5.2.7 Preventive Measures
EMI-related problems are more effectively alleviated during
the design and installation phases rather than after the
system is in service. Many of the steps listed here can be
implemented at a relatively low cost when compared to
the cost for identifying and fixing the problem later in the
field.
Grounding
Ground the adjustable frequency drive and motor solidly
to the equipment frame. A good high frequency
connection allows the high frequency currents to return to
the adjustable frequency drive rather than travel through
the power network. The ground connection is ineffective if
it has high impedance to high frequency currents. Make
the connection as short and direct as is practical. Flat-
braided cable has lower high-frequency impedance than
round cable. Mounting the adjustable frequency drive or
motor onto a painted surface does not create an effective
ground connection. In addition, running a separate ground
conductor directly between the adjustable frequency drive
and the running motor is recommended.
Cable Routing
Avoid routing motor wiring, line power wiring, and signal
wiring in parallel. If parallel routing is unavoidable, try to
maintain a separation of 200 mm (6–8 inches) 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 that are designed to minimize the
effects of EMI. While unshielded twisted-pair cables are
often adequate, shielded twisted-pair cables provide
another degree of protection. Terminate the shield on the
signal cable in a manner that is appropriate for the
connected equipment. Avoid terminating the shield
through a pigtail connection, which increases the high
frequency impedance and spoils the effectiveness of the
shield. Refer to
chapter 2.9 Grounding Shielded Cables
.
An alternative is to twist the existing single conductors to
provide a balanced capacitive and inductive coupling,
cancelling 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.
Check these conductors first when EMI problems occur.
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 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 effective means
to alleviate EMI problems. The 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
is terminated at both ends.
If a shielded motor cable is not available, then 3-phase
conductors plus ground in a conduit provide some degree
of protection. This technique is 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 in the market. Each recommends one or more
specific types of twisted-pair, shielded twisted-pair, or
proprietary cables. Refer to the manufacturer’s documen-
tation 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 more EMI protection, the
shield capacitance could reduce the maximum allowable
cable length at high data rates.
Adjustable Frequency Drive ...
Service Manual
MG94A222
Danfoss A/S © Rev. 2014-02-10 All rights reserved.
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Summary of Contents for VLT FC 103
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