Installation
Instruction handbook bmaXX 5000
Document No.: 5.09021.19
145
of 346
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
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).
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 ZToroidal cores
for motor cables– as from page 305 in chapter Accessories and Spare Parts.
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