New power switching revolutionizes medium-voltage drives

March 2, 1998 Design News

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New power switching revolutionizes medium-voltage drives

Faster switch with lower power losses leads to the development of reliable medium-voltage, variable-speed drives

Charles J. Murray, Senior Regional Editor

Zurich, Switzerland--By some estimates, only about 3% of all medium-voltage motors employ variable-speed drives. Although many customers would like variable-speed capabilities for their medium-voltage pumps, fans, and compressors, the technology hasn't been easily accessible. For many, medium-voltage drives are already too big, too costly, and too complex.

Now, however, that may be changing. Thanks to the development of a new power switching technology, medium-voltage motors and drives may be more accessible than ever before. A new medium-voltage ac drive, which employs the new power switching technology, brings adjustable-speed capabilities to motors in the 400- 6,000-hp range, for voltages of 2.3, 3.3, and 4.16 kV.

Engineers from ABB Industrial Systems, who spent 200 man-years and $33 million developing the ACS 1000 drive, believe it could change the way industry views the medium-voltage drive. "It gives all kinds of advantages over previous systems in terms of reliability, package size, and performance," notes Jim Nash, principal engineer for ABB Industrial Systems, Inc. The smallest of the ACS 1000 drives measures just under 10 ft in length, compared to 13 ft, 25 ft, and 33 ft for competing systems. The ACS 1000 also uses far fewer components than previous systems, which translates to greater reliability.

The key to this technological leap was the development of a power switching device that offers faster switching and low power loss. Known as the Integrated Gate Commutated Thyristor (IGCT), this silicon-based device makes it easier to switch up to 10 MW of power at medium voltages. "We liken this to the quest for the Holy Grail in power electronics," notes Eric Carroll, industry business segment manager for ABB Semiconductors AG, Switzerland. "It may not be the perfect switch, but it has taken us very close to that goal."

Silicon-based switches similar to the IGCT have long been used to control electrical motor speed. Up to now, however, the power silicon switches have had difficulty switching megawatts of electricity at medium voltages (2.3 to 6.9 kV). The two most prominent of those, Gate Turn-Off thyristors (GTOs) and Integrated Gate Bipolar Transistors (IGBTs), have exhibited different shortcomings. GTOs have proven to be comparatively slow, while IGBTs showed larger switching losses.

With the introduction of the IGCT, however, ABB engineers say they have licked those problems. They have employed a low-inductance gate unit to turn a low-loss conducting thyristor into a fast-switching transistor just prior to turn-off. By doing so, they reduced the thickness of the silicon wafer on which the device is fabricated. This permits the device's diode and switch to be fabricated on the same wafer, limiting the parts count and system cost. Result: Low-loss switching can occur at the device level.

In the development of the ACS 1000, the device's unique characteristics enabled designers to eliminate the need for so-called "snubbers," which are often used in conjunction with GTOs. Because snubbers are employed to compensate for non-uniform switching characteristics, and because the IGCT offers uniform switching characteristics, the ACS 1000 needed no snubbers.

Additionally, IGCT technology cut construction costs through thinner silicon wafer design and simplified power circuitry. It also vastly reduced the system's part count by enabling ABB engineers to build the medium-voltage drive using only 12 IGCTs. In contrast, competing medium-voltage drives employ as many as 60 IGBTs. More recently, one system has used 24 IGBTs, but up to now, no medium-voltage drive has employed as few as 12 power-switching devices.

Additional details?Contact Scott Conner, ABB Industrial Systems, 16250 W. Glendale Drive, New Berlin, WI 53151, (414) 780-5175.