6-2
Electromagnetic Interference
may cause problems with computer systems and distributed control
systems.
Containing Common Mode
Noise With Cabling
Cable type has a great effect on 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 may still have unintended contact with grid ground structure
due to straps, support, etc. The AC resistance characteristics of earth are
generally variable and unpredictable, making it difficult to predict how
noise current will divide between wire, conduit or the ground grid.
Shielded or Armored Power Cable
The predominant return path for common mode noise is the shield/armor
itself when using shielded or armored power cables. Unlike conduit, the
shield/armor is isolated from accidental contact with grounds by a PVC
outer coating. Making 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. Take care when
bonding the armor or shield to the drive PE. A low impedance cable or strap
is recommended when making this connection, as opposed to the smaller
gauge ground wire either supplied as part of the motor cable or supplied
separately. Otherwise, the higher frequencies associated with the common
mode noise will find this cable impedance higher and look for a lower
impedance path. The cable’s radiated emissions are minimal because the
armor completely covers the noisy power wires. Also, the armor prevents
EMI coupling to other signal cables that might be routed in the same cable
tray.
Another effective method of reducing common mode noise is to attenuate it
before it can reach the ground grid. Installing a common mode ferrite core
on the output cables can 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 located in a
relatively small area. For more information see the
1321-M Common Mode
Chokes Instructions
, publication 1321-5.0.
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: ......