5.2.7 Preventive Measures
EMI-related problems are more eectively alleviated during
the design and installation phases rather than after the
system is in service. Many of the listed steps can be
implemented at a relatively low cost compared to the cost
of identifying and
xing
the problem later.
Grounding
Ground the frequency converter and motor solidly to the
equipment frame. A good high-frequency connection is
necessary to allow the high frequency currents to return to
the frequency converter instead of traveling through the
power network. The ground connection is
ineective
if it
has high impedance to high frequency currents. Therefore,
it must be as short and direct as possible. Flat-braided
cable has lower high frequency impedance than round
cable. Mounting the frequency converter or motor onto a
painted surface creates an eective ground connection. In
addition, running a separate ground conductor directly
between the frequency converter and the running motor is
recommended.
Cable routing
Avoid parallel routing of:
•
Motor wiring.
•
Mains wiring.
•
Signal wiring.
If parallel routing is unavoidable, preferably maintain a
separation of 200 mm (6–8 in) between the cables or
separate them with a grounded conductive partition. Avoid
routing cables through free air.
Signal cable selection
Single conductor 600 V rated wires provide the least
protection from EMI. Twisted pair and shielded twisted-pair
cables are available which are designed to minimize the
eects of EMI. While unshielded twisted-pair cables are
often adequate, shielded twisted-pair cables provide
another degree of protection. Terminate the signal cable
shield in a manner that is appropriate for the connected
equipment. Avoid terminating the shield through a pigtail
connection as it increases the high frequency impedance
and spoils the eectiveness of the shield.
A simple alternative is to twist the existing single
conductors to provide a balanced capacitive and inductive
coupling. This operation cancels dierential mode
interference. While not as eective as true twisted-pair
cable, it can be implemented in the eld using the
materials at hand.
Motor cable selection
Motor conductors have the greatest inuence on the EMI
characteristics of the system. These conductors must
receive the highest attention whenever EMI is a problem.
Single conductor wires provide the least protection from
EMI emissions. Often, if these conductors are routed
separately from the signal and mains wiring, no further
consideration is needed. If the conductors are routed close
to other susceptible conductors, or if the system is
suspected to cause EMI problems, consider alternate motor
wiring methods.
Installing shielded power cable is the most eective way to
alleviate EMI problems. The cable shield forces the noise
current to ow directly back to the frequency converter.
Thus, the noise current cannot get back into the power
network or take other undesirable high frequency paths.
Unlike most signal wiring, the shielding on the motor cable
must be terminated at both ends.
If a shielded motor cable is not available, then 3-phase
conductors along with ground in a conduit provides some
degree of protection. This technique is not as eective as
shielded cable due to the unavoidable contact of the
conduit with various points within the equipment.
Serial communications cable selection
There are various serial communication interfaces and
protocols in the market. Each of these interfaces
recommends 1 or more specic types of twisted pair,
shielded twisted pair, or proprietary cables. Refer to the
manufacturer’s documentation when selecting these
cables. Similar recommendations apply to serial communi-
cation cables as to other signal cables. Using twisted-pair
cables and routing them away from power conductors is
encouraged. While shielded cable provides extra EMI
protection, the shield capacitance may reduce the
maximum allowable cable length at high data rates.
Frequency Converter and Mot...
44 02/2017 All rights reserved. BAS-SVM04B-EN
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