Rockwell Automation claims to be the first in the market with a transformer-less medium voltage (MV) ac drive - the Allen-Bradley PowerFlex 7000. Based on Rockwell's 'Direct-to-Drive' technology, which was developed to avoid having to include a drive isolation transformer, the new MV drive connects directly to the supply, thereby saving a considerable amount of panel space in confined control rooms.
Among PowerFlex 7000's other advantages are common mode voltage (CMV) elimination, reduced harmonic distortion, reduced complexity and lower capital purchase, shipping and installation costs. The technical innovations embodied in the new drive are CMV motor stress protection, and an active front end (AFE) rectifier and symmetrical gate commutated thyristor switching system that actively reduces the line current harmonics. Direct-to-Drive technology, moreover, uses zero sequence impedance to eliminate the CMV on the motor neutral, enabling the drive to be connected to standard motor and cable designs without an isolation transformer.
The new PowerFlex 7000 is smaller and lighter than drive technologies using isolation transformers. The volume and weight of a 950kW drive with isolation transformer are typically 13m3 and 4,200kg respectively. A transformer-less alternative of the same voltage and power takes up just 5.5m3 (60% less) and is 65% lighter at 1,350kg. Component count, too, is impressively reduced. For a typical 4,160V, 750kW motor installation, an average MV drive could have as many as 385 components in its power circuit. PowerFlex 7000 with Direct-to-Drive technology has just 30 components at 4,160V.