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VLT ® MICRO
79
Cs
I
4
I
3
I
2
I
1
Z
Z
Z
Z
L1
L2
L3
PE
Cs
U
V
W
PE
Line
Adjustable
Frequency
Drive
Motor Cable Shield
Motor
Cs
Ground wire
Shield
Cs
Cs Cs
I
4
Ground Plane
General Aspects of EMC Emissions
Electrical interference at frequencies in the range 150 kHz to 30 MHz are cableborne.
Airborne interference from the drive system in the range 30 MHz to 1 GHz is generated
from the inverter, the motor cable and the motor.
As the drawing below shows, capacitive currents in the motor cable together with a high
dV/dt from the motor voltage generate leakage currents.
The use of a shielded motor cable increases the leakage current (see drawing). This is
because shielded cables have higher capacitance to ground than unshielded cables. If
the leakage current is not filtered, it will cause greater interference on the AC line in the
radio frequency range below approximately 5 MHz. Since the leakage current (I
1
) is
carried back to the unit through the shield (I
3
), there will in principle only be a small
electro-magnetic field (I
4
) from the shielded motor cable according to the drawing below.
The shield reduces the radiated interference, but increases the low-frequency
interference on the AC line. The motor cable shield must be connected to the VLT
enclosure. The best way of doing this is by using integrated shield clamps so as to avoid
twisted shield ends (pigtails). If pigtails are used, make them as short and direct as
possible.
In the cases when the shield is to be placed on a mounting plate for the VLT adjustable
frequency drive, the shield currents are to be conveyed back to the unit. It is also
important to ensure good electrical contact from the mounting plate through the
mounting screws to the VLT adjustable frequency drive chassis.
With respect to installation, it is generally less complicated to use unshielded cables
than shielded ones.
NOTE: When shielded cables are used some emission requirements are not complied
with, although the immunity requirements are complied with.
In order to reduce the interference level from the system overall (unit + installation) as far
as possible, it is important to make motor and brake cables as short as possible. Cables
with a sensitive signal level must not be alongside motor and brake cables.
Radio interference higher than 50 MHz (airborne) will be generated especially by the
control electronics.