Output filter for variable speed drives


The Schaffner FN 530 Combination sine wave/EMC output filter (distributed in Australia by Westek) for variable speed drives solves three problems for motors as loads for variable speed drives. These comprise of high dv/dt pulses, which if not smoothed by effective filtering, can damage motor and drive cable insulation by building up voltage stress through reflection of voltage pulses; common mode high frequency components due to high frequency switching IGBTs in the drive inverter stage, which without common mode filtering, flow through the motor frame to earth, damaging bearings in the process; and RF filtering thus preventing RF interference being radiated or conducted back to the input of the drive.

The filter is so effective in its functions that it is possible to use very long drive cables up to 1000 metres in length – without shielding, thus providing an economical solution to applications including long conveyor belt systems, tank farms, pump stations and large motor numbers in parallel.

The filters provide traditional L-C sine wave filtering, in combination with common-mode (Y) and symmetrical (X) mode EMC filtering and reduces protective earth current contributions by feeding common mode components back to the drive’s DC link. The Schaffner FN 530 filters are suitable for operation for three-phase operation at line voltages to 500 V, and current ratings are available from 4 to 16 A.