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EXCEL 10 W7750A,B,C CONSTANT VOLUME AHU CONTROLLER
33 74-2958—1
Table 8. VA Ratings For Transformer Sizing.
For contactors and similar devices, the in-rush power ratin
g
s
should be used as the worst case values when performin
g
power bud
g
et calculations. Also, the application en
g
ineer
must consider the possible combinations of simultaneousl
y
ener
g
ized outputs and calculate the VA ratin
g
s accordin
g
l
y
.
The worst case, that uses the lar
g
est possible VA load, should
be determined when sizin
g
the transformer.
LINE LOSS
Excel 10 Controllers must receive a minimum suppl
y
volta
g
e
of 20 Vac. If lon
g
power or output wire runs are re
q
uired, a
volta
g
e drop due to Ohms Law
(
I x R
)
line loss must be
considered. This line loss can result in a si
g
nificant increase in
total power re
q
uired and thereb
y
affect transformer sizin
g
.
The followin
g
example is an I x R line-loss calculation for a
200 ft.
(
61m
)
run from the transformer to a W7750 Controller
drawin
g
37 VA usin
g
two 18 AWG
(
1.0 mm
2
)
wires.
The formula is:
Loss = [len
g
th of round-trip wire run
(
ft.
)
] X [resistance in
wire
(
ohms per ft.
)
] X [current in wire
(
amperes
)
]
From specification data:
18 AWG twisted pair wire has 6.52 ohms per 1000 feet.
Loss = [
(
400 ft.
)
X
(
6.52/1000 ohms per ft.
)
] X
[
(
37 VA
)
/
(
24V
)
] = 4.02 volts
This means that four volts are
g
oin
g
to be lost between the
transformer and the controller; therefore, to assure the
controller receives at least 20 volts, the transformer must
output more than 24 volts. Because all transformer output
volta
g
e levels depend on the size of the connected load, a
lar
g
er transformer outputs a hi
g
her volta
g
e than a smaller one
for a
g
iven load. Fi
g
. 21 shows this volta
g
e load dependence.
In the precedin
g
I x R loss example, even thou
g
h the
controller load is onl
y
37 VA, a standard 40 VA transformer is
not sufficient due to the line loss. From Fi
g
. 21, a 40 VA
transformer is
j
ust under 100 percent loaded
(
for the 37 VA
controller
)
and, therefore, has a secondar
y
volta
g
e of 22.9
volts.
(
Use the lower ed
g
e of the shaded zone in Fi
g
. 21 that
represents the worst case conditions.
)
When the I x R loss of
four volts is subtracted, onl
y
18.9 volts reaches the controller,
which is not enou
g
h volta
g
e for proper operation.
In this situation, the en
g
ineer basicall
y
has three alternatives:
1.
Use a lar
g
er transformer; for example, if an 80 VA
model is used, see Fi
g
. 21, an output of 24.4 volts
minus the four volt line loss supplies 20.4V to the
controller. Althou
g
h acceptable, the four-volt line-loss in
this example is hi
g
her than recommended. See the
followin
g
IMPORTANT
.
2.
Use heavier
g
au
g
e wire for the power run. 14 AWG
(
2.0
mm
2
)
wire has a resistance of 2.57 ohms per 1000 ft.
which, usin
g
the precedin
g
formula,
g
ives a line-loss of
onl
y
1.58 volts
(
compared with 4.02 volts
)
. This would
allow a 40 VA transformer to be used. 14 AWG
(
2.0
mm
2
)
wire is the recommended wire size for 24 Vac
wirin
g
.
3.
Locate the transformer closer to the controller, thereb
y
reducin
g
the len
g
th of the wire run, and the line loss.
The issue of line-loss is also important in the case of the
output wirin
g
connected to the Triac di
g
ital outputs. The
same formula and method are used. The rule to
remember is to keep all power and output wire runs as
short as practical. When necessar
y
, use heavier
g
au
g
e
wire, a bi
gg
er transformer, or install the transformer
closer to the controller.
IMPORTANT
No installation should be designed where the line
loss is greater than two volts to allow for nominal
operation if the primary voltage drops to 102 Vac
(120 Vac minus 15 percent).
To meet the National Electrical Manufacturers Association
(
NEMA
)
standards, a transformer must sta
y
within the NEMA
limits. The chart in Fi
g
. 21 shows the re
q
uired limits at various
loads.
With 100 percent load, the transformer secondar
y
must
suppl
y
between 23 and 25 volts to meet the NEMA standard.
When a purchased transformer meets the NEMA standard
DC20-1986, the transformer volta
g
e-re
g
ulatin
g
abilit
y
can be
considered reliable. Compliance with the NEMA standard is
voluntar
y
.
The followin
g
Hone
y
well transformers meet this NEMA
standard:
Transformer Type VA Rating
AT20A 20
AT40A 40
AT72D 40
AT87A 50
AK3310 Assembl
y
100
Device Description VA
W7750A Excel 10 W7750 Controller 6.0
W7750B,C Excel 10 W7750 Controllers 12.0
ML6161A/B Damper Actuator, 35 lb-in. 2.2
R8242A Contactor 21.0
M6410A Valve Actuator 0.7
MMC325 Pneumatic Transducer 5.0
ML684 Versadrive Valve Actuator 12.0
ML6464 Damper Actuator, 66 lb-in. 3.0
ML6474 Damper Actuator, 132 lb-in. 3.0
ML6185 Damper Actuator SR 50 lb-in. 12.0
ML7984B PWM Valve Actuator 6.0