Emc, Harmonics And Ground Leakage Protection; General Aspects Of Emc Emissions - Danfoss VLT AQUA Drive FC 202 Design Manual

System Integration
3.2 EMC, Harmonics and Ground Leakage
Protection

3.2.1 General Aspects of EMC Emissions

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Adjustable frequency drives (and other electrical devices)
generate electronic or magnetic fields that may interfere
with their environment. The electromagnetic compatibility
(EMC) of these effects depends on the power and the
harmonic characteristics of the devices.
Uncontrolled interaction between electrical devices in a
system can degrade compatibility and impair reliable
operation. Interference may take the form of line
harmonics distortion, electrostatic discharges, rapid voltage
fluctuations, or high-frequency interference. Electrical
devices generate interference along with being affected by
interference from other generated sources.
Electrical interference usually arises at frequencies in the
range 150 kHz to 30 MHz. Airborne interference from the
adjustable frequency drive system in the range 30 MHz to
1 GHz is generated from the inverter, motor cable, and the
motor.
Capacitive currents in the motor cable coupled with a high
dU/dt from the motor voltage generate leakage currents,
as shown in Figure 3.2.
The use of a shielded motor cable increases the leakage
current (see Figure 3.2) because shielded cables have
higher capacitance to ground than non-shielded cables. If
the leakage current is not filtered, it causes greater
interference on the line power in the radio frequency
range below approximately 5 MHz. Since the leakage
current (I ) is carried back to the unit through the shield
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VLT AQUA Drive FC 202
(I ), there is, in principle, only a small electro-magnetic field
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(I ) from the shielded motor cable according to Figure 3.2.
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The shield reduces the radiated interference, but increases
the low-frequency interference in the line power supply.
Connect the motor cable shield 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) Pigtails
increase the shield impedance at higher frequencies, which
reduces the shield effect and increases the leakage current
(I ).
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If a shielded cable is used for relay, control cable, signal
interface and brake, mount the shield on the enclosure at
both ends. In some situations, however, it is 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, to convey the shield currents back to the
unit. Moreover, ensure good electrical contact from the
mounting plate through the mounting screws to the
adjustable frequency driver chassis.
When using non-shielded cables, some emission
requirements are not complied with, although most
immunity requirements are observed.
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.
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