Insulated Bearing At The Non-Drive End (Nde End) Of The Motor; Other Measures - Siemens SINAMICS G130 Engineering Manual

Fundamental Principles and System Description
Engineering Information
The ground connections shown in black [0] represent the conventional grounding system for the drive components.
They are made with standard, heavy-power PE conductors without special high-frequency properties and ensure low
frequency equipotential bonding as well as protection against injury.
The connections shown in red inside the SINAMICS cabinets [1] provide solid bonding for high-frequency currents
between the metal housings of the integrated chassis components and the PE busbar and the EMC shield busbar of
the cabinet. These internal connections can be made via a large area using non-isolated metal construction
components of the cabinet. In this case, the contact surfaces must be bare metal and each contact area must have a
minimum cross-section of several cm
braided copper wires with a large cross-section ( ≥ 95 mm
The shields of the motor cables shown in orange [2] provide high-frequency equipotential bonding between the Motor
Modules and the motor terminal boxes. The finely stranded, braided copper cables shown in red can be routed in
parallel with the cable shields when cables with poor high-frequency properties are used or in installations with bad
grounding systems.
The connections shown in red [3], [4] and [5] provide a solid, high-frequency bond between the motor housing and
the motor terminal box or the gearbox and the driven machine.
The equipotential bonding measures described above can practically eliminate the rotor shaft currents. It is therefore
possible to dispense with insulated couplings between the motor and gearbox/driven machine. This is always an
advantage in cases where insulating couplings cannot be used for any number of reasons.

2. Insulated bearing at the non-drive end (NDE end) of the motor

Apart from an EMC-compliant installation which essentially prevents rotor shaft currents, the use of a motor with an
insulated bearing at the non-drive end is the second most important measure for reducing bearing currents.
Essentially, the insulated NDE bearing reduces the capacitive circular current in the motor by increasing the
impedance in the circuit, thus compromising the shaft – NDE bearing – motor housing – DE bearing – shaft.
Insulated NDE bearings are standard on all motors in the 1LA8 series designed for converter operation. An insulated
NDE bearing is available as an option for series 1LG4 and 1LG6 motors, frame size 225 and above. This option is
highly recommended at converter operation.
In systems with speed encoders, it must be ensured that the encoder is not installed in such a way that it bridges the
bearing insulation, i.e. the encoder mounting must be insulated or an encoder with insulated bearings must be used.

3. Other measures

Motor reactors or motor filters at the converter output
EMC-compliant installation and the use of a motor with insulated NDE bearing are generally perfectly adequate for
the purpose of maintaining bearing currents at a non-critical level, even under worst-case conditions when stochastic
disruptive discharges attributable to the EDM effect occur in the bearing.
In exceptional cases, it may be necessary to take additional measures to further reduce bearing currents.
This can be achieved with common mode filters consisting of toroidal cores made of highly permeable magnetic
material. They are mounted at the converter output and enclose all three phases of the motor cable. These filters
present a high resistance to the high-frequency currents (EDM current and rotor shaft current) flowing to ground and
reduce them.
As common mode filters are not generally needed on SINAMICS drives, they are not offered as a standard option.
They are available only on request.
As a general rule, all measures implemented at the converter output which serve to reduce the voltage rate of rise
dv/dt have a positive impact on bearing current levels in the motor.
Motor reactors reduce the voltage rate of rise on the motor as a function of the motor cable length. Although they help
in principle to reduce bearing currents, they cannot be regarded as a substitute to EMC-compliant installation and the
use of motors with insulated NDE bearings.
The capability of dv/dt filters and sine-wave filters to reduce the voltage rate of rise on the motor is generally not
affected by the motor cable length and achieves dv/dt values lower than those obtained with motor reactors. The
values attained with sine-wave filters in particular are markedly lower.
In consequence, it is possible to dispense with insulated motor bearings when dv/dt filters, or more particularly, sine-
wave filters are installed at the output of SINAMICS converters.
SINAMICS Engineering Manual – May 2008
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© Siemens AG
2
. Alternatively, these connections can be made with short, finely stranded,
2
).