THE DRIVE to a better standard

Much has been said, and written, in recent years about the advantages of powering process machinery through variable speed drives (VSD). Soft starts for pumps, compressors, conveyors and the like not only make process start-ups easier to control, but also contribute to the considerable energy savings achievable by matching motor speeds to actual demand from the process. And yet, according to ABB's Jouko Karvinen, industry surveys show that only 3 per cent of the installed base of medium-voltage motors are fitted with variable speed drives.

Karvinen is vice president of ABB's worldwide automation and drives division and was speaking in Switzerland at the launch of ABB's latest drive, the ACS 1000. The company is targeting this drive at a huge slice of that untouched 97 per cent of the existing market energy-intensive industries such as petrochemicals, mining, water, pulp and paper, cement, power generation and metals processing.

A survey of these industries by ABB showed that, as far as ac drives were concerned, what users wanted for the vast majority of their applications was a standard, packaged solution, rather than the dedicated, specially engineered drives that were traditionally available in the medium voltage and power range. 'About 85 per cent of all uses for medium-voltage drives are in standard applications such as fans, pumps, conveyors and compressors, where the customised engineering content is minimal,' says Ewan Morris, ABB's Asia-Pacific regional sales manager. 'It is not necessary to engineer a drive for the same applications over and over again, adding cost and extending delivery.'

INVESTING IN STANDARDS

Taking this customer feedback to heart, ABB has invested a total of $33 million over the past three years, researching, developing and now manufacturing the ACS 1000 in Turgi, Switzerland. Covering 315 to 5000kW in voltages of 2.3, 3.3 and 4.16kV, the ACS 1000 is first and foremost a standard product with, says ABB, all the attributes that standardisation brings shorter delivery times, faster installation and commissioning and lower cost than engineered solutions.

At up to eight weeks, delivery times will, says Karvinen, be 'some 50 per cent shorter than many other manufacturers of medium-voltage ac drives.'

Incorporated into the new drive are many of the standard components, software tools and design principles used by ABB in its successful low-voltage ACS 600 drive range introduced in 1995. Both the 600 and 1000 feature ABB's DTC (direct torque control) drive technology, the same communication buses (providing links to control systems such as ABB's Advant OCS), and the same operator interface.

A key target for the drive is the retrofit market, and here the ACS 1000 offers the added attraction of a much more compact design than traditionally engineered packages. Plant contractors surveyed by ABB also emphasised the importance of compact size to new installations, particularly on applications such as offshore platforms.

The key to the ACS 1000's compact size lies in ABB's development of a new power semiconductor switching device known as an IGCT (integrated gate commutated thyristor). The theory behind this is probably best left to electrical and electronic engineers, but as an example of its benefits ABB cites a 4.16kV drive. Using IGCT this would need only 12 such switching devices in the inverter, compared with double that number of conventional high-voltage devices, or up to 60 low-voltage devices.

Another space-limiting feature of the ACS 1000 is that its input isolation transformer needed to avoid common mode voltages whose high frequency spikes can destroy a motor's insulation can be installed either inside or outside the control room. Some drives simply don't have this option, because the transformer needs to be integrated into the drive to minimise cabling between the secondary windings and the converter an arrangement that can more than double a drive's footprint compared with the ACS 1000.

Another of the concerns expressed by contractors to ABB was over the uncertain reliability of early designs of medium-voltage drives. In addition to the DTC and IGCT developments which offer reliability through a reduction in the number of components in a drive ABB's engineers have also designed an output voltage filter for the ACS 1000. This produces a 'near perfect' sinusoidal voltage waveform to overcome many of the problems traditionally faced by medium-voltage motors, such as rate of change of voltage that stresses the motor insulation, torque pulsations, voltage reflections and noise. The filter also provides the compatibility link between the drive and any standard squirrel-cage motor.

As Ewan Morris explains: 'A key consideration in the design of the drive is that it is retrofit-friendly it can be fitted on to existing motors, no matter whose they are.'