Rockwell Automation 1747-L40 SLC 500 Fixed Hardware Style - Manual (Page 31)

Chapter 1
Selecting Your Hardware Components
1–12
If there is high frequency conducted noise in or around your distribution
equipment, we recommend that you use an isolation transformer in the AC
line to the power supply. This type of transformer provides isolation from
your power distribution system and is often used as a “step down”
transformer to reduce line voltage. Any transformer used with the controller
must have a sufficient power rating for its load. This power rating is
generally expressed in voltamperes (VA).
To select an appropriate isolation transformer, you must calculate the power
required by the fixed I//O chassis and any input circuits and output loads that
are connected through this transformer. The power requirement of any fixed
I/O unit is 50 VA.
The power requirement for the input circuits is determined by the number of
inputs, the operating voltage, and the nominal input current. The power
requirement for output loads is determined by the number of outputs, the
load voltage, and load current.
For example, if you have a 1747–L30B fixed unit with 18 AC inputs (12mA
at 120 VAC) and 12 triac outputs (0.5A at 120 VAC), the power consumed
would be:
50 + (18)(120)(0.012) + (12)(120)(0.5) = 796 VA
Important: In this case, 0.5 Amp is the maximum rating of the triac output
(at +30
°
C). If your load draws less than 0.5 Amp, this figure
may be reduced accordingly. The output portion of the VA
calculation should reflect the current requirements of your
loads.
In general, we recommend that the transformer is oversized to provide some
margin for line voltage variations and other factors. Typically a transformer
that is 25% larger than the calculated VA is sufficient.
Selecting Isolation
Transformers