Rockwell Automation 1763 MicroLogix 1100 Programmable Controllers User Manual - Manual (Page 26)

Publication 1763-UM001D-EN-P - March 2011
24 Installing Your Controller
Power Considerations The following explains power considerations for the micro controllers.
Isolation Transformers
You may want to use an isolation transformer in the AC line to the controller.
This type of transformer provides isolation from your power distribution
system to reduce the electrical noise that enters the controller 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. The power
rating is expressed in volt-amperes (VA).
Power Supply Inrush
During power-up, the MicroLogix 1100 power supply allows a brief inrush
current to charge internal capacitors. Many power lines and control
transformers can supply inrush current for a brief time. If the power source
cannot supply this inrush current, the source voltage may sag momentarily.
The only effect of limited inrush current and voltage sag on the MicroLogix
1100 is that the power supply capacitors charge more slowly. However, the
effect of a voltage sag on other equipment should be considered. For example,
a deep voltage sag may reset a computer connected to the same power source.
The following considerations determine whether the power source must be
required to supply high inrush current:
• The power-up sequence of devices in a system.
• The amount of the power source voltage sag if the inrush current
cannot be supplied.
• The effect of voltage sag on other equipment in the system.
If the entire system is powered-up at the same time, a brief sag in the power
source voltage typically will not affect any equipment.
Loss of Power Source
The power supply is designed to withstand brief power losses without
affecting the operation of the system. The time the system is operational
during power loss is called program scan hold-up time after loss of power. The
duration of the power supply hold-up time depends on the type and state of
the I/O, but is typically between 10 milliseconds and 3 seconds. When the
duration of power loss reaches this limit, the power supply signals the
processor that it can no longer provide adequate DC power to the system.