Until now, conventional thyristor rectifiers have usually been used for cathodic dip painting of car bodies, with which at best a coarse segmentation of the coating current is possible. This has the decisive disadvantage that if such a rectifier fails, a large part of the coating current is immediately lost - with the result that the paint finish is defective over a large area and the car body is then unusable. Dürr Systems, one of the world's leading machine and plant manufacturers, has now averted this danger. With EcoDC MACS, Dürr will in future divide the formerly large thyristor rectifiers into many small rectifier modules, each of which supplies voltage to only a small section of the dip bath. In the event of a defect in a single module, the coating quality remains unaffected. The division into many small units has another advantage: because EcoDC MACS can control each anode individually if necessary, the voltage in the dip tank can be regulated much more precisely and thus enables optimised voltage profiles - flexibly designed for a wide range of car body models. And since the smaller units cost only a fraction of the price of conventional thyristor rectifiers, the investment for replacement units is also significantly reduced.

Higher efficiency also saves energy and reduces operating and investment costs.

Furthermore, Dürr has increased the efficiency of EcoDC MACS to up to 96 percent by replacing conventional IGBT modules with modern semiconductors made of silicon carbide (SiC). Despite the increased performance, this results in measurable energy savings, which can be attributed to lower waste heat and a resulting reduction in the supply of cooling air. An example calculation for a painting plant with 58 car bodies per hour showed that the changeover to SiC semiconductors can generate energy cost savings of around 43,000 euros per year. This does not yet include the savings due to the better power quality and the resulting minimal reactive power.

Active harmonic compensation

But Dürr has not been satisfied with this yet either. Thanks to another innovation, the linear current consumption by the integrated Active Front End (AFE) technology, harmonic currents are compensated, which in the worst case could lead to equipment shutdown and downtime. AFE technology eliminates the need to invest in additional compensation equipment by increasing the power factor to approximately 1. In addition, the active harmonic compensation guarantees compliance with the legal limits for mains voltage quality.

New bus system facilitates troubleshooting

Finally, Dürr has also optimised data transmission from the rectifier module to the control unit. Whereas a special ring bus system was previously used for this purpose, data is now transmitted via a commercially available Profinet or Ethernet IP interface. This is primarily intended to simplify troubleshooting and thus ensure higher system availability. Moreover, a high-performance bus system is a prerequisite for Industry 4.0 solutions with additional diagnostic options.

Higher voltage for new coatings

With this overall package, especially with the new SiC rectifier modules, plant operators are also well positioned for the future, because new generations of paints may require higher coating currents or voltages. Dürr's EcoDC MACS is designed for a higher DC voltage of maximum 450 volts DC and for more current per rectifier mode up to a maximum of 120 amps DC.

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