Danfoss VLT Series Design Manual page 47

VLT ® HVAC Drive Design Guide
2.9. General aspects of EMC
2.9.1. General Aspects of EMC Emissions
Electrical interference is usually conducted at frequencies in the range of 150 kHz to 30 MHz.
Airborne interference from the drive system in the range of 30 MHz to 1 GHz is generated from
the inverter, motor cable and 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 shielded motor cable increases the leakage current (see illustration below), because
shielded cables have higher capacitance to ground than non-shielded cables. If the leakage current
is not filtered, it will cause greater interference on the line power supply in the radio frequency
range below approximately 5 MHz. Because the leakage current (I
through the shield (I
shielded motor cable according to the below figure.
The shield reduces the radiated interference, but increases the low-frequency interference in the
line power supply. The motor cable shield must be connected to the adjustable frequency drive
enclosure as well as on the motor enclosure. This is best done by using integrated shield clamps
so as to avoid twisted shield ends (pigtails). These increase the shield impedance at higher fre-
quencies, which reduces the shield effect and increases the leakage current (I
If a shielded cable is used for the serial communication bus, relay, control cable, signal interface
and brake, the shield must be mounted on the enclosure at both ends. In some situations, how-
ever, it will be necessary to break the shield to avoid current loops.
If the shield is to be placed on a mounting plate for the adjustable frequency drive, the mounting
plate must be made of metal, because the shield currents have to be conveyed back to the unit.
Moreover, ensure good electrical contact from the mounting plate through the mounting screws
to the adjustable frequency driver chassis.
), there will in principle only be a small electro-magnetic field (I
3
NOTE
When non-shielded cables are used, some emission requirements are not complied
with, although the immunity requirements are observed.
MG.11.B4.22 - VLT ® is a registered Danfoss trademark.
2. Introduction to the VLT HVAC Drive
) is carried back to the unit
1
).
4
2
) from the
4
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