Product Focus – Danfoss VLT Common Mode Filters
|Common mode HF Filters are designed to be placed between the frequency converter and the motor. They are nano-chrystalline cores that mitigate high frequency noise in the motor cable (shielded or unshielded) and eliminate bearing currents and hence Electro Discharge Machining (EDM) or bearing etching in the motor. Bearing currents caused by variable speed drives are also referred to as common mode currents.
As an aside, sinusoidal filters used on non-screened cables will not only ensure that the installation meets EMC compliance with non-screened cables, they will also reduce the dV/dt to nearly zero and limit Peak Voltages at the motor terminals to Square Root x 415Vac.
Both sinusoidal filters and common mode HF filters are often used together for installations where standard motor cables are used and common mode bearing currents could be a concern.
It should be noted that common mode bearing currents are different to axial motor circulating currents, which can occur in motors in frame sizes of 280 and above and flow between the rotor and the stator through the motor bearings. Motor manufacturers usually fit at least one insulated bearing and possibly a grounding brush.
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