Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 59)

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(
f)
/I
e(
50H
z
)
Fig. 2.4-3
Approximate load capacity of busbars at higher frequencies [12]
I
e50
Load capacity at 50 Hz
I
ef
Load capacity at frequency f
In order to reduce losses, no cage-type terminals should be used. This is especially important
with currents > 100 A!
For single phase loads over 400 Hz, the two outer poles of contactors should be used in parallel
for the feed line and the middle pole for the return line. This results in a partial mutual compen-
sation of the magnetic induction.
2.4.3.2 Effect of the supply frequency on the switching capacity
The starting currents of motors for higher frequencies are sometimes higher than those at
50/60 Hz. At 200 Hz this can result in 15 times the rated current and at 400 Hz up to 20 times.
The power factor may be significantly worse than for motors at 50/60 Hz. Allowance should be
made for the increased making currents when selecting devices.
Switching capacity of contactors and load switches
When breaking a.c. circuits, the clearance between open contacts must be sufficiently de-
ionized during the current zero crossover to prevent a re-ignition of the electric arc in the next
half cycle. At higher frequencies, the increase of the voltage after the zero crossover is usually
faster. The electric arc duration per half cycle and hence the ionization of the distance between
open contacts is however smaller. Therefore contactors and load switches (zero-point interrupt-
ers) up to around 400 Hz have virtually the same switching capacity as at 50/60 Hz.
Insofar as the making currents are permissible (see above) – a reduction of the rated current I
e
according to Fig. 2.4-2 for contactors and load switches is only required because of the larger
thermal loading at higher frequencies.
The circumstances at lower frequencies are less favorable. The effect of the strong ionization of
the switching chamber due to the longer presence of the electric arc is predominant with non
current limiting switchgear. The switching capacity falls at lower frequencies and becomes more
heavily dependent on the voltage and the inductance of the load.
The full rated operational current for three-pole operation at 400V and 50/60 Hz can be permit-
ted at 16 2/3 Hz and 400 V with two poles in series. For rated operational voltages up to 500 V
and 16 2/3 Hz all poles should be connected in series so that the full rated operational current
can be operated.
At frequencies below 16 2/3 Hz, the direct current switching capacity of switchgear in accor-
dance with catalog specifications must be applied.
Switching capacity of circuit breakers
The short-circuit currents in medium frequency supplies are comparatively low. Any reduction of
the switching capacities at frequencies over 400 Hz compared to 50/60 Hz does therefore not
cause problems in practice.
LVSAM-WP001A-EN-P - April 2009 2-31