Danfoss VLT series, Service Manual

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4-8
VLT is a registered Danfoss trademark
Motor cable selection. The management of the motor
conductors has the greatest influence on the EMI
characteristics of the system. These conductors should
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 AC line wiring then no further
consideration is needed. If the conductors are routed close
to other susceptible conductors, or if the system is
suspected of causing EMI problems then alternate motor
wiring methods should be considered.
Installing shielded power cable is the most effective means
to alleviate EMI problems. The cable’s shield forces the
noise current to flow directly back to the drive before it
gets back into the power network or takes other
undesirable and unpredictable high frequency paths. Unlike
most signal wiring, the shielding on the motor cable should
be terminated at both ends.
If shielded motor cable is not available, then 3 phase
conductors plus ground in a conduit will provide some
degree of protection. This technique will not be as effective
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 on
the market. Each of these recommends one or more specific
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 communication cables as to other signal cables. Using
twisted-pair cables and routing them away from power
conductors is encouraged. While shielded cable provides
additional EMI protection, the shield capacitance may
reduce the maximum allowable cable length at high data
rates.
Proper EMC Installation
Shown in Figure 4-5 is a correct installation with EMC
considerations in mind. Although most installations will not
follow all the recommended practices the closer an
installation resembles this example the better immunity the
network will have against EMI. Should EMI problems arise
in an installation, refer to this example. Attempt to replicate
this installation recommendation as closely as possible to
alleviate such problems.
Figure 4-5. Proper EMC Installation