Installation
Instruction handbook
b maXX
5000
Document No.: 5.09021.16
139
7
Avoiding bearing
damage
m
Basically the
grounding system
must be installed appropriately to ensure a forced re-
turn of the common mode current.
m
The cause of bearing current damage, that means the amplitude and slope of the com-
mon mode voltage is reduced by using toroidal cores.
The use of toroidal cores is therefore a
preferred measure
.
m
In addition the using of
current-isolated bearings
(standard for AC drives from motor
size 180 and higher) can reduce the effects of the common mode voltage.
m
The shaft can be grounded (and the bearing currents redirected) by using special
grounding rings or grounding brush(es).
m
Furthermore modified
motor cables
(for high frequencies, cable shield with low imped-
ance, symmetric cable design) can be used in order to lead the capacitive currents to
a large extent back to the converter via the cable shield.
Toroidal cores
NOTE
It is recommended to use toroidal cores in order to reduce/to avoid bearing currents.
Part numbers and the number of recommended toroidal cores, see
as from page 288 in chapter Accessories and Spare Parts.
m
The toroidal cores are made of nanocrystalline material. The toroidal cores cover all
three phases of the converter output. The time variable common mode current induces
a magnetic field into the toroidal core, which counteracts against the change of the
common mode current.
m
For this reason the toroidal core operates a current-compensated choke, which limits
the rate of change and the amplitude of the common mode voltage and therefore re-
duces the bearing currents significantly.
m
Because of the higher amplitude and frequency of the common mode voltage when us-
ing an active mains rectifier unit, there are used toroidal cores with a lower permeability
for optimized modulation of the cores (saturation and temperature characteristics).
