Danfoss VLT HVAC FC 100, Руководство по обслуживанию, Страница: 88 / 220

While these currents will tend to travel back to the adjustable frequency drive, imperfections in
the system will cause some current to flow in undesirable paths thus exposing other locations to
the EMI.
130BX140.11
Motor Cable AC Line
VFD
Motor
Stray capacitance
AC Line
Figure 5.4: Alternate Signal Conductor Currents
High frequency currents can be coupled into the line power supplying the adjustable frequency
drive when the line power conductors are located close to the motor cables.
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
The adjustable frequency drive and motor should be solidly grounded to the equipment frame. A
good high frequency connection is necessary to allow the high frequency currents to return back
to the adjustable frequency drive rather than to travel thorough the power network. The ground
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 adjustable frequency drive or motor onto a painted surface
will 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 6–8 in [200 mm] between the cables or separate
them with a grounded conductive partition. Avoid routing cables through free air.
Signal cable selection
Signal cable selection. Single conductor 600 volt rated wires provide the least protection from EMI.
Twisted-pair and shielded twist-pair cables are available which are specifically designed to mini-
mize the effects of EMI. While unshielded twisted-pair cables are often adequate, shielded twisted-
pair cables provide another degree of protection. The signal cable’s shield should be terminated
in a manner that is appropriate for the connected equipment. Avoid terminating the shield through
a pigtail connection as this increases the high frequency impedance and spoils the effectiveness
of the shield. Refer to Section 2.8, Grounding Shielded Cables.
High Power Service Manual for VLT
®
FC Series
Adjustable Frequency Drives
5 Adjustable Frequency Drive and Motor Ap-
plications
MG.90.L1.22 - VLT ® is a registered Danfoss trademark
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