The world market for dc drives is worth an estimated $370m. The USA has the largest installed base, closely followed by China, and companies throughout these regions are in no rush to convert their dc infrastructure to ac. But whereas ac drive technology has soared away in terms of functionality, sophistication and, of course uptake, progress on the dc drives front has, with few exceptions, been virtually stagnant. But users anxious to preserve their dc motor driven investments are demanding more from their dc drives suppliers.
Taking up this challenge has been a long haul for one of the more successful dc drive technology developers, but the result of this work has raised the bar for the sector as a whole. Control Techniques (CT) has taken its core SP platform - the control engine of its Unidrive SP ac drives series - and combined it with the well established Mentor dc drives power platform. The result is the Mentor MP, a dc drive with all the 'Smartcard' flexibility, digital control capability and communications options of its highly successful ac cousin.
CT launched Mentor, one of the first commercial digital dc drives, back in 1986. Though it shares the power platform of the latest Mentor II dc drive, Mentor MP is a very different animal. It has three universal option module slots, which can be populated by any combination of three from the 18 option modules currently assigned to the dc platform, including field communications, encoder feedback, I/O and control functions. Smartcard is a standard feature for all drive parameter and control program storage.
The Mentor MP's standard field controller is probably adequate for most needs, but for those duties that demand it, there is a new external field controller option - the FXMP25 (the numeral defining the maximum field current of 25A). This delivers field forcing for highly dynamic applications like the very fast reversing typical of machine tool spindle drives. And for lower cost systems that do not require fast reversal, there's a new feature that allows reverse jogging control with two-quadrant drives.
For older motors with very low field voltages and field currents greater than 25A, the Mentor MP itself has a field mode, allowing it to be implemented as a field controller with no additional components. Thus field current control is virtually unlimited.
Reliability assured
According to Mentor product manager, Richard Smith, the simulation and modelling tools used to develop the MP platform just weren't available a few years ago. Their use in the development of Mentor MP has given the design team a powerful means of investigating and testing the systems right up to the launch stage. The result, he says, is "market leading" reliability. Key areas targeted by the design team were over-voltage protection, surge protection and thermal management.
Galvanic isolation between power and control circuits is a common feature of ac drives. CT is now including this protection feature within the Mentor MP, but it has gone one better and designed a novel microprocessor-controlled optical isolation system (for which a patent is pending). This technology achieves the necessary galvanic isolation between power and control stages without compromising performance or reliability. CT's strategic planning vice president, John Murphy concludes:
"Despite the common perception that dc is in terminal decline, our research suggests that the global dc drives market is still growing. In fact, with the credit crisis, we expect continued growth as engineers decide to retain perfectly good plant such as dc motor based process lines. CT's dc market share reached 13% in 2007. We have a target of 18% by 2012, which seems eminently achievable."