A simple alternative is to twist the existing single conductors to provide a balanced capacitive and
inductive coupling thus canceling out differential mode interference. While not as effective as true
twisted-pair cable, it can be implemented in the field using the materials on hand.
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 line power wiring, then no further consideration is nee-
ded. 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 adjustable frequency 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 ca-
bles. Refer to the manufacturer’s documentation when selecting these cables. Similar recommen-
dations 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 ad-
ditional EMI protection, the shield capacitance may reduce the maximum allowable cable length
at high data rates.
5 Adjustable Frequency Drive and Motor Ap-
plications
High Power Service Manual for VLT
®
FC Series
Adjustable Frequency Drives
5-10
MG.90.L1.22 - VLT
®
is a registered Danfoss trademark
5
Summary of Contents for VLT HVAC FC 100
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