Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 157)

5.3.2.2 Double winding coils
Direct current contactors with double winding coils are contactors with alternating current
magnets and a pull-in and holding coil. The size is the same as that of alternating current
contactors. The contactors switch on by means of a pull-in winding with low impedance and a
correspondingly higher pull-in current. After the magnet circuit is closed, the excitation is
switched over by an auxiliary switch to the lower holding power. The auxiliary switch can be
integrated in the device or – usually with smaller contactors – be externally mounted.
5.3.2.3 Electronic coil control
Electronic coil control is also available for contactors with direct current supply. The characteris-
tics and advantages are similar to those with alternating current supply.
See Section
5.3.1.2 .
5.3.3 Electromagnetic compatibility and protective circuits
Conventional switchgear without active electronic circuits such as switched power supplies for
controlling contactor coils is considered from an EMC viewpoint as belonging to a normal control
environment. Although these devices can short-term generate very high and steep overvolt-
ages, from an EMC perspective no countermeasures are required. The levels of immunity tests
for industrial applications are set so that other devices usually function reliably at this level of
interferences.
Like all other electronic devices, devices with active electronic circuits such as contactors with
electronic coil control are subject to the relevant immunity and emission tests. For industrial
switchgear the levels correspond to the Environment A “Industry” (high immunity against
interferences, high interference emissions). If devices such as contactors with electronic coil
control are used in Environment B “Domestic /Commercial/Light industry Installations”
(low interference immunity, low emissions), it should be ensured that the devices have also
been tested for these areas of application. On the other hand, devices for the Environment B
are not suitable and permitted for use in industrial applications with their higher levels of
interference.
In addition to the selection of devices for the given environment, the instructions of the respec-
tive device manufacturer with respect to the installation and connections (for example shielded
cables) must be observed to achieve EMC compatibility of the switchgear assembly.
5.3.3.1 Protective circuits in coil circuits
When switching magnetic loads with high inductance such as for example contactor coils, in
spite of the above considerations, switching transients with magnitudes of several kV and with
rise-times in the range of
μs to ns can occur that may interfere with the proper functioning of
other devices. During the opening of the controlling contacts, there occur repeated restrikes
(shower discharges), as the inductance of the coil maintains the current flow and the opening
contact does not instantaneously attain its full withstand voltage (
Fig. 5.3-3 ). These shower
discharges also increase wear on the switching control contact. With respect to the interference
effect, it is not only the size of the overvoltage that is generated that is critical but also, in view of
the extremely short reaction times of electronic circuits, its rise and fall time. Rapid signals
couple via stray capacitances with other signal circuits.
LVSAM-WP001A-EN-P - April 2009 5-5