Electricity + Control September 2018

Replacing dc systems with new ac drives can be a significant task that requires machinery to be taken out of service for an extended period of time.

some engineers imagine. One further key advan- tage of dc drive technology is the relatively low cost of providing true four-quadrant operation. That is, the ability to generate torque in any direction, typically for motoring or decelerating, and return kinetic energy from the mechanical load and motor back to the power grid. Such attributes are consid- ered essential in applications where braking and reversing is required, such as wire drawing, wind- ers, rolling mills and any application where gravi- ty is a factor or where energy has to be removed from a rotating system. Whilst ac drives are inherently four-quadrant and typically use a ‘brake’ resistor to burn excess energy, they are considerably more cumbersome and costly than dc if energy is to be returned to the grid.To achieve full line regeneration with an ac drive a second grid facing inverter and filter com- ponents are required. However, it must be noted that the characteristics of a regenerative ac drive in terms of harmonics, mains dip ride through ca- pability and their ability to control power factor all outweigh those of a dc drive. Put simply, modern dc drives provide many benefits, and are often based on current ac drive technology, ensuring they provide enhanced mo- tor performance and reliability. For retrofit projects specifically, dc drives offer easy integration with ex- isting motors, power supplies, applications equip- ment and communication networks. The only ca- veat is choosing the right supplier. Many suppliers of drives simply no longer have specialists in the business who really understand dc technology.

Aside from a multitude of applications in the process industries, the retrofitting of dc drives on cranes is another market segment where demand for this technology remains strong. This statement is corroborated by a recent landmark project in the US involving the upgrade of 10 oil rig cranes. Here, two dc regenerative drives were used on each crane for the swing and hoist. The hoist motor drive system now features a Mentor MP dc drive rated at 420 A continuous, while a Mentor MP dc drive rated at 210 A con- tinuous was retrofitted to the swing motor drive system. All of the drives were purposely sized for twice the normal power required, which means the system never runs at its maximum capacity. Aside from the performance attributes of the dc drives, it was the technical advice offered by the supplier that was appreciated most by the crane user, with technicians addressing any issues quick- ly and comprehensively. The oil rig crane upgrade programme was completed at the end of 2017. Although dc was popular in years gone by for technical performance reasons, now that ac can offer comparable, if not better performance, the latter has become the technology of choice for new-build projects. However, there remain plenty of dc motors in widespread use that need com- plementary drive technology, a fact that means dc drives are likely to be in demand for a number of years to come. For those looking to select a part- ner in dc drive technology, give it careful and con- sidered thought.

Andy Green is the Business Development Manager - Automation at Nidec Control Techniques.

Electricity + Control

SEPTEMBER 2018

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