Rockwell Automation System Design for the Control of Electrical Noise - Manual (Page 53)

Publication GMC-RM001A-EN-P — July 2001
Chapter
6
Contact Suppression
Chapter Objectives This chapter describes how contact suppressors for solenoids, relays,
and various other switches can reduce electrical noise. This chapter
covers the following topics:
• Understanding contact suppression for AC circuits
• Understanding contact suppression for 24V dc circuits
• Contact suppression effects
Understanding Contact
Suppression for AC Circuits The one potential noise source that the you can reduce directly is a
contact switched load. Even circuits feeding resistive loads will
produce significant switching noise. This is because the wiring both
upstream and downstream of the contact is inductive. Thus, any
switch contact will benefit from suppression.
Examples of AC devices requiring contact suppression include:
• Contactor controlled motors
• Solenoid coils
• Contactor coils
• Relay coils
• Transformer primaries
• Transformer driven indicator lamps
• Fluorescent cabinet lights (also require line filters close to the
lamp)
• Line filters (often present an inductive load)
The only exception is a load driven by a Zero-Crossing Detector
circuit such as Allen Bradley solid-state (Triac) output modules.
Zero-crossing switches reduce noise generation virtually to zero.
Preferred for frequent operation or close to clean zones.
IMPORTANT
All switched, inductive loads in the system must be
suppressed. This is standard practice in any PLC
based control system.