General Aspects Of Emc - Danfoss VLT HVAC Basic Drive FC 101 Design Manual

Introduction to VLT® HVAC B...

2.8 General Aspects of EMC

2.8.1 General Aspects of EMC Emissions
Electrical interference is usually conducted at frequencies
in the range 150 kHz to 30 MHz. Airborne interference
from the frequency converter system in the range 30 MHz
to 1 GHz is generated from the inverter, motor cable, and
the motor.
As shown in Illustration 2.23, capacitive currents in the
motor cable coupled with a high dU/dt from the motor
voltage generate leakage currents.
The use of a screened motor cable increases the leakage
current (see Illustration 2.23) because screened cables have
higher capacitance to earth than unscreened cables. If the
leakage current is not filtered, it causes greater
interference on the mains in the radio frequency range
below approximately 5 MHz. Since the leakage current (I
FREQUENCY
LINE
CONVERTER
z
L1
z
L2
z
L3
z
PE
PE
Illustration 2.23 Situation that Generates Leakage Currents
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. Moreover, ensure good electrical contact
from the mounting plate through the mounting screws to
the frequency converter chassis.
When unscreened cables are used, some emission
requirements are not complied with, although the
immunity requirements are observed.
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
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VLT HVAC Basic Drive Design Guide
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1
MOTOR CABLE SCREENED
C
S
U
I
1
V
W
I
C
2
PE
I
3
C
S
I
4
Earth Plane
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MG18C402 - VLT is a registered Danfoss trademark
is carried back to the unit through the screen (I
will in principle only be a small electro-magnetic field (I
from the screened motor cable 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
is necessary to break the screen to avoid current loops.
S
Earth wire
Screen
I
4
generated by the control electronics. See 5.2.5 EMC-Correct
Electrical Installation for more information on EMC.
2.8.2 Emission Requirements
According to the EMC product standard for adjustable
speed frequency converters EN/IEC 61800-3:2004 the EMC
requirements depend on the intended use of the
frequency converter. Four categories are defined in the
EMC product standard. The definitions of the 4 categories
together with the requirements for mains supply voltage
conducted emissions are given in Table 2.9.
), there
3
)
4
).
4
MOTOR
C
S
C C
S
S
31
2 2