Table of Contents
Advertisement
Introduction to VLT Micro D...
3.2 General Aspects of EMC
3.2.1 General Aspects of EMC Emissions
Electrical interference is usually conducted at frequencies in the range 150kHz to 30MHz. Airborne interference from the
frequency converter system in the range 30MHz to 1GHz is generated from the inverter, motor cable, and the motor.
As shown in Illustration 3.3, 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 3.3) 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 approximately 5MHz. Since the leakage current (I
through the screen (I
), there will in principle only be a small electro-magnetic field (I
3
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.
FREQUENCY
LINE
CONVERTER
z
L1
z
L2
z
L3
z
PE
PE
Illustration 3.3 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 50MHz (airborne) is especially generated by the control electronics. Please see for more information on EMC.
VLT Micro Drive Design Guide
MOTOR CABLE SCREENED
C
S
U
I
1
V
W
I
C
2
PE
I
3
C
S
I
4
Earth Plane
®
MG02K102 - VLT
is a registered Danfoss trademark
) is carried back to the unit
1
) from the screened motor cable
4
).
4
S
Earth wire
Screen
I
4
MOTOR
C
S
C
C
S
S
21
3
3
Advertisement
Table of Contents
