General Aspects Of Emc - Danfoss VLT Series Design Manual

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VLT HVAC Drive Design Guide

2.10. General aspects of EMC

2.10.1. General Aspects of EMC Emissions
Electrical interference is usually conducted at frequences in the range 150 kHz to 30 MHz. Airborne
interference from the drive system in the range 30 MHz to 1 GHz is generated from the inverter,
motor cable, and the motor.
As shown in the illustration below, capacitive currents in the motor cable coupled with a high dV/
dt from the motor voltage generate leakage currents.
The use of a screened motor cable increases the leakage current (see illustration below) because
screened cables have higher capacitance to earth than unscreened cables. If the leakage current
is not filtered, it will cause greater interference on the mains in the radio frequency range below
approx. 5 MHz. Since the leakage current (I
there will in principle only be a small electro-magnetic field (I
according to the below figure.
The screen reduces the radiated interference but increases the low-frequency interference on the
mains. The motor cable screen must be connected to the frequency converter enclosure as well
as on the motor enclosure. This is best done by using integrated screen clamps so as to avoid
twisted screen ends (pigtails). These increase the screen impedance at higher frequencies, which
reduces the screen effect and increases the leakage current (I
If a screened cable is used for Fieldbus, relay, control cable, signal interface and brake, the screen
must be mounted on the enclosure at both ends. In some situations, however, it will be necessary
to break the screen to avoid current loops.
If the screen is to be placed on a mounting plate for the frequency converter, the mounting plate
must be made of metal, because the screen currents have to be conveyed back to the unit. More-
over, ensure good electrical contact from the mounting plate through the mounting screws to the
frequency converter chassis.
In order to reduce the interference level from the entire system (unit + installation), make motor
and brake cables as short as possible. Avoid placing cables with a sensitive signal level alongside
motor and brake cables. Radio interference higher than 50 MHz (airborne) is especially generated
by the control electronics.
NB!
When unscreened cables are used, some emission requirements are not complied
with, although the immunity requirements are observed.
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MG.11.B2.02 - VLT is a registered Danfoss trademark
2. Introduction to VLT HVAC Drive
) is carried back to the unit through the screen (I
1
) from the screened motor cable
4
).
4
2
),
3
41