Question:
How can currents occurring via the bearings of the motor rotor, so-called bearing currents, be reduced?
Answer:
Wiring and earthing according to the Manual is an important measure for bearing current attenuation. The earthing not only has to be of low impedance for operator protection but also has to be of low high-frequency impedance (with a surface as large as possible).
Generally, current attenuations can be decreased by a reduced switching frequency. Therefore, the switching frequency (C0018) should be set as small as possible. This could, however, lead to an increased noise emission of the drive unit.
As the main problems of bearing current occur in the power range of >15 kW, the Lenze accessories also include special bearing current-limiting chokes for this motor power range. With this the bearing currents are reduced by a ratio of 5 to 10, so that a sufficient life-time of the motor bearings is guaranteed.
There are no restrictions for operating frequency inverters and servo inverters in connection with the bearing chokes. The influence on the motor current control of the servo inverter can be neglected.
Additional improvements can be achieved by using symmetrical motor cables. This can be either a 3-conductor cable where the shield also acts as the PE conductor or a 6-conductor cable with three symmetrical PE conductors of reduced cross-section.
For reasons of economy (costs) Lenze recommends the application of insulated motor bearings for motor powers of 90 kW and more. These bearings have the same dimensions and tolerances as standard bearings. However, when applying insulated bearings, the shaft attachments (on load and encoder end) must also have an insulation, otherwise e. g. the bearings of an attached gear box may absorb the bearing current and thus take damage.
For optimum shield contact when applying additional output chokes such as motor chokes, motor filters or sinus filters, the bearing current-limiting choke must always be directly connected to the motor cable whereas the additional filters must be directly connected to the inverter.
The power connection has to comply with the Mounting Instructions requirements.
Attention should be paid to the following:
- Shielded motor cables must be applied.
- The cable shield must not be disconnected.
- The shield must be connected to both ends.
- The shield must be connected to the whole cable circumference (e.g. via clamp).
- The shield is connected to the device via clips on the shield sheet or via clamps (the bearing current-limiting chokes are provided with an appropriate shield clamp).
- The shield is connected to the motor via a special EMC cable routing through the terminal box.
Additional notes:
Bearing current-limiting chokes for preventing motor bearing damages:
Application Report No. 100.21.1Further information to the topic 'bearing current' see Doc-ID
20022649 