Siemens simovert masterdrives

05.2003 Instructions for Design of Drives in Conformance with EMC Regulations
Siemens AG 6SE7087-6QX70 (Version AD)
SIMOVERT MASTERDRIVES Compendium Motion Control 3-7
3.3.2 The frequency converter as a noise receiver
Noise can enter a unit either galvanically, inductively or capacitively.
The equivalent circuit diagram shows a noise source which causes
noise current I
S
in the unit due to capacitive coupling effects. The
magnitude of the coupling capacitance C
K
is determined by the cabling
and the mechanical design.
Z
i
IS
Noise
source
Unit
Board
Signal
cable
C
K
Fig. 3-5 Capacitive coupling for non-shielded signal cables
Noise current I
S
produces a voltage drop across impedance Z
i
. If the
noise current flows through a board with fast electronic components
(e.g. microprocessor), even a small spike in the µs area and an
amplitude of just a few volts can lead to disturbing noise.
The most effective way of preventing noise being coupled-in is to
rigorously separate power and signal cables.
Z
i
I
S
Noise
source
Unit
Board
Shielded
Signal
cable
C
K
Filter
Fig. 3-6 Increasing the noise immunity by using shielded signal cables
The inputs and outputs of the SIMOVERT MASTERDRIVES control
section are fitted with filters that keep noise currents I
S
separate from
the electronics. The filters also smooth the useful signal. In the case of
signal cables with extremely high-frequency signals, e.g. from the
digital tachometer, this smoothing has a disturbing effect. As no
smoothing is possible on account of its functionality, shielded signal
cables have to be used here. The noise current now flows back to the
noise source via the shield and the housing.
The shields of digital signal cables always have to be connected at
both ends, i.e. at the transmitter and at the receiver!
Ways in which
noise is received
Measures to
increase noise
immunity