Справочник Пользователя для Honeywell W7750A
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 ratings
should be used as the worst case values when performing
power budget calculations. Also, the application engineer
must consider the possible combinations of simultaneously
energized outputs and calculate the VA ratings accordingly.
The worst case, that uses the largest possible VA load, should
be determined when sizing the transformer.
should be used as the worst case values when performing
power budget calculations. Also, the application engineer
must consider the possible combinations of simultaneously
energized outputs and calculate the VA ratings accordingly.
The worst case, that uses the largest possible VA load, should
be determined when sizing the transformer.
LINE LOSS
Excel 10 Controllers must receive a minimum supply voltage
of 20 Vac. If long power or output wire runs are required, a
voltage drop due to Ohms Law (I x R) line loss must be
considered. This line loss can result in a significant increase in
total power required and thereby affect transformer sizing.
The following example is an I x R line-loss calculation for a
200 ft. (61m) run from the transformer to a W7750 Controller
drawing 37 VA using two 18 AWG (1.0 mm
of 20 Vac. If long power or output wire runs are required, a
voltage drop due to Ohms Law (I x R) line loss must be
considered. This line loss can result in a significant increase in
total power required and thereby affect transformer sizing.
The following example is an I x R line-loss calculation for a
200 ft. (61m) run from the transformer to a W7750 Controller
drawing 37 VA using two 18 AWG (1.0 mm
2
) wires.
The formula is:
Loss =
[length of round-trip wire run (ft.)] X [resistance in
wire (ohms per ft.)] X [current in wire (amperes)]
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
Loss = [(400 ft.) X (6.52/1000 ohms per ft.)] X
[(37 VA)/(24V)] = 4.02 volts
This means that four volts are going 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
voltage levels depend on the size of the connected load, a
larger transformer outputs a higher voltage than a smaller one
for a given load. Fig. 21 shows this voltage load dependence.
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
voltage levels depend on the size of the connected load, a
larger transformer outputs a higher voltage than a smaller one
for a given load. Fig. 21 shows this voltage load dependence.
In the preceding I x R loss example, even though the
controller load is only 37 VA, a standard 40 VA transformer is
not sufficient due to the line loss. From Fig. 21, a 40 VA
transformer is just under 100 percent loaded (for the 37 VA
controller load is only 37 VA, a standard 40 VA transformer is
not sufficient due to the line loss. From Fig. 21, a 40 VA
transformer is just under 100 percent loaded (for the 37 VA
controller) and, therefore, has a secondary voltage of 22.9
volts. (Use the lower edge of the shaded zone in Fig. 21 that
represents the worst case conditions.) When the I x R loss of
four volts is subtracted, only 18.9 volts reaches the controller,
which is not enough voltage for proper operation.
volts. (Use the lower edge of the shaded zone in Fig. 21 that
represents the worst case conditions.) When the I x R loss of
four volts is subtracted, only 18.9 volts reaches the controller,
which is not enough voltage for proper operation.
In this situation, the engineer basically has three alternatives:
1.
Use a larger transformer; for example, if an 80 VA
model is used, see Fig. 21, an output of 24.4 volts
minus the four volt line loss supplies 20.4V to the
controller. Although acceptable, the four-volt line-loss in
this example is higher than recommended. See the
following IMPORTANT.
model is used, see Fig. 21, an output of 24.4 volts
minus the four volt line loss supplies 20.4V to the
controller. Although acceptable, the four-volt line-loss in
this example is higher than recommended. See the
following IMPORTANT.
2.
Use heavier gauge wire for the power run. 14 AWG (2.0
mm
mm
2
) wire has a resistance of 2.57 ohms per 1000 ft.
which, using the preceding formula, gives a line-loss of
only 1.58 volts (compared with 4.02 volts). This would
allow a 40 VA transformer to be used. 14 AWG (2.0
mm
only 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
wiring.
3.
Locate the transformer closer to the controller, thereby
reducing the length of the wire run, and the line loss.
The issue of line-loss is also important in the case of the
output wiring connected to the Triac digital 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 necessary, use heavier gauge
wire, a bigger transformer, or install the transformer
closer to the controller.
reducing the length of the wire run, and the line loss.
The issue of line-loss is also important in the case of the
output wiring connected to the Triac digital 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 necessary, use heavier gauge
wire, a bigger 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).
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 stay within the NEMA
limits. The chart in Fig. 21 shows the required limits at various
loads.
(NEMA) standards, a transformer must stay within the NEMA
limits. The chart in Fig. 21 shows the required limits at various
loads.
With 100 percent load, the transformer secondary must
supply between 23 and 25 volts to meet the NEMA standard.
When a purchased transformer meets the NEMA standard
DC20-1986, the transformer voltage-regulating ability can be
considered reliable. Compliance with the NEMA standard is
voluntary.
supply between 23 and 25 volts to meet the NEMA standard.
When a purchased transformer meets the NEMA standard
DC20-1986, the transformer voltage-regulating ability can be
considered reliable. Compliance with the NEMA standard is
voluntary.
The following Honeywell transformers meet this NEMA
standard:
standard:
Transformer Type
VA Rating
AT20A
20
AT40A
40
AT72D
40
AT87A
50
AK3310 Assembly
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