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 à bearing at non-drive end (NDE bearing) à motor
housing à bearing at drive end (DE bearing) à shaft. This circular current therefore flows from the shaft to the
housing in one bearing and from the housing back to the shaft in the other. As the circular current value depends on
the capacitance C between winding and housing, it increases with the shaft height of the motor. It becomes the
wh
dominant bearing current type with motors of shaft height 225 and higher.
The EDM current
Each edge of the three phase-to-ground voltages on the winding (often referred to as the "common mode voltage")
charges the capacitance C
characteristic of the voltage on shaft and bearing is thus an image of the three superimposed phase-to-ground
voltages on the motor winding. The magnitude of this voltage is however reduced in accordance with the capacitive
BVR (Bearing Voltage Ratio) which can be calculated for each phase by the following equation
BVR
The resulting voltage on shaft and bearing is determined by the superimposition of the three phase-to-ground
voltages on the winding multiplied in each case by the Bearing Voltage Ratio BVR. It generally equals about 5 % of
the mean value of the phase-to-ground voltages on the winding on standard motors.
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.
1.9.4.1

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.
Of the measures described, implementation of the first two is mandatory for drives within the output power range of
the SINAMICS G130, G150, S120 (Chassis and Cabinet Modules) and S150 units which supply motors of shaft
heights 225 or greater, in other words, installation in compliance with EMC requirements in order to eliminate the
rotor shaft current in combination with an insulated bearing at the non-drive end of the motor in order to eliminate the
circular current. This combination generally provides adequate protection of the bearing against damage caused by
bearing currents in virtually all applications.
All the other measures described should be regarded as supplementary and essential only in the case of extremely critical
drive constellations where it is impossible to implement an EMC-compliant installation of satisfactory standard.
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, it can be worthwhile to install an additional
insulating coupling in order to eliminate the rotor shaft current. With high-output low-voltage motors, it is basically also
possible to install two insulated motor bearings combined with a shaft-grounding brush and an insulating coupling, as
is normal practice for converter-fed high-voltage motors.
SINAMICS Engineering Manual – November 2015
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Ó Siemens AG
between the winding and housing is also reversed with every switching edge. This
wh
of the bearing via the capacitance C
b
V
= Bearing
V
-
Winding / Phase Ground
Bearing
and C
b rh
between the winding and rotor. The time
wr
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
Housing