Electricity + Control September 2018

DRIVES + MOTORS + SWITCHGEAR

Enduring market demand for dc drives

Andy Green, Nidec Control Techniques

While it’s fair to say that almost all new-build machines and systems utilise the latest ac drive technologies, many legacy designs around the world mean that the demand for dc drives will continue for some years to come.

Take Note!

Modern dc drives provide many benefits, and are often based on current ac drive technology. Ac drives are are consid- erably more cumbersome and costly than dc if ener- gy is to be returned to the grid. To achieve full line re- generation a second grid facing inverter and filter components are required.

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O f course, for the manufacturers and users of these dc-based systems, getting the level of technology, innovation, product support and applications knowledge is easier said than done. Many of today’s suppliers of drives have simply not retained their dc expertise. Dc drives and motors remain common in indus- tries such as metals, plastics and rubber, cranes,

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Utilising the existing dc motor and upgrading the dc drive therefore becomes an attractive option. Dc motors are usually sturdily built and capable of of- fering many more years of service beyond that of a drive, while project costs are typically much lower, and disruption and risks are minimised. A lot of the major current projects in the drives market are for dc technology. This may come as something of a surprise as many industry commen- tators had already sounded the death knell for dc drive technology, predicting its demise within the coming two-to-three years. However, for the mar- ket’s more recognisable dc drive brands – those with extensive dc heritage – demand continues as the industrial world looks to support myriad legacy process machines, such as winders and extruders. The trick for manufacturers looking to continue supporting systems of this ilk and for end users who want to keep an obsolete machine running is to seek out a specialist drives supplier that of- fers high levels of knowledge, expertise and stock levels, all supported by a fair pricing policy. In ad- dition, those seeking a reliable dc drives partner should appoint one that can also supply dc motors, as this will help project and design continuity, and one who also manufactures ac drives so a bal- anced view can be drawn. Another point of note is that opting to continue with dc drives should not mean the technology is incompatible with the latest automation solutions. Well-supported dc drive technologies are able to take advantage of almost all modern automation systems that can be connected via a fieldbus. The latest digital dc drives support protocols such as EtherCAT, PROFINET and EtherNet/IP, making them a far cry from the archaic technology that

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mining, and paper and printing. The current trend is to replace dc systems with new ac drives and motors to reduce main- tenance overheads, however this can often be a significant task that requires machinery to be taken out of service for an extended period while me- chanical and electrical rework is undertaken. Another limitation here is that upgrading to ac drives obviously means upgrading to an ac motor. However, ac motors are typically larger than their dc counterparts, which would therefore require some design changes to the machine ―– hardly practical from a business or commer- cial perspective when dealing with legacy systems. Further- more, manufacturers of these machines will have know-how in maintaining dc technology and may be reluctant to start a new learning curve.

16 Electricity + Control

SEPTEMBER 2018