Rockwell Automation 1772-L8_LW_LWP_LX_LXP,D17726.5.8 User Manual - Manual (Page 45)

Installing Your
Programmable Controller
Chapter 4
4-6
To determine the required rating of the transformer, add the
external-transformer rating for the power supply and all other power
requirements (input circuits, output circuits). The power requirements
must take into consideration the surge currents of devices controlled by the
processor. Choose a transformer with the closest standard transformer
rating above the calculated requirements.
For example, the external-transformer rating of a 1771-P4S power supply
is 150VA. A 150VA transformer could be used if a 1771-P4S power
supply were the only load. A 500VA transformer should be used if there
were 350VA of load in addition to that of the 1771-P4S power supply.
Undervoltage Shutdown
Each power supply is designed to generate a shutdown signal if the ac line
voltage drops below its lower voltage limit. A shutdown in that situation is
necessary to ensure that only valid data is stored in memory. The
controller resumes operation when the line voltage reaches the lower
voltage limit again.