Service manual

to flow in undesirable paths, thus exposing other locations
to the EMI.
130BC345.10
1
3
4
5
6
2
Illustration 5.4 Alternate Signal Conductor Currents
NOTE
High frequency currents can be coupled into the mains
supplying the frequency converter when the mains
conductors are located close to the motor cables.
5.2.6 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 listed steps can be
implemented at a relatively low cost compared to the cost
of identifying and fixing the problem later.
Earthing
Earth 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 rather than to travel through the
power network. The earth connection is ineffective if it has
high impedance to high frequency currents. Therefore it
must be as short and direct as practical. Flat-braided cable
has lower high-frequency impedance than round cable.
Simply mounting the frequency converter or motor onto a
painted surface does create an effective earth connection.
In addition, running a separate earth conductor directly
between the frequency converter and the running motor is
recommended.
Cable routing
Avoid routing motor wiring, mains 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 an earthed 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 screened twist-pair
cables are available which are designed to minimize the
effects of EMI. While unscreened twisted-pair cables are
often adequate, screened twisted-pair cables provide
another degree of protection. The signal cable screen must
be terminated in a manner that is appropriate for the
connected equipment. Avoid terminating the screen
through a pigtail connection as it increases the high
frequency impedance and spoils the effectiveness of the
screen.
A simple alternative is to twist the existing single
conductors to provide a balanced capacitive and inductive
coupling. This operation cancels 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
Motor conductors have the greatest influence 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, 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 screened power cable is the most effective
means to alleviate EMI problems. The cable screen forces
the noise current to flow directly back to the frequency
converter before it gets back into the power network or
takes other undesirable high frequency paths. Unlike most
signal wiring, the screening on the motor cable must be
terminated at both ends.
If a screened motor cable is not available, then three-phase
conductors along with earth in a conduit provides some
degree of protection. This technique is not as effective as
screened 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 one or more specific types of twisted-pair,
screened 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 screened cable provides additional EMI
protection, the screen capacitance may reduce the
maximum allowable cable length at high data rates.
Frequency Converter and Mot...
VLT
®
HVAC Basic Drive Service Manual
MG90X202 - VLT
®
is a registered Danfoss trademark 45
5 5