MV drive update: 'No transformer' option offered by Rockwell Automation

Efficiency and ability to operate at lower current for a given power output are among the benefits propelling the growth of medium-voltage (MV) ac drives.


PowerFlex 7000 medium-voltage drives now offer "Direct-to-Drive technology," eliminating the need for an isolation transformer for applications starting at 200 hp (150 kW) with either new or existing motors.

Efficiency and ability to operate at lower current for a given power output are among the benefits propelling the growth of medium-voltage (MV) ac drives. Recently, Rockwell Automation has "sweetened the pot" by adding a "transformerless option" to its Allen-Bradley PowerFlex 7000 Medium-Voltage Drive. This so-called "Direct-to-Drive technology" lets users connect utility power directly to the MV drive without an isolation transformer. It makes the transformerless option possible, along with improved efficiency, reduced size and weight, and lower total cost of drive ownership, according to Rockwell. The new drive option was shown at the company’s Automation Fair in Orlando, FL, in late October.

New Direct-to-Drive technology combines three elements to eliminate the need for an isolation transformer. Power semiconductors used in PowerFlex 7000 are symmetrical gate commutated thyristors (SGCTs), which have high-voltage rating and can be connected in series, allowing the drive to be rated up to 6,600V. This comes without added equipment complexity, reducing total component count, and increasing reliability.

The drive’s active front-end (AFE) rectifier features active switching and Selective Harmonic Elimination to reduce harmonics. The common-mode voltage protection is the latest technology for mitigating common-mode voltage stress generated by variable-frequency drives without using an isolation transformer. Direct-to-Drive technology reportedly brings a lower total cost of ownership . It eliminates cost of the transformer and its protection relays, reduces cable costs, and eliminates the need for line and motor filtering devices.

Eliminating the isolation transformer on an MV drive:

  • Improves efficiency by eliminating transformer heat losses;

  • Reduces drive system size by 30-50%;

  • Reduces drive system weight by 50-70%; and

  • Eliminates the need for extra cooling capacity (normally needed by the transformer)

PowerFlex 7000 complies with applicable harmonic standards (IEEE 519, EN61000-2-4, and G5/4)—without using a phase-shifting transformer. Moreover, Direct-to-Drive technology maintains near unity power factor. PowerFlex 7000 air-cooled and liquid-cooled drives meet applications ranging from 200 to 8,500 hp (150-6,340 kW).

(Watch for an article in November 2004 Control Engineering on high-voltage motors from another manufacturer that do not use transformers.)

—Frank J. Bartos, executive editor, Control Engineering,

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