Turgi, Switzerland — Wider applications for medium-voltage (MV) drives are opening due to the substantial increase in power output of ACS 6000 drives, introduced by ABB Industrie AG in late May 2000. ACS 6000 expands power output with a 3-27 MW range at 3.3 kV (up to 6.6 kV for high-speed units). It reuses key technology and application experience of ACS 1000 drives introduced in late 1997. ACS 1000 targets lower-power applications in the 315 kW-5 MW range (see also CE, Feb. '00, pp. 85-90). ABB now markets the larger MV drives for specific industries, such as chemical, oil, and gas production; marine propulsion; and metal manufacturing. The latter industry is receiving special attention via ACS 6000 SD, a synchronous drive that targets the tough application of main drives in steel rolling mills (see photo).

Precise torque and speed control is crucial in rolling mills that must tightly control the thickness, flatness, and tension of the metal strip being processed. In fact, the drive's dynamic performance directly affects product quality. ACS 6000 (and ACS 1000) drives rely on a special motor control method called Direct Torque Control (DTC). "With DTC we are going beyond just speed control in the ACS 6000 drive," says Helmut Langlotz, ABB's metals drives product manager. He says torque response is 3 msec with DTC, for example, compared to 10 msec for other control methods.

DTC differs from PWM-based control and doesn't need sensors for speed and position feedback. Its switching technique reacts rapidly to load dynamics or power disturbances. DTC is reported to virtually eliminate torque harmonics and potential damage to the machine or motor from mechanical resonance.

A notable optional feature called Eccentricity Compensation (EC) adds to the drive's attractiveness for rolling mills. "In a coiler drive, coil eccentricity at the clamped head of the strip is unavoidable—especially with thicker strip materials," adds Mr. Langlotz. "The EC function maintains nearly constant tension within the strip, independent of coil diameter variations."

ACS 6000's design is based on modularity. Its 27-MW capacity for single drives comes from linking 6-MW and 9-MW base modules in parallel. A low-power, 3-MW module will be added by mid-2001. For multiple drives, the system configuration is borrowed from low-voltage drive methods. Several parallel drives can use the same dc bus, with an Active Rectifier Unit (ARU) operating between the bus and line supply. Power output can be three times 9 MW or other combinations; and induction, as well as synchronous motors, can be used. The ARU adds an optimized pulse pattern control that minimizes harmonics on the network side and runs the system at near-unity power factor.

DTC is carryover technology from ABB's low-voltage ac drives. The company stresses its extensive experience in applying DTC in that sector. Reuse of hardware elements—and especially software—helps increase reliability, lowers costs and the need for spare parts, and eases user training. Also integral to ACS 6000/1000 drives are IGCTs (integrated-gate commutated thyristors) that serve as the main power-switching device. ABB makes its own IGCTs. (For more on DTC, see CE, March '95, p. 9 and Sept. '96, pp.100-104. For more on IGCTs, see CE, July '97, p. 17; and April '98, pp. 91-101.).

Not to be overlooked, ACS 1000 MV drives are reportedly at work in growing numbers worldwide. They require adequate production facilities, and ABB's streamlined production and testing facility in Turgi can manufacture 550 voltage-source inverter drives annually. Capabilities of the plant are impressive, especially given the large units it produces. Assembly time for ACS 1000 is reported to be 38 hr (air-cooled) or 80 hr (water-cooled). Comparable figures are not available for the all water-cooled ACS 6000 line.

Quality goes along with quantity. ABB states that calculated MTBF (mean time between failure) for ACS 1000 is 5.7 years—even longer for the drive's inverter section at 17 years. "ACS 6000 builds on the same philosophy as ACS 1000 and, for single-drive units, has the same values," says Stefan Friedli, director, Product Management MV AC Drives. ACS 6000/1000 drives are available worldwide.

For more, see "Daily News," Aug. 17, at www.controleng.com .

For more information, visit www.abb.com/motors&drives or contact ABB MV ac Drives Marketing, Tel: +41 56 299 39 20; in North America, use Tel: 262/785-353, or visit www.controleng.com/freeinfo .