Brief Overview Of The Different Types Of Bearing Currents - Siemens SINAMICS G130 Engineering Manual

Grounding of the motor shaft with a grounding brush
A shaft-grounding brush at the motor drive end can also reduce bearing currents as the brush shorts the bearing.
However, there are problems associated with shaft-grounding brushes. They are very difficult to construct for smaller
motors, they are sensitive to contamination and also require a great deal of maintenance. As a result, shaft-grounding
brushes are not generally a recommended solution for low-voltage motors in the low to medium power range.
IT system
In operation on an IT system, the transformer neutral is not electrically connected to ground as it is with TN systems.
The ground connection is purely capacitive in nature, causing the impedance to increase in the circuit in which high-
frequency, common mode currents are flowing. The result is a reduction in the common mode currents and thus also
in the bearing currents. With respect to bearing currents, therefore, non-grounded IT systems are more beneficial
than grounded TN systems.

Brief overview of the different types of bearing currents

The following overview shows the different types of bearing currents depending on the shaft height and the grounding
conditions of stator and rotor.
Rotor grounding
Good rotor grounding achieved through
conductive coupling to the well-
grounded driven machine ?
SH ≤ 100
EDM currents
Counter measure:
EDM currents without super-imposed
circular and rotor shaft currents have a
tolerable magnitude. Therefore, usually
no measures are required.
Dominant bearing current types dependent on the shaft height and the grounding conditions of stator and rotor
With a good rotor grounding, achieved by means of a conductive coupling to the well-grounded driven machine and
simultaneous poor stator grounding due to poor installation, the rotor shaft currents can become very large and thus
easily damage the bearings of the motor and the load machine. Such a situation must be avoided with a good stator
grounding, achieved by means of an EMC-compliant installation and/or the use of an isolating coupling.
Fundamental Principles and System Description
yes
no
Shaft height
SH ≥ 225
100 < SH < 225
Circular currents
Counter measure:
•
Isolated bearing on the NDE side of the
motor (NDE = Non Drive End)
or, alternatively
•
dv/dt filter or sinusoidal filter at the
output of the converter / inverter
(dv/dt
phase to ground
Stator grounding
Good stator grounding through
shielded, symmetrical motor cable ?
and
shield connected on both sides and with a large surface area ?
and
HF potential bonding between motor housing and driven machine ?
From shaft heights of ≥ 225 also recommended:
HF potential bonding between motor terminal box and housing ?
yes
< 0,5 kV/μs)
SINAMICS Engineering Manual - May 2008
Engineering Information
Rotor shaft currents
Counter measure:
•
shielded, symmetrical motor cable
(shield connected on both sides)
•
HF potential bonding between
motor housing and driven machine
From shaft heights ≥225 recommended
•
HF potential bonding between
motor terminal box and motor housing
© Siemens AG
no
87/396