Allen-Bradley micrologix 1500, User Manual

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Publication 1764-UM001A-US-P
Installing Your Controller 2-5
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 1500 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 1500
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.
AB Spares