II F 2-23
Commutation and line chokes
(see also Section
Line reactors
in this chapter)
Due to the maximum power of public 400 V
transformers (P
MAX
= 1.2 MVA
⇒
I
MAX
= 1732 A)
and due to their relative voltage drop 6% or 4%
the maximum AC current which is available for a
converter is 346 A or 520 A (I
DC
≤
422 A or 633 A).
Separation transformers
A separation transformer makes line chokes un-
necessary because of its leakage inductance, and
a grounded screen between its windings saves an
EMC filter. The screen and the iron core must be
well connected with the mounting plate of the
converter.
Converter transformers
A converter transformer transfers high power
directly from a medium voltage line to a single
large converter or to a local low voltage line for
several converters. Furthermore it acts as separa-
tion transformer.
If such a converter transformer has no screen the
EMC demands are nevertheless fulfilled in most
cases because the RF interference energy can
hardly get via the medium-voltage line and the
transformer of the public line to the loads which
must be protected against pertubances.
PE
PE
Installation hints
All metal cubicles available on the market can
be used.
The mounting plate must be made from steel
with zinc surfaces and without any painting. It
shall be connected with the PE copper bar by
several bolts.
The converter, the line choke, fuses, contac-
tors and the EMC filter are to be placed on the
mounting plate so that the connections can be
made as short as possible, especially those
from the converter via the line choke to the
filter.
The cables for digital signals which are longer
than 3m and all cables for analogue signals
must be screened. Each screen must be con-
nected at
both
ends by metal clamps or
comparable means directly on clean metal
surfaces. In the converter cubicle this kind of
connection must be made directly on the sheet
metal close to the terminals.
The necessity of a screen depends on the
length of the cable and on the environmental
demands. If a screen is necessary then it must
be pressed by a well conducting metal clamp
directly against the mounting plate or the PE
bar of the converter cubicle.
Screened cables to the armature and to the
excitation winding cause the lowest noise
level. The armature current cable must contain
a third wire for a PE connection if the copper
cross section of the screen cannot fulfil the PE
safety demands.
If a screen is not necessary the arma-
ture current cable must be a four-
wire cable because two wires are
needed as conductors for the parasit-
ic RF currents from the motor to the RF filter in
the cubicle.
