVC operations and
menus. The sequence of screens that should be selected to
view any parameter in the ICVC can be found in Appendix A.
CAPACITY CONTROL — The PIC III controls the chiller
capacity by changing the compressor speed in response to
chilled liquid temperature deviation away from the CONTROL
POINT. The CONTROL POINT may be changed by a CCN
network device or is calculated by the PIC III adding any active
chilled liquid reset to the ECL or LCL SET POINT. The
CONTROL POINT may be viewed or overridden from the
MAINSTAT screen. See page 23.
Changes to the chiller capacity and system overrides are
achieved through the VFD SPEED OUTPUT.
The VFD SPEED OUTPUT is controlled by varying the
output from 0 to 100%. The TARGET VFD SPEED is forcible
and allows the operator manual control of the VFD SPEED
OUTPUT. The TARGET VFD SPEED will be allowed to
change every five seconds unless TARGET VFD SPEED is
forced.
The TARGET VFD SPEED is controlled between the VFD
MINIMUM SPEED and VFD MAXIMUM SPEED (refer to
SETUP2 screen) based on the Capacity Control algorithm. The
PIC III controls monitor the compressor oil properties and set a
COMPRESSOR MINIMUM SPEED to ensure sufficient com-
pressor bearing lubrication under all operating conditions. The
TARGET VFD SPEED shall not be allowed to be forced be-
low the COMP MINIMUM SPEED. A summary of all capaci-
ty control conditions and responses is shown in Table 4.
Changes to the VFD SPEED are implemented by the
following methods:
FORCED — The TARGET VFD SPEED can be forced from
the ICVC. The TARGET VFD SPEED is set to the forced
value and remains there until the force is removed or the chiller
is shut down. The forced value is limited between the VFD
MINIMUM SPEED and VFD MAXIMUM SPEED.
NORMAL CONDITIONS — VFD speed changes are based
on the calculated change in VFD DELTA multiplied by the
VFD GAIN. VFD DELTA is displayed in the COMPRESS
screen. The VFD GAIN increases or decreases the commanded
VFD speed change with respect to the VFD DELTA calculated
by the ICVC.
CAPACITY INCREASE — If the TARGET VFD SPEED is
less than the VFD MAXIMUM SPEED, the TARGET VFD
SPEED is increased by the VFD DELTA times the VFD
GAIN.
CAPACITY DECREASE — If the TARGET VFD SPEED
is greater than the VFD MINIMUM SPEED, the TARGET
VFD SPEED is decreased by the VFD DELTA times the VFD
GAIN.
Table 4 — Capacity Control Conditions
• VFD DELTA less than 0.2% will not cause the VFD
SPEED OUTPUT to increase or decrease.
• If the Capacity Decrease is in effect due to an override
then the VFD SPEED OUTPUT decreases at a rate of
2.0% per 5 seconds until the decrease condition is satis-
fied. This will occur regardless of VFD DELTA value.
• A VFD SPEED OUT OF RANGE fault will be declared
if the ACTUAL VFD SPEED exceeds the VFD SPEED
OUTPUT ±10% for 75 seconds when the chiller is
running.
• A COMP MIN SPEED LIMITED RUN CAPACITY
OVERRIDE alert will be declared when the TARGET
VFD SPEED is being limited by the COMPRESSOR
MINIMUM SPEED.
ECL CONTROL OPTION — If this option is enabled, the
PIC III modulates the compressor speed in response to the
entering chilled liquid temperature instead of the LEAVING
CHILLED LIQUID temperature. The ECL CONTROL
OPTION may be viewed on the TEMP_CTL screen, which is
accessed from the EQUIPMENT SERVICE screen.
CHILLED LIQUID DEADBAND — This is the tolerance
range on the chilled liquid/brine temperature control point. If
the liquid temperature goes outside the CHILLED LIQUID
DEADBAND, the PIC III increases or decreases compressor
speed until the temperature is within tolerance. The PIC III
may be configured with a 0.5 to 2 F (0.3 to 1.1 C) deadband.
CHILLE
D LIQUID DEADBAND may be viewed or modified
on the SETUP1 screen, which is accessed from the EQUIP-
MENT SERVICE table.
Example: A 1º F (0.6º C) CHILLED LIQUID DEAD-
BAND setting controls the liquid temperature within ±0.5º F
(0.3º C) of the CONTROL POINT. This may cause frequent
changes in compressor speed if the cooling load fluctuates
frequently. A value of 1º F (0.6º C) is the default setting.
PROPORTIONAL BANDS AND PROPORTIONAL
GAIN — The PIC III uses the PROPORTIONAL INC
(Increase) BAND, PROPORTIONAL DEC (Decrease) BAND,
and the PROPORTIONAL ECL (Entering Chilled Liquid)
GAIN to determine how fast or slow to respond. Proportional
band is the rate at which the compressor speed is changed in
proportion to how far the chilled liquid/brine temperature is
from the CONTROL POINT. Proportional gain determines
how quickly the VFD reacts to how quickly the temperature is
deviating from the CONTROL POINT. The proportional bands
and proportional gain may be viewed or modified from the
SETUP2 screen, which is accessed from the EQUIPMENT
SERVICE table.
The Proportional Band
— The PIC III controls can be config-
ured to respond differently to temperature deviations above the
control point and to temperature deviations below the control
point.
ENTER
EXIT
Capacity
DELTA
TARGET VFD
Forced
VFD Speed
= Min.
VFD Speed
= Max
VFD SPEED
CHANGE
Increase
X
TARGET VFD
= Forced Value
Decrease
X
TARGET VFD
= Forced Value
Increase
TARGET VFD
= TARGET VFD +
(VFD Delta *
VFD Gain)
Increase
X No Change
Decrease
X No Change
Decrease
TARGET VFD
= TARGET VFD +
(VFD Delta *
VFD Gain)