Rockwell Automation 1747-L5xx SLC 500 Modular Hardware Style User Manual - Manual (Page 46)

Publication 1747-UM011G-EN-P - June 2008
46 Selecting Your Hardware Components
Selecting Isolation
Transformers If there is high frequency conducted noise in or around your
distribution equipment, 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, calculate the power
required by the chassis power supply (or supplies if more than one
chassis in system) and any input circuits and output loads that are
connected through this transformer.
You can find the power requirement (VA rating) for the chassis power
supplies in the specifications starting on page 177. 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 1746-P1 power supply, a 16-point ac input
module, catalog number 1746-IA16, (12 mA at 120V ac) and a 16-point
ac triac output module, catalog number 1746-OA16, (0.5A at 120V ac),
the power consumed would be:
135VA + (16)(120V)(0.012 A) + (16)(120V)(0.5 A) = 1118VA
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.
Most industrial environments are susceptible to power transients or
spikes. To help insure fault-free operation and protection of
equipment, use suppression devices on power line to the equipment
in addition to the isolation equipment.
IMPORTANT
In this case, 0.5 A is the maximum rating of the triac output at 30
°C (86 °F). If the load draws less than 0.5 A, this figure may be
reduced accordingly. The output portion of the VA calculation
should reflect the current requirements of selected loads.