SBC PCD2.M5 Series - Carrying Inductive Loads; Details of Relay Manufacturers for Sizing of RC Elements

Hardware Manual for the PCD2.M5 Series│Document 26/856; Version EN 12│2014-07-24
Saia-Burgess Controls AG
Installation instructions and relay contacts
Annex
A-7
A
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 / 336 / EC).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certied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 / RL*√(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gure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ashoversinthecoilcausedbythe
productionprocess.Thisiswhyxed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.