46
Table 4 — Capacity Control Conditions
Changes to the TARGET 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 MIN-
IMUM SPEED and VFD MAXIMUM SPEED.
NORMAL CONDITIONS — The 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.
• 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 run-
ning.
• 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
controls may be configured with a 0.5 to 2 F (0.3 to 1.1 C)
deadband. CHILLED LIQUID DEADBAND may be viewed or
modified on the SETUP1 screen, which is accessed from the
EQUIPMENT 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.
The PIC III control’s response to temperatures above the
control point is affected by the PROPORTIONAL INC BAND.
This parameter will slow or quicken the rate at which the com-
pressor speed is changed in response to chilled liquid/brine
temperatures above the control point plus
1
/
2
times the
CHILLED LIQUID DEADBAND. The PROPORTIONAL INC
BAND can be adjusted from a setting of 2 to 10; the default
setting is 6.5. A smaller value of PROPORTIONAL INC
BAND will increase the rate at which the compressor speed is
increased.
The response below the control point is called the PRO-
PORTIONAL DEC BAND. This parameter will slow or quick-
en the rate at which the compressor speed is changed in re-
sponse to chilled liquid temperatures below the CONTROL
POINT minus
1
/
2
times the CHILLED LIQUID DEAD BAND.
The PROPORTIONAL DEC BAND can be adjusted on the
ICVC from a setting of 2 to 10. The default setting is 6.0. A
smaller value of PROPORTIONAL DEC BAND will increase
the rate at which the compressor speed is decreased.
The PROPORTIONAL ECL GAIN changes the amount the
compressor speed is changed each time the PIC III controls
command an adjustment. The PROPORTIONAL ECL GAIN
can be adjusted from 1 to 3. A larger value of PROPORTION-
AL ECL GAIN will increase the amount the compressor speed
changes each time the controls call for a change.
DEMAND LIMITING — The PIC III control module re-
sponds to the ACTIVE DEMAND LIMIT set point by limiting
the amps or kilowatts consumed by the chiller. It compares the
ACTIVE DEMAND LIMIT set point to the DEMAND LIMIT
SOURCE (either the actual AVERAGE LINE CURRENT or the
actual MOTOR KW), depending on how the control is config-
ured. DEMAND LIMIT SOURCE is on the RAMP_DEM
screen. The default DEMAND LIMIT SOURCE is the com-
pressor motor amps.
CHILLER TIMERS — The PIC III control module maintains
2 runtime clocks, known as COMPRESSOR ONTIME and
SERVICE ONTIME. COMPRESSOR ONTIME indicates the
total life-time compressor run hours. This timer can register up
to 500,000 hours before the clock turns back to zero. The
SERVICE ONTIME is a resettable timer that can be used to
indicate the hours since the last service visit or any other event.
The time can be changed from the ICVC to whatever value is
desired. The SERVICE ONTIME timer can register up to
32,767 hours before it rolls over to zero.
CAPACITY
DELTA
VFD SPEED CONTROL VFD SPEED CHANGE
INCREASE
TARGET VFD
FORCED
TARGET VFD = Forced Value
—
TARGET VFD = TARGET VFD +
(VFD Delta x FD Gain)
VFD SPEED
= MAX
No Change
DECREASE
TARGET VFD
FORCED
TARGET VFD = Forced Value
—
TARGET VFD = TARGET VFD +
(VFD Delta x VFD Gain)
VFD SPEED
= MIN.
No Change
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