Preventive Measures - Danfoss VLT HVAC Drive FC100 Service Manual

Frequency Converter and Mot...

5.3.4 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.
Earthing
The frequency converter and motor should be solidly
earthed to the equipment frame. A good high frequency
connection is necessary to allow the high frequency
currents to return back to the frequency converter rather
than to travel through the power network. The earth
connection will be ineffective if it has high impedance to
high frequency currents. Therefore it should 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
will not 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 a earthed conductive
partition. Avoid routing cables through free air.
Signal Cable Selection
Single conductor 600 volt rated wires provide the least
protection from EMI. Twisted-pair and screened twist-pair
cables are available which are specifically designed to
minimise the effects of EMI. While unscreened twisted-pair
cables are often adequate, screened twisted-pair cables
provide another degree of protection. The signal cable's
screen should be terminated in a manner that is
appropriate for the connected equipment. Avoid
terminating the screen through a pigtail connection as this
increases the high frequency impedance and spoils the
effectiveness of the screen. Refer to Section Earthing
Screened Cables.
A simple alternative is to twist the existing single
conductors to provide a balanced capacitive and inductive
coupling. This cancels 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
mains wiring, then no further consideration is needed. If
High Power Service Manual for Modular F Frame Drives
the conductors are routed close to other susceptible
conductors, or if the system is suspected to cause EMI
problems, alternate motor wiring methods should be
considered.
Installing screened power cable is the most effective
means to alleviate EMI problems. The cable's 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 and unpredictable high frequency
paths. Unlike most signal wiring, the screening on the
motor cable should be terminated at both ends.
If a screened motor cable is not available, then three—
phase conductors along with earth in a conduit will
provide some degree of protection. This technique will not
be 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 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 communication 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.
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