A US company has developed and patented a series of EMI (electromagnetic interference) filters that, it claims, resolve many of the issues associated with PWM (pulse width modulation) drives – such as the high-frequency currents that can destroy motors’ bearings. California-based OnFilter says that in comparison with conventional reactors, which reduce these leakage currents by a factor of 2–3, its SF series dV/dt filters can reduce the currents by 50–100 times or more.

And, unlike mechanical mitigation techniques that use contacts and brushes, the easy-to-install filters need no mechanical attachments or motor modifications, and are maintenance-free. They also reduce EMI (electromagnetic interference), which mechanical systems cannot do.

The new filters help to comply with IEC 60034-17/-25 by smoothing the sharp edges of potentially damaging drive pulses by providing “complete” filtering – common and differential mode and in ground – thus preventing currents from flowing through motor bearings. This will prolong the lives of motors powered by variable-speed drives or servodrives and cut maintenance costs, as well as reducing the occurrence of disruptive “no fault found” problems, according to OnFilter.

The new filters are rated from 3–20A, at up to 250V. They have ETL and CE approvals and are RoHS compliant. OnFilter has recently been awarded US Patent 10,263,591 which covers a “device and method for reduction of electrical noise from pulsed signal devices”.

One “unique” version of the filters (model SF20032) combines AC and motor EMI filtering in a single compact package said to solve both bearing damage and AC noise issues.

OnFilter designs and manufactures EMI filters to manage interference in a wide range of industries. Its products include filters for power lines, servomotors and variable-speed drives, data filters, and instrumentation for industrial, scientific, medical and other applications. The products are manufactured in the US.