Rockwell Automation 1756-L55M12, Selection Manual, Page: 71 / 88

69
1756-SG001G-EN-P — March 2004
Power
Requirements and
Transformer Sizing
Each ac-input power supply generates a shutdown signal on the backplane whenever
the ac line voltage drops below its lower voltage limit. It removes the shutdown signal
when the line voltage comes back up to the lower voltage limit. This shutdown is
necessary to help ensure that only valid data is stored in memory.
The external transformer rating (in VA) of each power supply is greater than its real
input power (in Watts) because a capacitor-input ac/dc supply draws power only from
the peak of the ac voltage wave form. If the transformer is too small, it clips the peak
of the sine wave; when the voltage is still above the lower voltage limit, the power
supply will sense this clipped wave form as low voltage and could prematurely shut
down modules in the chassis.
The following graphs display the backplane power load on the vertical axis. Since
these supplies have multiple outputs, the backplane power load is given in watts.
y
Use the real power value in watts for determining the amount of heat dissipation you
will have inside the enclosure.
y
Use the apparent power value in VA for estimating power distribution sizing.
y
Use the transformer load value in VA of each power supply plus all other loads on a
transformer to determine the required transformer size.