Siemens SINAMICS G130 Engineering Manual page 98

Fundamental Principles and System Description
Engineering Information
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 the motor terminal box
using an EMC gland
The shielded cable with well bonded shield at both ends ensures that interference currents can flow back easily 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
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 ineffective, with the result that the system would comply with category C4 only.
Compliance with category C4 in complex installations with rated currents ≥ 400 A in an industrial environment and in
IT systems (see section below) in accordance with EN 61800-3 is perfectly acceptable. In this case, the plant
manufacturer and the plant operator must agree upon an EMC plan, i.e. individual plant-specific measures to achive
electromagnetic compatibility. These could include, for example, plant-wide use of highly interference-immune
components (which would include SINAMICS devices and their system components), and strict separation of
interference sources and potentially susceptible equipment, for example, through systematic separate routing of
power and signal cables. Under the specified boundary conditions, use of shielded motor cables is no longer
essential in terms of EMC, but is still recommended for the purpose of reducing bearing currents in the motor in
installations where motor reactors or motor filters are not installed in the converter.
Basically it is also possible to reduce conducted interference emissions to the low values of category C3 according to
EN 61800-3 when unshielded motor cables are used. However, very complicated filtering mechanisms capable of
drastically reducing the voltage rate-of-rise and thus also the interference currents would have to be provided at the
inverter output. In view of the volume and the costs involved in implementing extensive filtering at the inverter output
as well as the negative impact of filters on the dynamic control response and accuracy of the drive, this option does
not in practice generally constitute a viable alternative to the use of shielded motor cables in cases where compliance
with the limits stipulated by catagory C3 or even category C2 is essential.
SINAMICS Engineering Manual – November 2015
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Ó Siemens AG
EMC gland
Shield bonding to the EMC shield busbar in the converter
using an EMC shield clip
2
. Alternatively, this connection can
2
). These must be designed to have a low