Publication 1763-UM001E-EN-P - June 2015
24 Installing Your Controller
Periodic Tests of Master Control Relay Circuit
Any part can fail, including the switches in a master control relay circuit. The
failure of one of these switches would most likely cause an open circuit, which
would be a safe power-off failure. However, if one of these switches shorts
out, it no longer provides any safety protection. These switches should be
tested periodically to assure they will stop machine motion when needed.
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.
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