Rockwell Automation 1336T Wiring and Grounding Guide, (PWM) AC Drives - Manual (Page 76)

76 Rockwell Automation Publication DRIVES-IN001M-EN-P - March 2014
Chapter 6 Electromagnetic Interference
Containing Common Mode
Noise With Cabling The type of cable that is used can affect the ability to contain common mode
noise in a system that incorporates a drive.
Conduit
The combination of a ground conductor and conduit contains most capacitive
current and returns it to the drive without polluting the ground grid. A conduit
can still have unintended contact with grid ground structure due to straps,
support, and so on. The AC resistance characteristics of earth are generally
variable and unpredictable, making it difficult to know how noise current is
divided between wire, conduit, or the ground grid.
Shielded or Armored Power Cable
The predominant return path for common mode noise is the shield or armor
itself when you use shielded or armored power cables. Unlike conduit, the shield
or armor is isolated from accidental contact with grounds by a PVC outer
coating. The coating makes the majority of noise current flow in the controlled
path and very little high frequency noise flows into the ground grid.
Noise current returning on the shield or safety ground wire is routed to the drive
PE terminal, down to the cabinet PE ground bus, and then directly to the
grounded neutral of the drive source transformer. When bonding the armor or
shield to the drive PE, use a low impedance cable or strap, as opposed to the
smaller gauge ground wire supplied as part of the motor cable or supplied
separately. Otherwise, the higher frequencies associated with the common mode
noise can find this cable impedance higher and look for a lower impedance path.
The radiated emissions of the cable are minimal because the armor completely
covers the noisy power wires. Also, the armor prevents EMI coupling to other
signal cables that are routed in the same cable tray.
Another effective method of reducing common mode noise is to attenuate the
noise before it can reach the ground grid. Install a common mode ferrite core on
the output cables to reduce the amplitude of the noise to a level that makes it
relatively harmless to sensitive equipment or circuits. Common mode cores are
most effective when multiple drives are in a relatively small area. For more
information, refer to 1321-M Common Mode Chokes Instructions, publication
1321-5.0 .
Follow these guideline as a general rule for installing common mode chokes:
• If the distance between the drive and motor, or the drive and input
transformer, is greater than 22.8 m (75 ft), and
• If sensitive circuits with leads greater then 22.8 m (75 ft), such as encoders,
analog or capacitive sensors, are routed in or out of the cabinet near the
drive or transformer, then
Install common mode chokes.