High-frequency common-mode core filters reduce electromagnetic interference and eliminate bearing damage by electrical discharge. High-frequency common-mode (HF-CM) cores are special nanocrystalline magnetic cores which have superior filtering performance compared to regular ferrite cores. They act like a common-mode inductor (between phases and ground).
Installed around the three motor phases (U, V, W), they reduce highfrequency common-mode currents. As a result, high-frequency electromagnetic interference from the motor cable is reduced. However, the core kit should not be used as the sole mitigation measure. Even when the cores are used, the EMC installation rules should be followed.
Prevent motor bearing currents
The most important function is to reduce high-frequency currents associated with electrical discharges in the motor currents. These discharges contribute to the premature wear-out and failure
of motor bearings. By reducing or even eliminating discharges, the bearing wear is reduced and the lifetime extended. Thus, maintenance and down-time costs are lowered.
Bearing current problems are most often discovered after commissioning. Therefore, cores with an oval shape are ideal for retrofitting and for installation in restricted places.
A flexible solution
The cores can be combined with other output filters, especially in combination with dU/dt filters. They offer a low cost solution for protection of both motor bearings and insulation.
The cores can be installed at the frequency converter’s output terminals (U, V, W) or in the motor terminal box.
When installed at the frequency converter’s terminals, the cores reduce bearing stress and high-frequency electromagnetic interference from the motor cable. The number of cores to use depends on motor cable length and frequency converter voltage.
From Danfoss’ VLT High Power Drives Selection Guide
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