Siemens SIMOTICS M-1PH8 - Page 97

For speci󹪝c applications, the grounding of the motor Z
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can be more unfavorable than the
grounding of the connected loads Z
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, e.g. for long motor cables and when the motor is
mounted in an insulated fashion. In this case, the capacitive discharge (leakage) current of
the motor 󹪞ows from the motor frame through the motor shaft to the connected load and
from there to ground.
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Figure4-9 Bearing current due to the grounding situation (=rotor ground current)
The rotor ground current should be avoided by using an electrically insulating coupling. If
such a coupling cannot be used for mechanical reasons, then the motor frame must be
connected to the load through the largest possible surface area. The capacitive discharge
(leakage) current then 󹪞ows from the motor frame to the load and not through the bearings.
The connection between the motor frame and load is only e󹪜ective if it represents an
extremely low impedance for the high-frequency discharge (leakage) current. To achieve this,
use several 󹪞at straps, e.g. grounding straps, metal plates.
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Figure4-10 Connection between the motor frame and load to avoid rotor ground currents
Mechanical properties
4.4Types of bearing
1PH8 SIMOTICS M main motors
Con󹪝guration Manual, 12/2022, A5E51895839A 95