Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 16)

Transformers up to approx. 1 kVA at 230 V n ≈ 20
at 400 V n
≈ 15
larger transformers at 400 V n ≈ 15 ... 30
Note
The thermal continuous current I
th(e)
may not be exceeded.
Transformers in welding machines are usually designed so that inrush current peaks and the
short-circuit current with electrodes short-circuited are limited (n
≈ 10). The contactor is selected
for switching these currents operationally.
If the individual welding current surges are not switched by power semiconductors but by the
primary contactor, this means that the latter has a high switching frequency and a very high
number of operations. It is essential that the contactor selected is checked with respect to the
permitted frequency of operation and the electrical life span. For the electrical endurance the
selection can be based on approx. 70 % of the AC-1-ratings as long as the inrush currents are
limited.
1.5 Reactive power compensation and switching of capacitors
1.5.1 Reactive power compensation
In electrical networks in which inductive consumers (e.g. motors) are switched on and off, the
power factor cos
φ often changes with each switching operation. The Power Utilities demand
from their consumers that the ratio of the consumed effective power P to the drawn apparent
power S does not fall below a certain value, as the transmission of apparent power is uneco-
nomic.
The reactive power of motors, luminescent lamps with series chokes and other inductive loads
is therefore frequently compensated by connecting capacitors, in order to reduce the additional
load of transformers and lines by the reactive current.
In deciding whether it is more advantageous to compensate individual consumers with fixed
capacitors or to provide central compensation units, economic and technical considerations are
definitive. Control units for central compensation have a higher price per power unit (kVA). If
allowance is made however for the fact that in most operations not all consumers are switched
on at the same time, a lower installed capacitor power is often sufficient for central compensa-
tion.
1.5.1.1 Individual compensation
For individual compensation ( Fig. 1.5-1 a) the capacitors are directly connected to the terminals
of the individual consumer (e.g. motor, transformer, induction heater, luminescent lamp) and
switched together with these via a common switchgear unit. Single compensation is recom-
mended with large consumers with constant power consumption and long ON-times. They offer
the advantage that the lines to the consumers are also relieved of load. The capacitors can
frequently be connected directly to the terminals of the individual consumer and be switched on
and off with a common switchgear device.
~
~ ~
~ ~ ~
~ ~
Ne Na Xe
a) Individual compensation b) Group compensation c) Central compensation
Fig. 1.5-1
Compensation types
LVSAM-WP001A-EN-P - April 2009 1-6