Siemens SINAMICS S120 Equipment Manual page 51

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Avoiding bearing currents
When operating multiphase induction machines on a converter, an electrical bearing stress
results from a capacitive induced voltage via the bearing lubricating film, depending on the
principle being used.
The physical cause of this is the common-mode voltage at the converter output: The sum of
the three phase-to-neutral voltages is not zero at all times, unlike with direct on-line
operation. The high-frequency, pulse-shaped common-mode voltage brings about a residual
current, which closes back to the converter's DC link via the machine's internal capacitances,
the machine housing, and the grounding circuit. The machine's internal capacitances include
the main-insulation winding capacitance, the geometric capacitance between the rotor and
stator, the lubricating-film capacitance, and the capacitance of any bearing insulation that
may be present. The magnitude of the currents induced via the internal capacitances is
proportional to the common-mode-voltage regulation (i(t) = C x dv/dt).
In order to apply currents to the motor which are sinusoidal as far as possible (smooth
running, oscillation torques, stray losses), a high pulse frequency is required for the
converter's output voltage.
The related (very steep) switching edges of the converter output voltage (and also, therefore,
of the common-mode voltage) cause correspondingly high capacitive currents and voltages
on the machine's internal capacitances.
In the worst-case scenario, the capacitive voltage induced via the bearing can lead to
random punctures of the bearing lubricating film, thus damaging the bearing/causing
premature wear.
The current pulses caused by the breakdown in the lubricant film are referred to as EDM
(electrostatic discharge machining) currents, although this is not primarily a question of an
electrostatic effect, but more of (partial) breakdowns of insulating material, i.e. of partial
discharges.
This physical effect, which occurs in isolated cases, has mostly been observed in connection
with larger motors.
EMC-compliant installation of the drive system is a basic prerequisite for preventing
premature bearing damage via bearing currents.
S120 Cabinet Modules Equipment Manual
Manual, (GH5), 10/2008, A5E00427098A
Rated voltage < 1000 V; unrestricted use in environment 1
Rated voltage for stationary drive systems < 1000 V; for use in environment 2 For use in
environment 1 only when sold and installed by skilled personnel.
Rated voltage < 1000 V; for use in environment 2 only
Rated voltage ≥ 1000 V or for rated currents ≥ 400 A in complex systems in
environment 2
Electrical installation
4.3 Introduction to EMC
51