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Replacement bridge bypasses drive harmonics

01 September, 2001

Replacement bridge bypasses drive harmonics

Mitsubishi has launched a replacement input circuit for its variable speed drives which it claims will almost eliminate unwanted harmonics. The company says that the "high-current active bridge" is a direct response to the impact of the G5/4 regulations recently published by the UK electricity industry that place strict limits on the levels of harmonics that can be fed onto the electricity network.

The bridge, consisting essentially of six power transistors that switch in phase with the supply voltage sinewave, replaces the usual six input diodes on a drive. Whereas a standard VSD produces a total harmonic distortion of around 30% across all of the harmonic orders, the new circuit is said to result in a distortion value of around 1% in the driving mode and 0.7% in the regenerative mode.

The ability to feed power back into the grid will save energy, especially on overhauling loads, says Mitsubishi. It also allows VSDs to replace traditional four-quadrant controllers in many applications, removing the need to use DC drives.

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