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EXCEL 10 W7750A,B,C CONSTANT VOLUME AHU CONTROLLER
74-2958—160
STAGED COOLING CONTROL
The Excel 10 W7750 Controller supports up to four sta
g
es of
D/X coolin
g
. As space temperature rises above the current
Coolin
g
Setpoint, the controllers mode of operation is
switched to the COOL mode. When in the COOL mode, all
heatin
g
outputs are driven closed or off
(
with the exception
that occurs durin
g
IAQ Override Operation, see above
)
, and
the sta
g
ed coolin
g
outputs are enabled for use. When in the
COOL mode, the PID coolin
g
control al
g
orithm compares the
current space temperature to the EffectiveCoolSetPt, and
calculates a PID error si
g
nal. This error si
g
nal causes the
coolin
g
sta
g
e outputs to be c
y
cled as re
q
uired to drive the
space temperature back to the setpoint. Fi
g
. 50 illustrates the
relationship between PID error and sta
g
ed output activit
y
. As
the error si
g
nal increases and the space temperature is
g
ettin
g
farther awa
y
from setpoint, or is remainin
g
above
setpoint as time elapses, additional sta
g
es of coolin
g
are
ener
g
ized until, if PID error reaches 100 percent, all
confi
g
ured sta
g
es are on.
Fig. 50. Staged output control versus PID Error.
If economizer dampers are confi
g
ured, and the outdoor air is
suitable for free coolin
g
, the economizer operates as the first
sta
g
e of coolin
g
. For example, if a controller was confi
g
ured
with two sta
g
es of mechanical coolin
g
and an economizer, the
application should be viewed in Fi
g
. 50 as a
three
-sta
g
e
s
y
stem.
Setpoints for the PID
g
ains allow for unit-b
y
-unit ad
j
ustment of
the control loop, if re
q
uired; however, an
y
chan
g
e from the
default values should be minimal.
The PID control al
g
orithm used to control sta
g
ed coolin
g
is
anticipator-driven, and is similar to the al
g
orithm used in the
T7300 commercial thermostat. All sta
g
in
g
events are sub
j
ect
to a minimum intersta
g
e time dela
y
, which is based on the
c
y
cles per hour user settin
g
(
CoolC
y
cHr
)
. The minimum
intersta
g
e time dela
y
ran
g
es from 90 seconds
(
at 12 c
y
cles
per hour
)
to 8.5 minutes
(
at two c
y
cles per hour
)
, see Table
17. The user has the option to disable the minimum run timer
(
DisMinClTimer for coolin
g)
. If the minimum run timer is
disabled, the intersta
g
e time dela
y
is fixed at 20 seconds. The
c
y
clin
g
rate is separatel
y
selectable for heatin
g
and coolin
g
between 2 and 12 c
y
cles per hour
(
cph
)
.
Table 17. Interstage Minimum Times
STAGED HEATING CONTROL
The Excel 10 W7750B,C Controller supports up to four sta
g
es
of heatin
g
. As space temperature falls below the current
Coolin
g
Setpoint, the controller mode of operation is switched
to the HEAT mode. When in the HEAT mode, all coolin
g
outputs are driven closed or off, and the sta
g
ed heatin
g
outputs are enabled for use. When in the HEAT mode, the PID
NO. STAGES
CONFIGURED
PID
ERROR
ONE STAGE
TWO STAGES
THREE STAGES
FOUR STAGES
0% 25% 33% 50% 66% 75% 100% > 100%
CYCLING
STAGE 1
CYCLING
STAGE 1 LOCKED ON
STAGE 2 CYCLING
STAGE 1
CYCLING
STAGE 1 LOCKED ON
STAGE 2 CYCLING
STAGE 1,2 LOCKED ON
STAGE 3 CYCLING
ALL STAGES
LOCKED ON
ALL STAGES
LOCKED ON
ALL STAGES
LOCKED ON
ALL STAGES
LOCKED ON
STAGE 1
CYCLING
STAGE 1
LOCKED ON
STAGE 2
CYCLING
STAGE 1,2
LOCKED ON
STAGE 3
CYCLING
STAGE 1,2,3
LOCKED ON
STAGE 4
CYCLING
M10112
Cycles/Hour Selection Minimum On/Off time (Min.)
28.5
35.5
44.0
53.5
63.0
72.5
82.0
92.0
10 2.0
11 1.5
12 1.5