Siemens SIMOVERT MASTERDRIVES Manual page 49

05.2003
Measures to
reduce noise
emission
Siemens AG 6SE7087-6QX70 (Version AD)
SIMOVERT MASTERDRIVES
Instructions for Design of Drives in Conformance with EMC Regulations
Z
N
Z
E
U
I
S
Fig. 3-2 Block diagram showing output voltage V and fault current Is
The source of the fault current I
why the fault current must also flow back to the inverter. Impedance Z
and ground impedance Z
forms parasitic capacitances between the supply cable and ground,
which is connected in parallel with the impedance (between phase and
ground) of the supply transformer. The noise current itself and the
voltage drops across Z
affect other electrical units.
Frequency converters generate the high-frequency noise currents
which have already been described. In addition, low-frequency
harmonics should be taken into account. As a result of rectification of
the line supply, a non-sinusoidal line current is drawn which causes a
distortion of the line supply voltage.
Low-frequency harmonics are reduced using line reactors.
The high-frequency noise emission can only be reduced if the
generated noise current is correctly routed. Using non-shielded motor
cables, the noise current flows in an undefined fashion back to the
frequency converter, e.g. via foundation/base frame grounders, cable
ducts, cabinet frames. These current paths have a very low resistance
for currents with a frequency of 50 or 60 Hz. However, the noise current
induces a high-frequency component, which can result in problematical
voltage drops.
A shielded motor cable is absolutely necessary to enable the fault
current to flow back to the frequency converter in a defined fashion.
The shield must be connected to the housing of the frequency
converter and to the motor housing through a large surface area. The
shield now forms the easiest path for the noise current to take when
returning to the frequency converter.
Compendium Motion Control
U
I
S
is the inverter, which is the reason
S
act in the return flow path. Impedance Z
E
and Z caused by the noise current can also
N E
M
3~
C
P
C
P
t
t
N
N
3-5