Siemens SINAMICS G130 Engineering Manual page 59

Effective shield bonding is achieved if EMC cable glands are used to create a solid 360° contact between the shield
and motor terminal box and, at the other side in the converter cabinet, a solid 360° contact with the EMC shield
busbar using EMC shield clips. An alternative shield connection to the PE busbar in the converter using only long,
braided "pigtails" is less suitable, particularly if the pigtails are very long, as this type of shield bond represents a
relatively high impedance for high-frequency currents. Further additional shield bonds between the converter and
motor, e.g. in intermediate terminal boxes, must never be created as the shield will then become far less effective in
preventing interference currents from spreading beyond the drive system.
Motor terminal box
Shield bonding to motor terminal box using
an EMC gland
The cable symmetry combined with the concentric shield with good bonding connections at both sides ensures the
interference currents can flow easily back to the cabinet.
The housing of the SINAMICS Chassis containing the standard, category C3 line filter must be connected inside the
cabinet to the PE busbar and the EMC shield busbar with a low-inductance contact. The connection can be made
over a large area using metal construction components of the cabinet. In this case, the contact surfaces must be bare
metal and each contact point must have a minimum cross-section of several cm
be made with short ground conductors with a large cross-section ( ≥ 95 mm
low impedance over a wide frequency range, e.g. made of finely stranded, braided round copper wires or finely
stranded, braided flat copper strips.
The same rules apply to the connection of the optional category C2 line filter to the PE busbar and the EMC shield
busbar.
The optional line filter must always be combined with a line reactor, otherwise it cannot achieve its full filtering effect.
If the motor cable used were unshielded rather than shielded, the high-frequency leakage currents would be able to
return to the cabinet via an indirect path, i.e. across the motor cable capacitance. They would inevitably flow to the
cable rack and thus to system ground. From here they would continue to the transformer neutral point along
undefined paths and finally via the three phases of the supply system back to the converter. They would bypass the
line filter, rendering it largely ineffective.
It is basically also possible to reduce conducted interference emissions to low values with unshielded motor cables.
However, this would require very sophisticated filtering measures at the inverter output, i.e. filters which would
achieve a major reduction in voltage rates of rise and thus the interference current. The only filter suitable for this
purpose is the sine-wave filter and this would be effective only if the permissible motor cable lengths were not fully
utilized.
Line filters and IT systems
The standard line filters for category C3 and the optional line filters for category C2 are suitable for use only in
grounded supply systems (TN and TT supply systems). Where SINAMICS equipment is to be operated on an
ungrounded (IT) supply system, the following must be noted:
•
In the case of standard line filters, the connection between the filter and ground must be interrupted when
the equipment is installed or commissioned. This can be done simply by removing a metal clip as described
in the operating instructions.
•
Optional line filters for category C2 must not be used at all.
If these rules are not followed, the line filters will be overloaded and irreparably damaged in the event of a ground
fault at the motor side. Once the ground connection for the standard RFI suppression filter has been removed, the
devices meet the criteria of category C4 in accordance with the EMC product standard EN 61800-3. For more
information, refer to the chapter "EMC Installation Guideline".
Fundamental Principles and System Description
EMC gland
Shield bonding to the EMC shield busbar in the converter
using an EMC shield clip
Engineering Information
2
. Alternatively, this connection can
2
). These must be designed to have a
SINAMICS Engineering Manual - May 2008
© Siemens AG
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