Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 155)

U
2
+
_
U
2
+
_
.
.
U
DC
= U
2rms
0,9 - 2U
D
U
DC
= U
2rms
1,35 - 2U
D
Fig. 5.2-1
Rectifier circuits for supplying electromagnetic loads
For controlling and supplying contactors with electronic coil control it should be noted that the
instantaneous value of the direct voltage may not fall below a certain minimum value. This is in
regard to the proper functioning of the electronic circuit. The specifications of the product in
question with respect to the quality of the direct voltage should be observed. Especially with
large contactors and a small control voltage (for example 24 V), the current consumption of the
devices may short-term extend way beyond the 10 A range. In such cases it should be ensured
that correspondingly powerful supply units are used and that the voltage drops on the connec-
tions between the supply and contactor are as small as possible (short lines, large cross
sections, no loops, good quality terminations). Unstabilized supply devices with two-way
rectification and smoothing capacitor soon reach their limits in these applications.
Fig. 5.2-2
Simple Graetz rectifier with capacitive smoothing result at higher load currents to strong discharge
between the half-cycles (dashed line) that can cause malfunctions in electronically controlled contactors
and other electronic devices.
5.3 Switching contactors
5.3.1 Alternating current magnets
5.3.1.1 Conventional alternating current magnets
Alternating current magnetic drives are characterized by a high pull-in current that flows when
the magnet system is open and is determined by the low coil impedance (large air gap). The
utilization categories AC-14 and AC-15 take this characteristic into account. The high pull-in
current surge thermally loads the coil and restrict the permissible frequency of operation.
Especially at small control voltages with large contactors, attention must be paid to voltage
drops in the control circuit to ensure reliable switching.
When contactors are switched off, the inductance is large because of the small size of the
residual air gap. This results in a corresponding arcing of the control contacts and to switching
transients. External overvoltage protection measures may be required (see Section
5.3.3.1 ).
5.3.1.2 Electronic coil control
By means of electronic circuits, the operating conditions for contactor magnet systems can be
optimized and the operation optimally adjusted to user requirements. Thus the magnet system
can be isolated from voltage variations so that the current drawn is optimized and the pull-in and
drop-out values clearly defined. As required by the user, control can be performed convention-
ally by application of a control voltage or by a PLC signal, either directly or via a control input.
LVSAM-WP001A-EN-P - April 2009 5-3