Rockwell Automation 1336T Wiring and Grounding Guide, (PWM) AC Drives - Manual (Page 75)

Rockwell Automation Publication DRIVES-IN001M-EN-P - March 2014 75
Chapter
6
Electromagnetic Interference
This chapter discusses types of electromagnetic interference and its impact on
drive systems.
What Causes Common Mode
Noise Faster output dV/dt transitions of IGBT drives increase the possibility for
increased common mode (CM) electrical noise. Common mode noise is a type of
electrical noise induced on signals with respect to ground.
Electrical noise from drive operation can interfere with adjacent sensitive
electronic equipment, especially in areas where many drives are concentrated.
Generating common mode currents by varying frequency inverters is similar to
the common mode currents that occur with DC drives, although AC drives
produce a much higher frequency then DC drives (250 kHz…6 MHz). Inverters
have a greater potential for exciting circuit resonance because of very fast turn on
switches causing common mode currents to look for the lowest impedance path
back to the inverter. The dV/dt and di/dt from the circulating ground currents
can couple into the signal and logic circuits, causing improper operation and
possible circuit damage. When conventional grounding techniques do not work,
you must use high frequency bonding techniques. If installers do not use these
techniques, motor bearing currents increase and system circuit boards have the
potential to fail prematurely. Currents in the ground system can cause problems
with computer systems and distributed control systems.
X
0
R
S
T
U
V
W
PE
A
B
C
PE
C
lg-m
C
lg-c
V
ng
Input Transformer
AC Drive
Motor Frame
Feedback
Device
Motor
System Ground
Path for Common
Mode Current
Path for Common
Mode Current
Path for Common
Mode Current
Path for
Common Mode
Current
Path for Common
Mode Current