Rockwell Automation 1398-DDM-xxx USE MNL/ULTRA 200 DIG.SERVO.DR - Manual (Page 285)

Publication 1398-5.0 – October 1998
Electromagnetic Compatibility Guidelines for Machine Design E-5
Grounding
High frequency (HF) grounding is different from safety grounding. A
long wire is sufficient for a safety ground, but is completely
ineffective as an HF ground due to the wire inductance. As a rule of
thumb, a wire has an inductance of 20 nH/in regardless of diameter. At
low frequencies it acts as a constant impedance, at intermediate
frequencies as an inductor, and at high frequencies as an antenna. The
use of ground straps is a better alternative to wires. However the
length to width ratio must be 5:1, or better yet 3:1, to remain a good
high frequency connection.
The ground system’s primary purpose is to function as a return current
path. It is commonly thought of as an equipotential circuit reference
point, but different locations in a ground system may be at different
potentials. This is due to the return current flowing through the ground
systems finite impedance. In a sense, ground systems are the sewer
systems of electronics and as such are sometimes neglected.
The primary objective of a high frequency ground system is to provide
a well defined path for HF currents and to minimize the loop area of
the HF current paths. It is also important to separate HF grounds from
sensitive circuit grounds. “Single Point Ground Types” shows single
point grounds for both series (daisy chain) and parallel (separate)
connections. A single point, parallel connected ground system is
recommended.
A ground bus bar or plane should be used as the “single point” where
circuits are grounded. This will minimize common (ground)
impedance noise coupling. The ground bus bar (GBB) should be
connected to the AC ground, and if necessary, to the enclosure. All
circuits or subsystems should be connected to the GBB by separate
connections. These connections should be as short as possible, and
straps should be used when possible. The motor ground conductor
must return to the ground terminal on the drive, not the GBB.
Figure E.2 Single Point Ground Types
Intro
CIRCUIT
1
CIRCUIT
2
CIRCUIT
3
CIRCUIT
1
CIRCUIT
2
CIRCUIT
3
PARALLEL CONNECTION SERIES CONNECTION
Ground Bus Bar