Technology news: November 2004
MV drive ditches the transformer
Rockwell has introduced a medium-voltage drive that eliminates the usual isolation transformer, thus cutting space and weight, boosting efficiency, and avoiding the cost not only of the transformer, but also its protection relays and filters.
The PowerFlex 7000 drives (above) can be connected straight to the MV supply, using a combination of three technologies that, together, Rockwell calls Direct-to-Drive. They are:
high-voltage symmetrical gate commutated thyristors (SGCTs) that can be connected in series to allow ratings up to 6.6kV;
an active-front-end rectifier that reduces harmonics; and
common-mode voltage protection that mitigates stress produced by variable speed drives, without using an isolating transformer.
In addition to improving efficiency by eliminating transformer heat losses (as well as the need for air-conditioning to cool the transformer), the new drive is said to cut the size of drive systems by 30-50% and weight by 50-70%.
The transformer-less drive is said to meet most global harmonics standards and to have a near-unity power factor.
Potential applications for the PowerFlex 7000 include fans, pumps, compressors, extruders and conveyors.