Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 50)

With intermittent or short-time duty, the loading current can be higher than in continuous duty,
without resulting in the permitted temperature being exceeded. Therefore, for example, for
switching ohmic loads and rotor contactors for slip-ring motors smaller contactors can be
selected than would be required according to the rated current of the load.
When switching squirrel-cage induction motors, transformers, capacitors and incandescent
lamps, the required contact rating is however the main selection criterion. The size of contactors
for these applications is therefore determined by the rated operational current and the respec-
tive utilization category for all service types.
2.3.7.1 Intermittent duty and relative ON-time
In order to define a specific intermittent duty, in addition to the value of the current either the
load and cycle time or the frequency of operation per hour together with the relative ON-time are
preferably stated.
Fig. 2.3-12
Intermittent duty
t
t
I I
S
B
B S
⋅ =
I
S
Average current loading (r.m.s. value during a switching cycle and hence also during
the whole service time) [A]
I
B
Current during period under load [A]
t
B
B
B
t
S
Load duration [s]
t
S
Switching cycle = load duration + de-energized interval [s]
ED relative ON-time = t
B/
t
S
[%]
I
B
I
S
t
B
The relative ON-time – usually expressed as a percentage – is the ratio of the load duration to
the cycle-time, whereby the cycle-time is the sum of the load duration and the de-energized
interval.
The average current loading I
S
must always be somewhat lower than the thermal continuous
current, so that the temperature rise peaks at the end of each period under load do not exceed
the permitted values. With stator contactors of slip-ring motors, especially with short switching
cycles, the higher current during the starting time (see
Fig. 2.3-10 ) as well as the additional
heating effect of the electric arcs must be taken into account.
At high frequency of operation the heating effect of the starting current and the switching arc is
greater than the cooling effect of the de-energized intervals so that contactors with higher
ratings must be chosen than would normally be required according to the rated operational
current. The selection is made based on the graphs for the permissible frequency of operation.
Even without electrical loading, the frequency of operation of contactors is limited by the
maximum permissible temperature of the coil or the electronic coil control circuit, if any. The in-
rush currents of the coil (
Fig. 2.3-13 ) make a considerable contribution at higher frequencies of
operation to the overall heating of the coil and of the contactor. This applies both for alternating
current and for direct current magnets with series resistance or contactors with double-winding-
coils (economy circuit) and also to electronic coil control circuits.
LVSAM-WP001A-EN-P - April 2009 2-22