Lenze PCM 310 PCMatic Slot Controller - Handbuch (Seite 50)


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48
3.3. Switching of external components
Direct current driven magnetic coils
When magnetic coils are switched off, high voltage peaks occur as a result of self-induction. The digital
outputs from Fieldbus modules usually contain protective diodes, which short-circuit these transient
pulses. Nevertheless, for a short period high equalisation currents flow through the connecting wires,
which may be very long. This leads to the emission of interference energy. If measures are taken to
counteract these peak currents directly at the magnetic coil, these currents cannot disperse along the
connecting wires.
To suppress the voltage peaks when the coil is switched off, the following measures may be adopted:
•
switching by means of diodes
•
switching by means of varistors
•
switching by means of resistor-capacitor units
If a diode switch is applied, the peak current is short-circuited and the current flow disperses with the
time constant L/R
Diode
. In consequence, for example in the case of contacts or relays, the decay time
may be prolonged. This can be counteracted by means of a resistor placed in series with the diode.
Varistors and resistor-capacitor switches ensure minimal voltage peaks and short switch-off times.
Resistor-capacitor units can be calculated very approximately in accordance with the following simple
formula:
R
≈
2
L
C
- R
L
+
-
L
+
-
L
+
-
L
+
-
L
R
C
Switching with a resistor-capacitor unit Switching with a varistor
Switching with a diode and resistor
Switchi ng with a diode
Alternating current driven magnetic coils
Because of the changing polarity of the alternating current, only varistors and resistor-capacitor units
are suitable for switching purposes. The switching system and the dimensional aspects are identical to
the methods for direct current driven magnetic coils (see
direct current driven magnetic coils ).