Table of Contents
Advertisement
Saia-Burgess Controls AG
A.3.3 Carrying inductive loads
Because of the physical properties of inductance, it cannot be cut off without
interference. This interference must be minimised as far as possible. Although the
PCD is immune to this interference, there are other devices that could be affected.
Please note that, as a result of harmonisation of norms within the EU, the EMC
standards have been in effect since 1996 (EMC Directive 89 / 3 36 / E C). Two principles
can therefore be set out:
● INTERFERENCE SUPPRESSION IS ESSENTIAL FOR INDUCTIVE LOADS
● INTERFERENCE SHOULD BE ELIMINATED AT SOURCE WHERE POSSIBLE
The relay contacts on this module are switched. However, it is still recommended to
connect a suppressor element to the load.
(Often obtainable as a standard component of certified gates and valves).
When carrying direct current, it is recommended to install a protective diode across
the load, even where an ohmic load is theoretically present. In practice, there will
always be an inductive portion (connecting cable, resistor winding etc.). Note that the
switch-off time will be extended.
(Tau approx. L / R L * √ (RL * IL / 0 .7).
For direct current, the transistor output modules are recommended.
A.3.4 Details of relay manufacturers for sizing of RC elements
Contact protection circuits:
The purpose of contact protection circuits is to suppress arcing and hence to
increase the service life of the contact elements. Any protective circuit may have
disadvantages as well as advantages. For how to suppress arcing with an RC
element, see figure opposite.
When switching off load circuits with inductive components (e.g. relay coils and
magnet windings), the break in current to the contacts causes an overvoltage
(self-inductance voltage), which may be many times the operating voltage and
compromise the isolation of the load circuit. The resulting sparks will cause the
relay contacts to wear out quickly. For this reason, the contact protection circuit is
particularly important with inductive load circuits. The values for the RC combination
can also be determined from the diagram, but for voltage U the overvoltage arising
when the current is interrupted (measured e.g. with an oscillograph) should be used.
The current can be calculated from this voltage and the known resistance against
which it was measured.
In suppression elements, suppression condensers conforming to VDE 0565 T1 Class
X2 must be used. These condensers are switch-tolerant and designed for particularly
high overvoltages. Direct operation at mains voltage is still possible.
The resistors used must be able to withstand high voltages (impulse resistance).
Particularly at low resistances, there may be flashovers in the coil caused by the
production process. This is why fixed carbon resistors are most often used for
suppression elements. However, vitreous enamelled wire-wound resistors and
cement resistors with large pitch windings are also suitable.
Hardware Manual for the PCD2.M5 Series│Document 26/856; Version EN 1 2│2014-07-24
Installation instructions and relay contacts
Annex
A
A-7
Advertisement
Table of Contents
