Rockwell Automation 1336T Wiring and Grounding Guide, (PWM) AC Drives User Manual
Page 77
Rockwell Automation Publication DRIVES-IN001M-EN-P - March 2014
77
Electromagnetic Interference Chapter 6
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 with electronic logic circuits, are susceptible to this type of
interference.
Examine this circuit model for a switch controlling an inductive load. Both the
load and the wiring have inductance that 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.
How to Prevent or Mitigate
Transient Interference from
Electromechanical Switches
The most effective way to avoid this type of transient interference, is to use a
device like an Allen-Bradley Bulletin 156 contactor to switch inductive AC loads.
These devices feature zero cross switching.
V
C
Load
Power
Wiring
Capacitance
Load
Inductance
Wiring Inductance
AC
A1
A2
L1
T1
Bulletin 156
Contactor
Load