Switching Of Inductive Loads - ProMinent DULCOMETER Assembly And Operating Instructions Manual

Assembly and installation
5.5

Switching of inductive loads

If you connect an inductive load,
i.e. a consumer which uses a coil
(e.g. an alpha motorised pump),
then you must protect your con‐
troller with a protective circuit. If in
doubt, consult an electrical techni‐
cian for advice.
The RC member protective circuit is a
simple, but nevertheless very effective,
circuit. This circuit is also referred to as a
snubber or Boucherot member. It is pri‐
marily used to protect switching contacts.
When switching off, the connection in
series of a resistor and capacitor means
that the current can be dissipated in a
damped oscillation.
Also when switching on, the resistor acts
as a current limiter for the capacitor
charging process. The RC member pro‐
tective circuit is highly suitable for AC
voltage supplies.
The magnitude of the resistance R of
the RC member is determined according
to the following equation:
R=U/I
L
(Where U= Voltage across the load and
I = current through the load)
L
The magnitude of the capacitor is deter‐
mined using the following equation:
Units: R = Ohm; U = Volt; I
C = µF
40
= Ampere;
L
C=k * I
L
k=0,1...2 (dependent on the application).
Only use capacitors of class X2.
Units: R = Ohm; U = Volt; I
C = µF
If consumers are connected which
have a high starting current (e.g.
plug-in, switched mains power sup‐
plies), then a means of limiting the
starting current must be provided.
The switching-off process can be investi‐
gated and documented using an oscillo‐
scope. The voltage peak at the switch
contact depends on the selected RC com‐
bination.
Fig. 15: Switching-off process shown on
the oscillogram.
Fig. 16: RC protective circuit for the relay
contacts
= Ampere;
L
A0842
A0835