Electromagnetic Interference
6-3
As a general rule:
IF
the distance between the drive and motor or the distance between drive
and input transformer is greater than 75 feet.
AND
IF
sensitive circuits with leads greater then 75 feet such as: encoders,
analog, or capacitive sensors are routed, in or out of the cabinet, near the
drive or transformer
THEN
Common mode chokes should be installed.
How Electromechanical
Switches Cause
Transient
Interference
Electromechanical contacts cause transient interference when switching
inductive loads such as relays, solenoids, motor starters, or motors. Drives,
as well as other devices having electronic logic circuits, are susceptible to
this type of interference.
Examine the following circuit model for a switch controlling an inductive
load. Both the load and the wiring have inductance, which prevents the
current from stopping instantly when the switch contacts open. There is also
stray capacitance in the wiring.
Interference occurs when the switch opens while it is carrying current. Load
and cable inductance prevents the current from immediately stopping. The
current continues to flow, and charges the capacitance in the circuit. The
voltage across the switch contacts (VC) rises, as the capacitance charges.
This voltage can reach very high levels. When the voltage exceeds the
breakdown voltage for the space between the contacts, an arc occurs and the
voltage returns to zero. Charging and arcing continues until the distance
between the contacts is sufficient to provide insulation. The arcing radiates
noise at an energy levels and frequencies that disturb logic and
communication circuits.
If the power source is periodic (like AC power), you can reduce the
interference by opening the contact when the current waveform crosses
zero. Opening the circuit farther from zero elevates the energy level and
creates more interference.
Load
Power
Wiring Inductance
Load
Inductance
Wiring
Capacitance
V
C
Summary of Contents for Allen-Bradley 1305-AA02A
Page 4: ...ii Summary of Changes Notes ...
Page 40: ...2 18 Power Distribution Notes ...
Page 48: ...3 8 Grounding Notes ...
Page 68: ...4 20 Practices Notes ...
Page 78: ...6 8 Electromagnetic Interference Notes ...
Page 94: ...Glossary 4 UL Underwriters Laboratories ...
Page 100: ...Index 6 ...
Page 101: ......