Cutler-Hammer D320 PLC, User Manual

Page: 201 / 258

Chapter 8: Troubleshooting Noise Problems
189
Methods to Handle I/O Inductive Loads
Several methods exist for handling I/O inductive loads.
DC Input Module
Attach a diode in a reverse biased direction parallel to the inductive load, as close as possible to the
load.
AC Input Module
Attach an RC network parallel to the inductive load.
Handling Long Cable Runs
When a long cable run is needed to attach the AC input module to an external input device attach a
surge suppressor parallel to the input module. When possible, convert the application so a DC input
module can be used instead of the AC input module. The input circuitry of DC input modules
inherently have filters that suppress noise and therefore are less affected by the noise from inductive
loads and stray wiring capacitance.
Protecting Against Arcing
When a relay output module switches an inductive load, a surge voltage measured in thousands of
volts is generated across the relay contacts. This causes arcing (an electrical discharge between two
contact points that can vaporize the contact material) and shortens the contact life of the relay.
Eventually this arcing can destroy the relay contacts. Below is a chart of some methods to protect the
relay contacts.
DC input module
DC power
COM
IN
Inductive
load
Flywheel
diode
AC input module
COM
IN
Inductive
load
R
C
AC input module
In case of AC power
COM
IN
R
C RIN
wiring is long