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Regenerative drive `eliminates harmonic distortion`

01 July, 2002

Regenerative drive `eliminates harmonic distortion`

ABB claims to have cut the cost of eliminating harmonic distortion caused by AC drives by developing a regenerative four-quadrant version of its ACS600 drive. It says that that the regen drive performs better than conventional 12- or 18-pulse systems, while occupying up to 75% less space.

By combining a high-frequency LCL filter with its DTC (Direct Torque Control) technology, which removes frequencies below 1kHz, ABB says it can eliminate the need for multi-pulse rectifiers, along with their transformers - which can weigh up to 2 tonnes - and associated cabling. According to James Haigh, ABB`s senior vice-president responsible for drives in the UK, the absence of transformers also boosts the system efficiency from around 96% for a conventional 12-pulse installation, to around 98%.

He estimates that the savings for a 500kW installation over a ten-year lifecycle could amount to more than £28,000.

In addition, the low line harmonic current and the system`s unity power factor can allow the use of smaller supply transformers. The regenerative drive has a current distortion level of less than 4%, thus complying with the UK electricity industry`s G5/4 regulations on power line distortion.

Conventional AC drives use an uncontrolled diode bridge in the line input, resulting in a current waveform that is not sinusoidal. In the regenerative four-quadrant drive, the line voltage is switched in a modulated way to produce the desired sinusoidal shape.

To achieve this, ABB has modified its DTC technology to compensate for voltage variations, thus producing the sinusoidal waveform. The LCL filter reduces voltage switching ripple caused by the drive`s IGBT power semiconductors, making it easier to use in weak networks.

In standard drives, the motor voltage can sometimes fall below the line voltage because of voltage drops in the transformers, cables and other components. To achieve full torque in such circumstances, the motor often has to be over-sized. ABB says that its new drive avoids the need to oversize by boosting the output voltage even when the line voltage is low. It can therefore be used with motors up to 10% smaller than those needed for 12-pulse installations.

According to Haigh, a conventional 12-pulse system for a 315kW installation would need a 1.2m enclosure for the drive, as well as about 1m to accommodate the transformer. With the regenerative version, everything would fit in a 1.2m enclosure.

The regenerative drives come in ratings from 15kW-3.6MW and are intended for applications such as centrifuges, punch and press drives, and fan drives. Haigh predicts that these drives "will rapidly replace 12-pulse systems".

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