Measures For Reducing Bearing Currents - Siemens SINAMICS G130 Engineering Manual

Fundamental Principles and System Description
Engineering Information
The circular current
In the same way as the motor cable capacitance changes its polarity with every switching edge at the inverter output,
the polarity of the capacitance C
creates a kind of high-frequency, capacitive "leakage current" between the winding and the housing and thus to
ground. This leakage current leads to a magnetic imbalance in the motor which induces a high-frequency shaft
voltage V . If the insulating capacity of the lubricating film on the motor bearing cannot withstand this shaft voltage,
Shaft
a capacitive circular current flows through the circuit: Shaft
housing bearing at drive end (DE bearing)
housing in one bearing and from the housing back to the shaft in the other.
The EDM current
Each edge of the phase-to-ground voltage on the winding (also referred to as "common mode voltage") charges the
capacitance C in the bearing via capacitance C
b
voltage on the bearing is thus an image of the phase-to-ground voltage on the motor winding. The amplitude of this
voltage is however reduced in accordance with the capacitive BVR (Bearing Voltage Ratio) which is generally of the
order of about 5 % on standard motors:
BVR
In the worst-case scenario, the bearing voltage V
bearing breaks down and the capacitance C
pulse is referred to as the EDM current (Electrostatic Discharge Machining).
The rotor shaft current
The high-frequency, capacitive "leakage current" flowing through the capacitance C
to cause the circular current must flow from the motor housing back to the inverter. If the motor housing is badly
grounded for the purpose of high-frequency currents, the high-frequency "leakage current" encounters a significant
resistance between the motor housing and grounding system across which a relatively high voltage drop V
occurs. If the coupled gearbox or driven machine is more effectively grounded for the purposes of high-frequency
current, however, the current may flow along the following path to encounter the least resistance: Motor housing via
the motor bearing – motor shaft – coupling – gearbox or driven machine to the grounding system and from there to
the inverter. With a current following this path, there is not only a risk of damage to the motor bearings, but also to the
bearings of the gearbox or the driven machine.

Measures for reducing bearing currents

Since there is a range of different bearing current types caused by different physical phenomena, it is generally
necessary to take a series of measures in order to reduce the resultant bearing currents to a non-critical level. These
measures are described in detail on the following pages.
For drives in the power rating range of SINAMICS G130, G150, S150 and S120 (Chassis and Cabinet Modules), the
first two of the described measures are absolutely mandatory, i.e. installation in accordance with EMC guidelines
combined with an insulated bearing at the non-drive end of the motor. This combination generally provides sufficient
protection against bearing damage.
All the other measures described should be regarded as supporting precautions, although it can certainly be
worthwhile to implement them in certain critical drive constellations. This applies particularly if it should not be
possible to achieve high-quality implementation of the first two measures.
If it is not practically possible to achieve EMC-compliant installation standards when extending an existing plant which
already features a poor grounding system and/or unshielded cables, for example, it can be very worthwhile to provide
in addition to an insulated motor bearing, filters at the converter output. Alternatively, a shaft-grounding brush can be
installed in combination with an insulated coupling. Which of the alternative measures is the most beneficial will
depend on the weak points of the individual installation.
SINAMICS Engineering Manual – May 2008
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© Siemens AG
between the winding and housing is also reversed with every switching edge. This
wh
shaft. This circular current therefore flows from the shaft to the
between the winding and rotor. The time characteristic of the
wr
V
=
Bearing
V
−
Winding / Phase Ground
Bearing
and
C
b rh
bearing at non-drive end (NDE bearing)
C
=
wr
+ +
C C C
wr rh b
can reach such high values that the lubricating film on the
are discharged by a short, high current pulse. This current
between winding and housing
wh
motor
Housing