Capacitive Coupling - Siemens SINAMICS G130 Engineering Manual

EMC Installation Guideline
Engineering Information

Capacitive coupling

Capacitive coupling occurs between conductors that are isolated against each another and that have different potentials.
This difference in potential generates an electrical field between the conductors, known as capacitance C
of the capacitance C depends on the geometry of the conductors and on the distance between the conductors with
c
different potential.
The diagram below shows a source of interference that is coupling an interference current I
susceptible equipment by means of capacitive coupling. The interference current I
impedance Z of the potentially susceptible equipment and, in turn, an interference voltage.
i
Capacitive coupling of an interference current into a signal cable
If, for example, a motor cable and an unshielded signal cable were routed in parallel close to each another on a long
cable rack the small distance between the cables would result in a high coupling capacitance C
inverter of the frequency converter, which acts as source of interference, couples via the capacitance C
interference current into the signal cable with each switching edge. If this interference current flows via the digital
inputs into the Control Unit of the converter, the generated small interference pulses lasting only a few microseconds
with an amplitude of only a few volts can affect the microprocessor-based digital control of the converter and can
cause the converter to malfunction.
Measures for reducing capacitive coupling
•
Maximize the distance between the cable causing the interference and the cable affected by the interference
•
Minimize the length of the parallel cable routing
•
Use shielded signal cables.
The most effective method is to ensure systematic separation of power and signal cables in combination with a
shielding of the signal cables. This ensures that the interference current I
flows to ground via shield and housing of the device or converter without affecting the internal electrical circuits.
Reducing the interference coupled into the potentially susceptible equipment by using a shielded signal cable
To ensure that the shield is as effective as possible it is necessary to establish a low-inductance shield bonding using
a large contact area. When digital signal cables are used, shield bonding hat to be established at both ends (i.e. at
the transmitter side and at the receiver side) using a large contact area. When analog signal cables are used, shield
bonding at both ends can result in low-frequency interference (hum loops). In this case, shield bonding should only
be carried out at one end (i.e. the converter side). The other side of the shield should be grounded by means of a
MKT-type capacitator with approximately 10 nF/100 V. When the capacitator is used, this means that the shield is
bonded for high frequencies at both ends.
SINAMICS Engineering Manual – May 2008
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© Siemens AG
. The magnitude
c
into the potentially
I
generates a voltage drop at
I
. The motor-side
c
is coupled into the shield and that it
I
a pulsating
c