Siemens SINAMICS S 1PH7 - Motor Grounding; Bearing Current Due to Grounding Situation (= Rotor Ground Current)

Information for using the motors
7.11 Insulated bearings (NDE) (option L27)
1PH7 induction motors (Machine tools)
234 Configuration Manual, (APH7W), 04/2009, 6SN1197-0AD72-0BP0
Motor grounding
In order to avoid rotor ground currents, the motor frame should be well grounded - e.g. by
using shielded motor cables. The motor cable shield should be connected at both ends
through the largest possible surface area.
For specific applications, the grounding of the motor Z
hg
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 flows from the motor frame through the motor shaft to the connected load and from
there to ground.
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Figure 7-3 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 flows from the motor frame to the load and not through the bearings.
The connection between the motor frame and load is only effective if it represents an
extremely low impedance for the high-frequency discharge (leakage) current. To achieve
this, use several flat straps, e.g. grounding straps, metal plates.
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Figure 7-4 Connection between the motor frame and load to avoid rotor ground currents