Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 51)

Fig. 2.3-13
Coil current at closing a contactor a.c. magnet
I
S
Rated current of coil
I
S1
Inrush current of coil (depending on contactor 6... 15 · I
S
)
T1 ON-command (coil circuit closed)
T2 Magnet closed
The permissible frequency of operation of conventional coils can be exceeded short-time
without risk, as the time constants for the heating of coils is 5 to 20 minutes depending on
contactor size.
True direct current magnets do not exhibit in-rush currents. Therefore with these the ON and
OFF delay times, which in this case are notably longer, determine the maximum frequency of
operation.
With electronically controlled coils, the permissible frequency of operation is determined by the
thermal load capacity of the electronic components and may not be exceeded.
With electrical loading, the temperature rise of the contacts must also be taken into account for
determining the permissible frequency of operation. Although, heat is dissipated during the de-
energized intervals, the contacts are additionally heated to a considerable degree by arcing and
by the starting currents when switching motors. The permissible frequency of operation is
therefore dependent on the relative ON-time, the size and duration of the motor starting current
and on the breaking current. Corresponding diagrams are provided in the catalogs for typical
applications (
Fig. 2.3-14 ).
Contactor size I
e(AC-3
[A]
9/12/16 A
23/30/37/43 A
60/72/85 A
Fig. 2.3-14
Example of a frequency of operation characteristic for contactors. The frequency of operation for small
loads is limited by the temperature rise of the coil.
At higher frequencies of operation, the contacts are predominantly loaded by the starting
currents. This also applies to motor windings.
LVSAM-WP001A-EN-P - April 2009 2-23