Despite the advantages of ac, dc drives still have a future

Paul Boughton
For years, pundits have been forecasting the demise of the dc drive. The advantages of ac are many - reduced maintenance, cheaper motors. So, are they right? Richard Smith reports.

While there is no doubt that, particularly for the OEM market and for new installations in paper mills, steel mills, cranes and in many more areas, ac is becoming the drive of choice, the statistics for worldwide sales seem to defy the apparently unassailable logic. The fact is that dc sales are, in fact, holding up very well.

The simple reason is the massive existing installed base of dc around the world. When their plant is suffering breakdowns, struggling to meet required tolerances or simply too slow, users are often faced with a stark choice - rip it all out and start again or look to an upgrade option. Dc motors are both efficient and robust and offer years upon years of service, providing adequate performance for all but the most demanding of applications. Retaining an existing dc motor and upgrading the drive makes good environmental sense and, from a commercial perspective, provides an opportunity to enhance the overall system performance and reliability whilst minimising the risk to production and capital investment; an attractive proposal in today's economic environment.

At Control Techniques, for instance, dc drive sales have proved to be steady and even expanding slightly in some world markets. Over several years, their market share has been increasing by 1 per cent per year.

In Europe, for instance, there is a large installed dc base that dates back several decades, representing good opportunities for older outdated devices to be replaced with newer ones that provide more functionality and less maintenance. This market is not going away but represents a totally different market with a different set of dynamics compared with ac.

Clearly, the world recession will have some effect on this, perhaps in a negative way, but conceivably could boost the dc market, at the expense of ac as the purse-strings tighten.

Control Techniques' launched the world's first digital dc drive, Mentor, in 1986. It believes in the future of the dc market and committed considerable research and development resources - 14 man years - into a new dc drive - Mentor MP.

Mentor MP offers the same flexible control platform as Control Techniques' intelligent ac drive, Unidrive SP, to make Mentor MP the most advanced dc drive on the market, giving energy efficient operation, high performance and flexible system interfacing capabilities.

Each Mentor MP has three option modules slots offering users unrivalled flexibility to choose from any combination of the 18 option modules available including additional feedback options and I/O as well as onboard programmable control. The optional fieldbus connectivity is unique, with Profibus, Interbus, DeviceNet, CANopen and Various Ethernet protocols such as Modbus TCP/IP, Ethernet/IP and EtherCAT.

The built-in no-cost field controller is all most users will ever need and a new external field controller, FXMP25, will provide field forcing for highly dynamic applications, such as where very fast reversing may be required.

Control Techniques has a patent (pending) to protect a unique aspect of the Mentor MP design. Galvanic isolation between power and control is a standard feature in ac drives and, in the case of power-side failure, protects the control circuits and customer connected equipment. Other dc drives feature a form of isolation that rely on impedance only isolation, which has limitations; Mentor MP does not, its novel technology achieves Galvanic Isolation without compromising performance or reliability.

Mentor MP spans 25A to 1850A as a single module - 7400A when paralleled - is available in 12 and 18-pulse configurations for low harmonics.

Despite the effects of the credit crunch, Control Techniques predicts the dc market will continue at a healthy level. In the long term, it is inevitable that dc will eventually decline, but it would be a very brave market analyst who predicts when this will happen.

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Richard Smith, is Product Manager High Performance Drives, Control Techniques, Newton, Powys, Wales.

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