Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 58)

conductor is virtually de-energized and the current only flows in a relatively thin layer at the
conductor surface.
This means that with increasing frequency, the resistance of the circuit increases. In addition,
due to magnetic induction, higher hysteresis and eddy current losses are created in adjacent
metal parts. Especially ferromagnetic materials (arc extinguishing parts, screws, cage terminals,
magnets, base plates) can reach unacceptably high temperatures. Special care should be taken
at frequencies > 400 Hz.
Because of the variable cross-sections of the conductive parts as well as the different nature
and distances to adjacent metal parts, additional heating effects and especially the local
overtemperatures vary according to device type. This has the following consequences for the
load capacity of the switchgear and the switchgear combinations.
Individual clarification is required for each individual application as a general statement cannot
be provided due to the widely differing design features, especially at frequencies > 400 Hz.
Load capacity of contactors, load switches and circuit breakers
Devices that are designed for a frequency of 50/60 Hz can, from a thermal viewpoint, be used
for the same rated current at a lower frequency. Approximate values for permissible operational
currents are stated in
Fig. 2.4-2 . The actual reduction factors vary according to design and the
rated current range of the devices. It should be noted that cage-type terminals have an adverse
effect on heating at higher frequencies.
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
10 100 1000 10000
f [Hz]
I e
AC-
1
(f
)
/I
e
A
C
-1(
50
/60
H
z
)
small devices
medium size
devices
( ≈100 A)
big devices
big devices with
cage-type
terminals
Fig. 2.4-2
Approximate values for permissible operational currents AC-1 of contactors, load switches and circuit
breakers at higher frequencies relative to I
e
at 50…60 Hz
Higher frequencies and installation provisions
For installation at higher frequencies, special attention must be paid to the effects of current
distribution (skin effect), hysteresis and eddy current losses:
The conductors to be connected should be rated according to the higher frequency (larger
cross-section, flat or tube conductors). The load capacity of circuits at higher frequencies can be
roughly estimated by help of
Fig. 2.4-3 . It depends on the geometry of the rails and their
arrangement and should be measured separately in each individual case. The conductors
should be positioned as far as possible from conductive (especially ferromagnetic) parts, to
minimize inductive effects.
LVSAM-WP001A-EN-P - April 2009 2-30