Honeywell W7750A VA Ratings For Transformer Sizing Device Description, ML7984B PWM Valve Actuator

EXCEL 10 W7750A,B,C CONSTANT VOLUME AHU CONTROLLER

Table 8. VA Ratings For Transformer Sizing.

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

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.

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 mm2) wires.

The formula is:

Loss = [length 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 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.

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) 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.

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.

2.Use heavier gauge wire for the power run. 14 AWG (2.0 mm2) 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 mm2) 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.

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 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.

The following Honeywell transformers meet this NEMA

standard:
Transformer Type	VA Rating
AT20A	20
AT40A	40
AT72D	40
AT87A	50
AK3310 Assembly	100

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